EO (Earth Observation) Topics on Climate Change
EO (Earth Observation) Topics on Climate Change
Since the start of the space age, Earth observation is providing its share of evidence for a better perception and understanding of our Earth System and its response to natural or human-induced changes.
Earth is a complex, dynamic system we do not yet fully understand. The Earth system comprises diverse components that interact in complex ways. We need to understand the Earth's atmosphere, lithosphere, hydrosphere, cryosphere, and biosphere as a single connected system. Our planet is changing on all spatial and temporal scales.
Over the years, the entire Earth Observation community, the space agencies as well as other governmental bodies, and many international organizations (UN, etc.) are cooperating on a global scale to come to grips with the modeling of the Earth system, including a continuous process of re-assessment and improvement of these models. The goal is to provide scientific evidence to help guide society onto a sustainable pathway during rapid global change.
In the second decade of the 21st century, there is alarming evidence that important tipping points, leading to irreversible changes in major ecosystems and the planetary climate system, may already have been reached or passed. Ecosystems as diverse as the Amazon rainforest and the Arctic tundra, may be approaching thresholds of dramatic change through warming and drying. Mountain glaciers are in alarming retreat and the downstream effects of reduced water supply in the driest months will have repercussions that transcend generations. 1)
Table 1: Overview of some major international bodies involved in global-change research programs 2)
The UN Framework Convention on Climate Change (UNFCCC) is an intergovernmental treaty developed to address the problem of climate change. The Convention, which sets out an agreed framework for dealing with the issue, was negotiated from February 1991 to May 1992 and opened for signature at the June 1992 UN Conference on Environment and Development (UNCED) — also known as the Rio Earth Summit. The UNFCCC entered into force on 21 March 1994, ninety days after the 50th country's ratification had been received. By December 2007, the convention had been ratified by 192 countries. 3)
In the meantime, there were many UN conferences on Climate Change, starting with the UN climate conference in Kyoto, Japan, in December 1997. The Kyoto Protocol set standards for certain industrialized countries. Those targets expired in 2012.
Meanwhile, greenhouse gas emissions from both developed and developing countries have been increasing rapidly. Even today, those nations with the highest percentage of environment pollution, are not willing to enforce stricter environmental standards in their countries in order to protect their global business interests. It's a vicious cycle between these national interests and the deteriorating environment, resulting in more frequent and violent catastrophes on a global scale. All people on Earth are effected, even those who abide by their strict environmental rules.
The short descriptions in the following chapters are presented in reverse order on some topics of climate change to give the reader community an overview of research results in this wide field of global climate and environmental change.
Winds Trigger Pond Growth
July 6, 2017: Wind is a force to be reckoned with. It can stir up monsoons, carry dust thousands of miles, and sculpt rock into sinuous arches. But sometimes, the effects of wind go unnoticed for years, like when it carves away slowly at the edges of a pond. 4)
A new study shows that winds are responsible for the widespread growth of ponds in three watersheds along the Mississippi River. The paper, published in April 2017 in Geophysical Research Letters, shows that wind-driven waves can erode pond banks, leading them to migrate in the direction of the wind. In effect, researchers have shown that wind-driven erosion, which nibbles away coastlines and the edges of larger bodies of water, can also happen inland on small scales. 5)
The researchers analyzed roughly 10,000 satellite images taken between 1982 and 2016, examining land and water pixels to look for inland change across the Mississippi River Delta. "Up until now, a lot of focus has been on coastal retreat," said Alejandra Ortiz, a marine geologist at Indiana University, Bloomington. Instead, Ortiz and colleagues focused on internal fragmentation; that is, what happens when land becomes subdivided by inland erosion processes. "Our thinking was, can you see this on large scale?"
Ortiz and her co-authors found that ponds in the Mississippi Delta tended to expand in a southwesterly direction, which is the same direction as the prevailing winds (which blow out of the northeast). This was especially true in Terrebonne and Barataria basins, where 80 percent of the ponds are expanding. The other study basin, the Atchafalaya-Vermillion, was deemed stable, with nearly as many ponds contracting as expanding—roughly 30 percent.
The false-color image of Figure 1 shows the area of study along the Atchafalaya Delta. It was captured on December 1, 2016, by OLI (Operational Land Imager) on Landsat 8. The colors emphasize the difference between land and water while allowing viewers to observe waterborne sediment, which is typically absent from false-color imagery.
Figure 1: OLI image on Landsat-8 of ponds in three watersheds along the Mississippi River south of New Orleans, acquired on December 1, 2016 (image credit: NASA Earth Observatory, images by Joshua Stevens, using Landsat data from the USGS and data from Ortiz, A. C., Roy, S., & Edmonds, D. A. (2017), story by Pola Lem)
The images of Figures 2 and 3 illustrate ponds that have grown (blue) or receded (orange) near the delta. In areas like Houma, Louisiana, the size of ponds increased significantly. The Terrebone and Barataria basins have much higher pond density, making them more susceptible to pond merging—when two or more ponds migrate toward each other and produce one larger body of water.
Figure 2: Landsat images (Landsat-7, Landsat-4, Landsat-5 and Landsat-8) of ponds that have grown (blue) or receded (orange) near the delta, acquired in the period 1982 - 2016 (image credit: NASA Earth Observatory, images by Joshua Stevens, using Landsat data from the USGS and data from Ortiz, A. C., Roy, S., & Edmonds, D. A. (2017), story by Pola Lem)
Some ponds were too small to generate waves strong enough to erode the shoreline. The researchers found that critical pond width was about 300 meters. Ponds at least that wide offer enough open space—for wind to gather momentum, or "fetch" as sailors and meteorologists call it—to create waves big enough to nibble away the shore.
Ortiz said the findings could affect the management of erosion-prone water bodies. For instance, managers could create physical barriers to prevent ponds from growing. "One possibility is thinking about putting in something that stops wave generation," she said.
Figure 3: Landsat images (Landsat-7, Landsat-4, Landsat-5 and Landsat-8) of ponds that have grown (blue) or receded (orange) near the delta, acquired in the period 1982 - 2016 (image credit: NASA Earth Observatory, images by Joshua Stevens, using Landsat data from the USGS and data from Ortiz, A. C., Roy, S., & Edmonds, D. A. (2017), story by Pola Lem)
Increasing rate of GMSL (Global Mean Sea Level) rise during 1993-2014
June 2017: Ocean levels rose 50 percent faster in 2014 than in 1993, with meltwater from the Greenland ice sheet now supplying 25 percent of total sea level increase compared with just five percent 20 years earlier, researchers reported on June 26, 2017. The findings add to growing concern among scientists that the global watermark is climbing more rapidly than forecast only a few years ago, with potentially devastating consequences. 6)
- The findings add to growing concern among scientists that the global watermark is climbing more rapidly than forecast only a few years ago, with potentially devastating consequences. Hundreds of millions of people around the world live in low-lying deltas that are vulnerable, especially when rising seas are combined with land sinking due to depleted water tables, or a lack of ground-forming silt held back by dams.
- Major coastal cities are also threatened, while some small island states are already laying plans for the day their drowning nations will no longer be livable.
- "This result is important because the IPCC (Intergovernmental Panel on Climate Change), the UN science advisory body, makes a very conservative projection of total sea level rise by the end of the century," at 60 to 90 cm, said Peter Wadhams, a professor of ocean physics at the University of Oxford who did not take part in the research.
- That estimate, he added, assumes that the rate at which ocean levels rise will remain constant. "Yet there is convincing evidence — including accelerating losses of mass from Greenland and Antarctica — that the rate is actually increasing, and increasing exponentially."
- Greenland alone contains enough frozen water to lift oceans by about 7 m, though experts disagree on the global warming threshold for irreversible melting, and how long that would take once set in motion.
- "Most scientists now expect total rise to be well over 1 m by the end of the century," Wadhams said.
- The new study, published in Nature Climate Change, reconciles for the first time two distinct measurements of sea level rise. 7)
- GMSL has been rising at a faster rate during the satellite altimetry period (1993–2014) than previous decades, and is expected to accelerate further over the coming century. However, the accelerations observed over century and longer periods2 have not been clearly detected in altimeter data spanning the past two decades. Here we show that the rise, from the sum of all observed contributions to GMSL, increases from 2.2 ± 0.3 mm yr-1 in 1993 to 3.3 ± 0.3 mm yr-1 in 2014. This is in approximate agreement with observed increase in GMSL rise, 2.4 ± 0.2 mm yr-1 (1993) to 2.9 ± 0.3 mm yr-1 (2014), from satellite observations that have been adjusted for small systematic drift, particularly affecting the first decade of satellite observations. 8)
- The mass contributions to GMSL increase from about 50% in 1993 to 70% in 2014 with the largest, and statistically significant, increase coming from the contribution from the Greenland ice sheet, which is less than 5% of the GMSL rate during 1993 but more than 25% during 2014. The suggested acceleration and improved closure of the sea-level budget highlights the importance and urgency of mitigating climate change and formulating coastal adaption plans to mitigate the impacts of ongoing sea-level rise.
Lightning Sparking More Boreal Forest Fires
June 26, 2017: A new NASA-funded study finds that lightning storms were the main driver of recent massive fire years in Alaska and northern Canada, and that these storms are likely to move farther north with climate warming, potentially altering northern landscapes. 9)
The study, led by Vrije Universiteit Amsterdam and the University of California, Irvine, examined the cause of the fires, which have been increasing in number in recent years. There was a record number of lightning-ignited fires in the Canadian Northwest Territories in 2014 and in Alaska in 2015. The team found increases of between two and five percent a year in the number of lightning-ignited fires since 1975. 10)
To study the fires, the team analyzed data from NASA's Terra and Aqua satellites and from ground-based lightning networks.
Lead author Sander Veraverbeke of Vrije Universiteit Amsterdam, who conducted the work while at UC Irvine, said that while the drivers of large fire years in the high north are still poorly understood, the observed trends are consistent with climate change. "We found that it is not just a matter of more burning with higher temperatures. The reality is more complex: higher temperatures also spur more thunderstorms. Lightning from these thunderstorms is what has been igniting many more fires in these recent extreme events," Veraverbeke said.
Study co-author Brendan Rogers at Woods Hole Research Center in Falmouth, Massachusetts, said these trends are likely to continue. "We expect an increasing number of thunderstorms, and hence fires, across the high latitudes in the coming decades as a result of climate change." This is confirmed in the study by different climate model outputs.
Study co-author Charles Miller of NASA's Jet Propulsion Laboratory in Pasadena, California, said while data from the lightning networks were critical to this study, it is challenging to use these data for trend detection because of continuing network upgrades. "A spaceborne sensor that provides high northern latitude lightning data that can be linked with fire dynamics would be a major step forward," he said.
The researchers found that the fires are creeping farther north, near the transition from boreal forests to Arctic tundra. "In these high-latitude ecosystems, permafrost soils store large amounts of carbon that become vulnerable after fires pass through," said co-author James Randerson of the University of California, Irvine. "Exposed mineral soils after tundra fires also provide favorable seedbeds for trees migrating north under a warmer climate."
"Taken together, we discovered a complex feedback loop between climate, lightning, fires, carbon and forests that may quickly alter northern landscapes," Veraverbeke concluded. "A better understanding of these relationships is critical to better predict future influences from climate on fires, and from fires on climate."
Figure 4: A lightning-caused wildfire burns in Alberta, Canada (image credit: The Government of Alberta)
Sea level as a metronome of Earth's history
May 19, 2017: Sedimentary layers contain stratigraphic cycles and patterns that precisely reveal the succession of climatic and tectonic conditions that have occurred over millennia. Researchers have been working on an analytical method that combines observing deep-water sedimentary strata and measuring in them the isotopic ratio between heavy and light carbon. They have discovered that the cycles that punctuate these sedimentary successions are ascribable to sea level changes. 11) 12) 13)
Sedimentary layers record the history of the Earth. They contain stratigraphic cycles and patterns that precisely reveal the succession of climatic and tectonic conditions that have occurred over millennia, thereby enhancing our ability to understand and predict the evolution of our planet.
Researchers at the University of Geneva (UNIGE), Switzerland, - together with colleagues at the University of Lausanne (UNIL) and American and Spanish scientists - have been working on an analytical method that combines observing deep-water sedimentary strata and measuring in them the isotopic ratio between heavy and light carbon.
They have discovered that the cycles that punctuate these sedimentary successions are not, as one might think, due solely to the erosion of mountains that surround the basin, but are more ascribable to sea level changes. This research, which you can read in the journal Geology, paves the way for new uses of isotopic methods in exploration geology.
The area south of the Pyrenees is particularly suitable for studying sedimentary layers. Rocks are exposed over large distances, allowing researchers to undertake direct observation. Turbidites can be seen here: large sediment deposits formed in the past by underwater avalanches consisting of sand and gravel.
The ups and downs of oceans regulate sedimentation cycles:
The variations in the ratio helped us explore the possible link with the sea level". The research team found that the turbidite-rich intervals were associated with high 12C levels, and almost always corresponded to periods when the sea level was low. It seems that sedimentary cycles are mainly caused by the rise and fall of the sea level and not by the episodic growth of mountains.
When the sea level is high, continental margins are flooded under a layer of shallow water. Since the rivers are no longer able to flow, they begin to deposit the sediments they carry there. This is why so little material reaches the deep basins downstream. When the sea level is low, however, rivers erode their beds to lower the elevation of their mouth; they transfer their sediment directly to the continental slopes of the deep basins, creating an avalanche of sand and gravel.
Consequently, if the variations of the sea level are known, it is possible to predict the presence of large sedimentary accumulations created by turbidites, which often contain large volumes of hydrocarbons, one of the holy grails of exploration geology.
Measuring stable carbon isotopes: a new indicator of reservoir rocks:
In addition, this measurement is relatively simple to perform and it provides accurate data - a real asset for science and mining companies. The study also highlights the importance of sea levels, which are a real metronome for the Earth's sedimentary history.
"Of course," concludes Honegger, "tectonic deformation and erosion are important factors in the formation of sedimentary layers; but they play a secondary role in the formation of turbidite accumulations, which are mainly linked to changes in the sea level".
Study of Ice-shelf Channel Formation in Antarctica
May 15, 2017: A team of scientists led by the ULB (Universite Libre de Bruxelles) Belgium and the Bavarian Academy of Sciences (Munich,Germany) have discovered an active hydrological system of water conduits and sediment ridges below the Antarctic ice sheet. Their study reveals that the scale of these subglacial features is five times bigger than those seen in today's deglaciated landscapes. 14)
The newly discovered, oversized sediment ridges actively shape the ice hundreds kilometers downstream, by carving deep incisions at the bottom of the ice. This is of interest for the stability of the floating ice shelves, as numerous studies show that ice shelf thinning has major consequences for ice sheet stability.
Subglacial conduits form under large ice sheets as part of their basal hydrological system. These tunnels have a typical diameter of several meters to tens of meters, and they funnel the subglacial melt water towards the ocean. However, new geophysical observations by the Laboratoire de Glaciologie of the ULB show that these conduits widen considerably the closer they come to the ocean. A new mathematical model explains this widening with the vanishing overburden pressure at the location where the ice becomes afloat on the ocean.
As the conduits widen, the outflow velocity of the subglacial water decreases, which leads to increased sediment deposition at the conduit's portal. Over thousands of years, this process builds up giant sediment ridges - comparable in height with the Eiffel tower - below the ice. Active sedimentation in subglacial water conduits seems to drive the formations of Eskers - elongated ridges of gravel which are commonly observed today in areas where former ice sheets have retreated. However, the remainders of today's Eskers are considerably smaller in size than those now discovered in Antarctica.
Ice-shelf channels are long curvilinear tracts of thin ice found on Antarctic ice shelves. Many of them originate near the grounding line, but their formation mechanisms remain poorly understood. The study team uses ice-penetrating radar data from the Roi Baudouin Ice Shelf, East Antarctica, to infer that the morphology of several ice-shelf channels is seeded upstream of the grounding line by large basal obstacles indenting the ice from below. The team interprets each obstacle as an esker ridge formed from sediments deposited by subglacial water conduits, and calculates that the eskers' size grows towards the grounding line where deposition rates are maximum. Relict features on the shelf indicate that these linked systems of subglacial conduits and ice-shelf channels have been changing over the past few centuries. Because ice-shelf channels are loci where intense melting occurs to thin an ice shelf, these findings expose a novel link between subglacial drainage, sedimentation and ice-shelf stability. 15)
Water beneath the Antarctic Ice Sheet promotes the formation of ice streams that rapidly slide over wet sediments and a lubricated base. Ice streams discharge the majority of Antarctic ice into floating ice shelves, which surround about 74% of the Antarctic perimeter. Ice shelves occupying embayments buttress the continental mass flux. The buttressing strength depends on the pattern of basal mass balance (i.e., the sum of melting and refreezing), which in turn influences ice-shelf geometry. Measurements show that basal melting is concentrated by ice-shelf channels, which are typically a few kilometers wide and extend for up to hundreds of kilometers along the shelf flow. Ice is thinnest along their central axes (sometimes thinner than half of the ice thickness), and basal melt rates are elevated at their onsets near the grounding line. Theory and satelliteborne observations suggest that such ‘subglacially sourced' ice-shelf channels are formed by buoyant melt-water plumes forced by basal melt water exiting from subglacial conduits at the grounding line. Hitherto, no such conduits have been observed, presumably because they are too small to be detected with ice-penetrating radar.
The study team surveyed three hydrologically predicted subglacial water-outlet locations at the Roi Baudouin Ice Shelf in Dronning Maud Land, Antarctica, all with corresponding ice-shelf channels seawards (Sites A–C, Figure 5a,b). Airborne radar data collected upstream of the satellite-inferred grounding line show distinct radar reflectors situated several hundred meters above the adjacent ice-bed interface (reflectors A–C, Figure 6 c). Using additional ground-based radar data from 2016, the team examined the reflectors' geometry in order to deduce their identity and evaluate three different scenarios for ice-shelf channel formation.
Figure 5: Overview of the study area: (a) Location of airborne (2011) and ground-based (2016) radar profiles of the Roi Baudouin Ice Shelf, East Antarctica, with Landsat image in the background. Grounding lines are marked for 1996, 2007 and 2016. The dashed white box delineates the area in b where radar-profile locations are shown with TanDEM-X surface elevation (5m contours), image credit: Study Team)
Figure 6: Overview of the study area: (c) Airborne radar profile EuA-EuA' covering the grounded ice sheet. Internal reflection hyperbolas reaching hundreds of meters above the ice-bed interface are evident (reflectors A–C), and are aligned with ice-shelf channels located seawards (into page). Reflectors A and C are beneath surface ridges (image credit: Study Team)
Giant conduits that can sap the ice from below: The evolving sediment ridges leave scars at the bottom of the ice as the ice flows over them. These scars are transmitted to the floating ice shelves farther downstream forming ice-shelf channels. Ice in these channels is up to half as thin as their surroundings, making them a weak spot when exposed to melting from the warmer ocean.
It was originally thought that ice-shelf channels are carved by melting due to the ocean only, but this seems only part of the story: "Our study shows that ice-shelf channels can already be initiated on land, and that the size of the channels significantly depends on sedimentation processes occurring over hundreds to thousands of years" indicates Reinhard Drews, lead author of the study.
The novel link between the subglacial hydrological system, sedimentation, and ice-shelf stability, offers new opportunities to unravel key processes beneath the Antarctic ice sheet, and also improves our ability to reconstruct the ice-sheet extent in the Northern Hemisphere during the last ice ages.
More information on this topic is provided in Ref. 15).
Glacial lakes grow in the Himalayas as well as the risks
• May 9, 2017: For people living around the Himalayas, the effects of global warming are anything but distant or abstract. As air temperatures have risen in the past half-century, glaciers have melted and retreated in these mountains. Between 1990 and 2015, Landsat satellites have documented a significant increase in both the number and average size of glacial lakes throughout the range. 16) 17) 18)
- Expanding lakes mean greater risks for the people living in valleys downstream. Specifically, there is a greater risk of GOLFs (Glacial Lake Outburst Floods)—a type of flash flood that occurs when ice or sediment dams collapse beneath glacial lakes. Landslides, avalanches, earthquakes, and volcanic eruptions often trigger GLOFs.
- After analyzing hundreds of satellite images, a research team from the Chinese Academy of Sciences and UCLA (University of California, Los Angeles) concluded that the number of Himalayan glacial lakes increased from 4,459 in 1990 to 4,950 in 2015, with a total area gain of 56 km2, or 14 percent.
- The degree of change varied by region. The size and number of lakes in the southern central Himalayas increased the most, particularly in Nepal, at elevations between 4,200 and 5,800 meters. In the map of Figure 7, regions where lakes expanded the most (20 percent or more) are shown with dark blue; regions where lakes grew only slightly (10 percent or less) are light blue. Lakes in the western Himalayas are generally more stable. Some glaciers in the Karakorum, for instance, are advancing. In contrast, rapid warming in the central Himalayas—as well as more soot being deposited on ice—may explain the rapid retreat of glaciers there.
Figure 7: Himalayan region effected by an increasing number of glacial lakes (image credit: NASA Earth Observatory, images by Jesse Allen, using Landsat data from the USGS, caption by Adam Voiland)
- The researchers observed changes to lakes at both the terminus of glaciers (proglacial lakes) and on top of them (supraglacial lakes). Between 1990 and 2015, the number of proglacial lakes increased by 227; the number of supraglacial lakes rose by 144. About 81 percent of the expansion in lake area was caused by changes to proglacial lakes.
- The researchers also identified 118 proglacial lakes that pose a particularly high risk to people living downstream. These lakes—many of them in the central Himalayas near Kathmandu, Nepal, and in the eastern Himalayas near Thimphu, Bhutan—grew by more than 1 percent each year.
- One rapidly expanding proglacial lake, Cirenmaco, highlights the risks. It stands at the base of Amaciren Glacier in the Zhangzangbo Valley of Nepal, and it was the scene of outburst floods in 1964 and 1981. The flood in 1981 was particularly destructive, killing hundreds of people, knocking out a power plant, and destroying bridges and roads. As seen in the false-color Landsat images (Figure 8), Cirenmaco's size more than doubled between 1988 and 2015.
- While proglacial lakes generally grew steadily each year, the lakes emerging on top of the glaciers were small, short-lived, and fast-changing. Many supraglacial lakes are perched on debris-covered ice, meaning lake water can quickly drain deeper into the glacier when cracks emerge.
Figure 8: Left: Image of Lake Cirenmaco acquired with Landsat-5 on Oct. 12, 1988; Right: Image of the same region acquired with Landsat-8 on Oct. 7, 2015 (image credit: NASA Earth Observatory, images by Jesse Allen, using Landsat data from the USGS, caption by Adam Voiland)
- Nepal's Ngozumpa Glacier (Figure 9), which lies about 25 km west of Mount Everest, has seen a significant increase in the number and size of supraglacial lakes on its surface. The pair of Landsat images below show the surface of the glacier in 1989 and 2015. Many of the lakes that existed in 1990 had drained by 2015, while many new lakes emerged in other areas.
- Researchers point to rising temperatures and melting glaciers as the primary cause for the increase in the size and number of lakes in the Himalayas. Rates of warming vary by region, but match the increases in total lake area. For instance, the team noted that between 1979 and 2014, temperatures rose by 5.9ºC at Nyalam in the central Himalaya; 1.0 ºC at Shiquanhe in the western Himalaya; and 0.4ºC at Yadong in the eastern Himalaya.
- "As the region continues to warm, it is urgent that scientists continue to monitor the most rapidly expanding glacial lakes with satellites because many are remote and very difficult to access," said Yong Nie, lead author of the study. "If we know which lakes pose the greatest risk, then authorities can take steps to develop early warning systems, drain specific high-risk lakes, and educate people on how to minimize their exposure to flash floods."
- Yongwei Sheng, a geography professor at UCLA and one of the study authors, added: "While we have reported on widespread lake expansion across the Himalayas, region-specific driving mechanisms, lake-specific risk assessments, as well as future change prediction all deserve further investigation."
Figure 9: Left: Landsat image of the Ngozumpa Glacier, acquired on Nov. 9, 1089; Right: Landsat-8 image of the same region acquired on Sept. 30, 2015 (image credit: NASA Earth Observatory, images by Jesse Allen, using Landsat data from the USGS, caption by Adam Voiland)
Regional Sea Level Scenarios for Coastal Risk Management
• May 2017: Sea level rise is occurring worldwide, but not at the same rate everywhere. Differences will also likely continue in the future, so decision-makers need local information to assess their community's vulnerability. These new scenarios integrate updated global sea level rise scenarios with regional factors, such as changes in land elevations and ocean circulation, that influence sea level regionally.
"The ocean is not rising like water would in a bathtub," said William Sweet, a NOAA oceanographer and lead author of the report detailing the scenarios. "For example, in some scenarios sea levels in the Pacific Northwest are expected to rise slower than the global average, but in the Northeast they are expected to rise faster. These scenarios will help communities better understand local trends and make decisions about adaptation that are best for them." 19) 20)
In the USA, the "Sea Level Rise and Coastal Flood Hazard Scenarios and Tools Interagency Task Force", jointly convened by the USGCRP (U.S. Global Change Research Program) and the NOC (National Ocean Council), began its work in August 2015. The Task Force has focused its efforts on three primary tasks: 1) updating scenarios of GMSL (Global Mean Sea Level) rise, 2) integrating the global scenarios with regional factors contributing to sea level change for the entire U.S. coastline, and 3) incorporating these regionally appropriate scenarios within coastal risk management tools and capabilities deployed by individual agencies in support of the needs of specific stakeholder groups and user communities.
Long-term sea level rise driven by global climate change presents clear and highly consequential risks to the United States over the coming decades and centuries. Today, millions of people in the United States already live in areas at risk of coastal flooding, with more moving to the coasts every year . Rising seas will dramatically increase the vulnerability of this growing population, along with critical infrastructure related to transportation, energy, trade, military readiness, and coastal ecosystems and the supporting services they provide.
GMSL (Global Mean Sea Level) has increased by about 21 cm to 24 cm since 1880, with about 8 cm occurring since 1993. In addition, the rate of GMSL rise since 1900 has been faster than during any comparable period over at least the last 2800 years. Scientists expect that GMSL will continue to rise throughout the 21st century and beyond, because of global warming that has already occurred and warming that is yet to occur due to the still-uncertain level of future emissions. GMSL rise is a certain impact of climate change; the questions are when, and how much, rather than if. There is also a long-term commitment (persistent trend); even if society sharply reduces emissions in the coming decades, sea level will most likely continue to rise for centuries.
While the long-term, upward shift in sea level is an underlying driver of changes to the nation's coasts, impacts are generally expressed through extreme water levels (short-period, lower-probability events both chronic and acute in nature) occurring against the background of this shifting baseline. Higher sea levels worsen the impacts of storm surge, high tides, and wave action, even absent any changes in storm frequency and intensity. Even the relatively small increases in sea level over the last several decades have led to greater storm impacts at many places along the U.S. coast. Similarly, the frequency of intermittent flooding associated with unusually high tides has increased rapidly (accelerating in many locations) in response to increases in RSL (Relative Sea Level ) as shown in Figure 10. At some locations in the United States, the frequency of tidal flooding (events typically without a local storm present) has increased by an order of magnitude over the past several decades, turning it from a rare event into a recurrent and disruptive problem. Significant, direct impacts of long-term RSL rise, including loss of life, damage to infrastructure and the built environment, permanent loss of land, ecological regime shifts in coastal wetlands and estuary systems, and water quality impairment, also occur when key thresholds in the coastal environment are crossed. Some of these impacts have the potential to ‘feedback' and influence wave impacts and coastal flooding. For example, there is evidence that wave action and flooding of beaches and marshes can induce changes in coastal geomorphology, such as sediment build up, that may iteratively modify the future flood risk profile of communities and ecosystems.
Figure 10: a) Multi-year empirical (smoothed) distributions for daily highest water levels in Norfolk, VA for the 1960s and 2010s, showing extent that local RSL rise has increased the flood probability relative to impact thresholds defined locally by the National Weather Service for minor (~0.5 m nuisance level), moderate (~0.9 m) and major (~1.2 m local level of Hurricane Sandy in 2012) impacts, relative to MHHW (Mean Higher High Water) tidal datum of the National Tidal Datum Epoch (1983–2001). b) Due to RSL rise, annual flood frequencies (based upon 5-year averages) in Norfolk for recurrent nuisance tidal floods with minor impacts are accelerating, as shown by the quadratic trend fit (goodness of fit [R2]=0.84). Flood rates are rapidly increasing in similar fashions along dozens of coastal cities of the U.S. (image credit: NOAA, USGS, EPA)
In this context, there is a clear need—and a clear call from states and coastal communities (White House, 2014)—to support preparedness planning with consistent, accessible, authoritative and more locally appropriate knowledge, data, information, and tools about future changes in sea level and associated coastal risks. In response to this need, the White House Council on Climate Preparedness and Resilience in 2015 called for the establishment of the Federal Interagency Sea Level Rise and Coastal Flood Hazard Scenarios and Tools Task Force, a joint task force of the NOC (National Ocean Council) and the USGCRP (U.S. Global Change Research Program ). The Task Force's charge is to develop and disseminate, through interagency coordination and collaboration, future RSL and associated coastal flood hazard scenarios and tools for the entire United States. These scenarios and tools are intended to serve as a starting point for on-the-ground coastal preparedness planning and risk management processes, including compliance with the new FFRMS (Federal Flood Risk Management Standard).
The Task Force is charged with leveraging the best available science; incorporating regional science and expertise where appropriate; building this information into user-friendly mapping, visualization, and analysis tools; and making it easily accessible through established Federal web portals. Part of the motivation for forming the Task Force was to bring together key efforts within individual agencies, such as the FEMA (Federal Emergency Management Agency), NOAA (National Oceanic and Atmospheric Administration), USACE (U.S. Army Corps of Engineers), USGS (U.S. Geological Survey), DoD (Department of Defense), EPA (Environmental Protection Agency) and NASA (National Aeronautics and Space Administration), that could serve as building blocks of an overall Federal system of sea level information and decision support, and to provide synthesis and coverage of the entire United States coastline.
• May 4, 2017: Thanks in large part to satellite measurements, scientists' skill in measuring how much sea levels are rising on a global scale - currently 3.4 mm per year - has improved dramatically over the past quarter century. But at the local level, it's been harder to estimate specific regional sea level changes 10 or 20 years away - the critical timeframe for regional planners and decision makers. 21)
That's because sea level changes for many reasons, on differing timescales, and is not the same from one place to the next. Developing more accurate regional forecasts of sea level rise will therefore have far-reaching benefits for the more than 30 percent of Americans who currently reside along the Pacific, Atlantic or Gulf Coasts of the contiguous United States.
New research published this week in the Journal of Climate reveals that one key measurement — large-scale upper-ocean temperature changes caused by natural cycles of the ocean — is a good indicator of regional coastal sea level changes on these decadal timescales. Such data may give planners and decision makers a new tool to identify key regions of U.S. coastlines that may be vulnerable to sea level changes on 10- to 20-year timescales. 22)
"Decision makers need a diverse set of tools with different informational needs," said lead author Veronica Nieves of UCLA and NASA's Jet Propulsion Laboratory in Pasadena, California. "Having a better understanding of the chances of local flood damage from rising seas in coastal areas is a key factor in being able to assess vulnerability, risk and adaptation options." Such tools could help planners decide whether a given part of a coastline would be better served by "soft" techniques, such as beach replenishment or preservation of wetlands, or by "hard" techniques, such as construction of sea walls or levees.
Nieves' team, which included participation from the IMEDEA (Mediterranean Institute for Advanced Studies) in Esporles, Spain, set out to detect decadal sea level changes over large U.S. coastal ocean regions. They compared existing NOAA records of upper-ocean temperatures in coastal waters for each U.S. ocean coastline with records of actual sea level changes from 1955 to 2012, and data from U.S./European satellite altimeter missions since 1992. They identified those sea level changes that have a large impact at regional scales in many locations, including largely populated cities. Sea level along the U.S. East Coast and West Coast can rise and fall by an inch or two (several centimeters) over the course of a decade or two because of fluctuations in upper ocean temperatures.
Their method was able to explain about 70 percent of regional sea level variability on decadal time scales for the West Coast, about 80 percent for the East Coast, and about 45 percent for the Gulf Coast. Along the Gulf Coast, the authors say other factors, such as tidal effects and the ongoing subsidence, or sinking, of the land, can play a more important role.
"Our study shows that large-scale upper-ocean temperature changes provide a good way to distinguish decade-long natural ocean signals from longer-term global warming signals," said Nieves. "This is important for regional planning, because it allows policymakers to identify places where climate change dominates the observed sea level rise and places where the climate change signal is masked by shorter-term regional variability caused by natural ocean climate cycles."
Nieves said an example is the U.S. West Coast, where the phase of a multi-decadal ocean climate pattern, called the Pacific Decadal Oscillation, has helped keep sea level rise lower during the past two decades. With the recent shift of this oscillation to its opposite phase, scientists expect sea level rise along the West Coast to accelerate in coming years.
"Scientists have worked hard to understand the really fast changes in sea level, such as storm surges, because they cause major damage, and the really slow changes because long-term sea level rise will shape the coastlines of the future," said study co-author Josh Willis of JPL. "But in between these fast and slow changes, there's a gap in our understanding. The results of our study help fill that gap."
Figure 11: Correlations in U.S. coastal sea level rise between the new sea level indicator tool and reconstructed decade-scale estimates of sea level (image credit: NASA/JPL-Caltech/UCLA/IMEDEA)
Sea Ice Extent Sinks to Record Lows at Both Poles
• March 22, 2017: Arctic sea ice appears to have reached on March 7 a record low wintertime maximum extent, according to scientists at NASA and the NASA-supported NSIDC (National Snow and Ice Data Center ) in Boulder, Colorado. And on the opposite side of the planet, on March 3 sea ice around Antarctica hit its lowest extent ever recorded by satellites at the end of summer in the Southern Hemisphere, a surprising turn of events after decades of moderate sea ice expansion. 23) 24) 25)
Figure 12: On March 7, 2017, Arctic sea ice hit a record low wintertime maximum extent in 2017. At 14.4 million km2, it is the lowest maximum extent in the satellite record, and 1.18 million km2 below the 1981 to 2010 average maximum extent (image credit: NASA/GSFC Scientific Visualization Studio, L. Perkins)
On Feb. 13, 2017, the combined Arctic and Antarctic sea ice numbers were at their lowest point since satellites began to continuously measure sea ice in 1979. Total polar sea ice covered 16.21 million km2, which is 2 million km2 less than the average global minimum extent for 1981-2010 – the equivalent of having lost a chunk of sea ice larger than Mexico.
Figure 13: These line graphs plot monthly deviations and overall trends in polar sea ice from October 1978 to March 7, 2017 as measured by satellites. The top line shows the Arctic; the middle shows Antarctica; and the third shows the global, combined total. The graphs depict how much the sea ice concentration moved above or below the long-term average (they do not plot total sea ice concentration). Arctic and global sea ice totals have moved consistently downward over 38 years. Antarctic trends are more muddled, but they do not offset the great losses in the Arctic (image credit: Joshua Stevens, NASA Earth Observatory)
The ice floating on top of the Arctic Ocean and surrounding seas shrinks in a seasonal cycle from mid-March until mid-September. As the Arctic temperatures drop in the autumn and winter, the ice cover grows again until it reaches its yearly maximum extent, typically in March. The ring of sea ice around the Antarctic continent behaves in a similar manner, with the calendar flipped: it usually reaches its maximum in September and its minimum in February.
This winter, a combination of warmer-than-average temperatures, winds unfavorable to ice expansion, and a series of storms halted sea ice growth in the Arctic. This year's maximum extent, reached on March 7 at 14.42 million km2, is 96,000 km2 below the previous record low, which occurred in 2015, and 1.22 million km2 smaller than the average maximum extent for 1981-2010.
Figure 14: On March 3, 2017, the sea ice cover around the Antarctic continent shrunk to its lowest yearly minimum extent in the satellite record, in a dramatic shift after decades of moderate sea ice expansion (image credit: NASA/GSFC Scientific Visualization Studio, L. Perkins)
Figure 15: Alternate view view of Arctic sea ice extend, acquired on March 7, 2017 (image credit: NASA Earth Observatory, image by Joshua Stevens using AMSR-2 sensor data on GCOM-W1)
"We started from a low September minimum extent," said Walt Meier, a sea ice scientist at NASA/GSFC in Greenbelt, Maryland. "There was a lot of open ocean water and we saw periods of very slow ice growth in late October and into November, because the water had a lot of accumulated heat that had to be dissipated before ice could grow. The ice formation got a late start and everything lagged behind – it was hard for the sea ice cover to catch up."
The Arctic's sea ice maximum extent has dropped by an average of 2.8 % per decade since 1979. The summertime minimum extent losses are nearly five times larger: 13.5% per decade. Besides shrinking in extent, the sea ice cap is also thinning and becoming more vulnerable to the action of ocean waters, winds and warmer temperatures.
This year's record low sea ice maximum extent might not necessarily lead to a new record low summertime minimum extent, since weather has a great impact on the melt season's outcome, Meier said. "But it's guaranteed to be below normal."
In Antarctica, this year's record low annual sea ice minimum of 2.11 million km2 was 184,000 km2 below the previous lowest minimum extent in the satellite record, which occurred in 1997.
Antarctic sea ice saw an early maximum extent in 2016, followed by a very rapid loss of ice starting in early September. Since November, the daily Antarctic sea ice extent has continuously been at its lowest levels in the satellite record. The ice loss slowed down in February.
This year's record low happened just two years after several monthly record high sea ice extents in Antarctica and decades of moderate sea ice growth. "There's a lot of year-to-year variability in both Arctic and Antarctic sea ice, but overall, until last year, the trends in the Antarctic for every single month were toward more sea ice," said Claire Parkinson, a senior sea ice researcher at Goddard. "Last year was stunningly different, with prominent sea ice decreases in the Antarctic. To think that now the Antarctic sea ice extent is actually reaching a record minimum, that's definitely of interest."
"It is tempting to say that the record low we are seeing this year is global warming finally catching up with Antarctica," Meier said. "However, this might just be an extreme case of pushing the envelope of year-to-year variability. We'll need to have several more years of data to be able to say there has been a significant change in the trend."
NASA, NOAA data show 2016 warmest year on record globally
• January 18, 2017: Earth's 2016 surface temperatures were the warmest since modern record-keeping began in 1880, according to independent analyses by NASA and NOAA (National Oceanic and Atmospheric Administration). Globally-averaged temperatures in 2016 were 0.99 degrees Celsius warmer than the mid-20th century mean. This makes 2016 the third year in a row to set a new record for global average surface temperatures. 26)
The 2016 temperatures continue a long-term warming trend, according to analyses by scientists at NASA/GISS ( Goddard Institute for Space Studies) in New York. NOAA scientists concur with the finding that 2016 was the warmest year on record based on separate, independent analyses of the data.
Figure 16: Global temperature anomalies averaged from 2012 through 2016 in degrees Celsius (image credit: NASA/GSFC Scientific Visualization Studio; data provided by Robert B. Schmunk, NASA/GSFC GISS)
Because weather station locations and measurement practices change over time, there are uncertainties in the interpretation of specific year-to-year global mean temperature differences. However, even taking this into account, NASA estimates 2016 was the warmest year with greater than 95 percent certainty. "2016 is remarkably the third record year in a row in this series," said GISS Director Gavin Schmidt. "We don't expect record years every year, but the ongoing long-term warming trend is clear." The planet's average surface temperature has risen about 1.1 degrees Celsius since the late 19th century, a change driven largely by increased carbon dioxide and other human-made emissions into the atmosphere.
Most of the warming occurred in the past 35 years, with 16 of the 17 warmest years on record occurring since 2001. Not only was 2016 the warmest year on record, but eight of the 12 months that make up the year – from January through September, with the exception of June – were the warmest on record for those respective months. October, November, and December of 2016 were the second warmest of those months on record – in all three cases, behind records set in 2015.
Phenomena such as El Niño or La Niña, which warm or cool the upper tropical Pacific Ocean and cause corresponding variations in global wind and weather patterns, contribute to short-term variations in global average temperature. A warming El Niño event was in effect for most of 2015 and the first third of 2016. Researchers estimate the direct impact of the natural El Niño warming in the tropical Pacific increased the annual global temperature anomaly for 2016 by 0.12 degrees Celsius.
Weather dynamics often affect regional temperatures, so not every region on Earth experienced record average temperatures last year. For example, both NASA and NOAA found the 2016 annual mean temperature for the contiguous 48 United States was the second warmest on record. In contrast, the Arctic experienced its warmest year ever, consistent with record low sea ice found in that region for most of the year.
Figure 17: The planet's long-term warming trend is seen in this chart of every year's annual temperature cycle from 1880 to the present, compared to the average temperature from 1880 to 2015. Record warm years are listed in the column on the right (image credit: NASA/Earth Observatory, Joshua Stevens)
NASA's analyses incorporate surface temperature measurements from 6,300 weather stations, ship- and buoy-based observations of sea surface temperatures, and temperature measurements from Antarctic research stations. These raw measurements are analyzed using an algorithm that considers the varied spacing of temperature stations around the globe and urban heating effects that could skew the conclusions. The result of these calculations is an estimate of the global average temperature difference from a baseline period of 1951 to 1980.
NOAA scientists used much of the same raw temperature data, but with a different baseline period, and different methods to analyze Earth's polar regions and global temperatures.
GISS is a laboratory within the Earth Sciences Division of NASA's Goddard Space Flight Center in Greenbelt, Maryland. The laboratory is affiliated with Columbia University's Earth Institute and School of Engineering and Applied Science in New York.
Changing rainfall patterns linked to water security in India
• January 11, 2017: Changing rainfall is the key factor driving changes in groundwater storage in India, according to a new study led by the IIT (Indian Institute of Technology) Gandhinagar published in the journal Nature Geoscience. The study shows that changing monsoon patterns - which are tied to higher temperatures in the Indian Ocean - are an even greater driver of change in groundwater storage than the pumping of groundwater for agriculture. 27) 28)
Agriculture in India relies heavily on groundwater for irrigation, particularly in the dry northern regions where precipitation is scarce. Groundwater withdrawals in the country have increased over tenfold since the 1950's, from 10-20 km3 per year in 1950, to 240-260 km3 per year in 2009. And satellite measurements have shown major declines in groundwater storage in some parts of the country, particularly in northern India.
"Groundwater plays a vital role in food and water security in India. Sustainable use of groundwater resources for irrigation is the key for future food grain production," says study leader Vimal Mishra, of the IIT Gandhinagar. "And with a fast-growing population, managing groundwater sustainably is going become even more important. The linkage between monsoon rainfall and groundwater can suggest ways to enhance groundwater recharge in India and especially in the regions where rainfall has been declining, such as the Indo-Gangetic Plain."
Groundwater acts like a bank for water storage, receiving deposits from surface water and precipitation, and withdrawals as people pump out water for drinking, industry, and irrigating fields. If withdrawals add up to more than the deposits, eventually the accounts could run dry, which could have disastrous consequences.
"This study adds another dimension to the existing water management framework. We need to consider not just the withdrawals, but also the deposits in the system," says Yoshihide Wada, a study coauthor and the deputy director of the Water program at the IIASA (International Institute for Applied Systems Analysis) in Austria.
The issue of groundwater depletion has been a topic of much discussion in India, but most planning has focused on pumping, or the demand side, rather than the deposit side. By looking at water levels in wells around the country, the researchers could track groundwater replenishment following the monsoons. They found that in fact, variability in the monsoons is the key factor driving the changing groundwater storage levels across the country, even as withdrawals increase.
In addition, the researchers found that the monsoon precipitation is correlated with Indian Ocean temperature, a finding which could potentially help to improve precipitation forecasts and aid in water resource planning.
"Weather is uncertain by nature, and the impacts of climate change are extremely difficult to predict at a regional level," says Wada "But our research suggests that we must focus more attention on this side of the equation if we want to sustainably manage water resources for the future."
Figure 18: Changing precipitation in India in the period 1980-2013 (image credit: IIT)
Ozone Hole 2016, and a Historic Climate Agreement
• October 27, 2016: The size and depth of the ozone hole over Antarctica was not remarkable in 2016. As expected, ozone levels have stabilized, but full recovery is still decades away. What is remarkable is that the same international agreement that successfully put the ozone layer on the road to recovery is now being used to address climate change. 29)
The stratospheric ozone layer protects life on Earth by absorbing ultraviolet light, which damages DNA in plants and animals (including humans) and leads to health issues like skin cancer. Prior to 1979, scientists had never observed ozone concentrations below 220 Dobson Units. But in the early 1980s, through a combination of ground-based and satellite measurements, scientists began to realize that Earth's natural sunscreen was thinning dramatically over the South Pole. This large, thin spot in the ozone layer each southern spring came to be known as the ozone hole.
The image of Figure 19 shows the Antarctic ozone hole on October 1, 2016, as observed by the OMI (Ozone Monitoring Instrument) on NASA's Aura satellite. On that day, the ozone layer reached its average annual minimum concentration, which measured 114 Dobson Units. For comparison, the ozone layer in 2015 reached a minimum of 101 Dobson Units. During the 1960s, long before the Antarctic ozone hole occurred, average ozone concentrations above the South Pole ranged from 260 to 320 Dobson Units.
The area of the ozone hole in 2016 peaked on September 28, 2016, at about 23 million km2. "This year we saw an ozone hole that was just below average size," said Paul Newman, ozone expert and chief scientist for Earth Science at NASA's Goddard Space Flight Center. "What we're seeing is consistent with our expectation and our understanding of ozone depletion chemistry and stratospheric weather."
The image of Figure 20 was acquired on October 2 by the OMPS (Ozone Mapping Profiler Suite) instrumentation during a single orbit of the Suomi-NPP satellite. It reveals the density of ozone at various altitudes, with dark orange areas having more ozone and light orange areas having less. Notice that the word hole isn't literal; ozone is still present over Antarctica, but it is thinner and less dense in some areas.
Figure 19: Image of the Antarctic Ozone Hole acquired with OMI on Aura on October 1, 2016 (image credit: NASA Earth Observatory, Aura OMI science team)
Figure 20: An edge-on (limb) view of Earth's ozone layer, acquired with OMPS on the Suomi-NPP on October 2, 2016 (image credit: NASA Earth Observatory, image by Jesse Allen, using Suomi-NPP OMPS data)
In 2014, an assessment by 282 scientists from 36 countries found that the ozone layer is on track for recovery within the next few decades. Ozone-depleting chemicals such as chlorofluorocarbons (CFCs)—which were once used for refrigerants, aerosol spray cans, insulation foam, and fire suppression—were phased out years ago. The existing CFCs in the stratosphere will take many years to decay, but if nations continue to follow the guidelines of the Montreal Protocol, global ozone levels should recover to 1980 levels by 2050 and the ozone hole over Antarctica should recover by 2070.
The replacement of CFCs with hydrofluorocarbons (HFCs) during the past decade has saved the ozone layer but created a new problem for climate change. HFCs are potent greenhouse gases, and their use — particularly in refrigeration and air conditioning — has been quickly increasing around the world. The HFC problem was recently on the agenda at a United Nations meeting in Kigali, Rwanda. On October 15, 2016, a new amendment greatly expanded the Montreal Protocol by targeting HFCs, the so-called "grandchildren" of the Montreal Protocol.
"The Montreal Protocol is written so that we can control ozone-depleting substances and their replacements," said Paul Newman, who participated in the meeting in Kigali. "This agreement is a huge step forward because it is essentially the first real climate mitigation treaty that has bite to it. It has strict obligations for bringing down HFCs, and is forcing scientists and engineers to look for alternatives."
NASA Releases First Map of Thawed Areas under Greenland's Ice Sheet
• August 2016: NASA researchers have helped produce the first map showing what parts of the bottom of the massive Greenland Ice Sheet are thawed – key information in better predicting how the ice sheet will react to a warming climate. 30)
Greenland's thick ice sheet insulates the bedrock below from the cold temperatures at the surface, so the bottom of the ice is often tens of degrees warmer than at the top, because the ice bottom is slowly warmed by heat coming from the Earth's depths. Knowing whether Greenland's ice lies on wet, slippery ground or is anchored to dry, frozen bedrock is essential for predicting how this ice will flow in the future, But scientists have very few direct observations of the thermal conditions beneath the ice sheet, obtained through fewer than two dozen boreholes that have reached the bottom. Now, a new study synthesizes several methods to infer the Greenland Ice Sheet's basal thermal state –whether the bottom of the ice is melted or not– leading to the first map that identifies frozen and thawed areas across the whole ice sheet.
"We're ultimately interested in understanding how the ice sheet flows and how it will behave in the future," said Joe MacGregor, lead author of the study and a glaciologist at NASA's Goddard Space Flight Center in Greenbelt, Md. "If the ice at its bottom is at the melting point temperature, or thawed, then there could be enough liquid water there for the ice to flow faster and affect how quickly it responds to climate change."
For this study, published last month in the Journal of Geophysical Research – Earth Surface, MacGregor's team combined four different approaches to investigate the basal thermal state. First, they examined results from eight recent computer models of the ice sheet, which predict bottom temperatures. Second, they studied the layers that compose the ice sheet itself, which are detected by radars onboard NASA's Operation IceBridge aircraft and suggest where the bottom of the ice is melting rapidly. Third, they looked at where the ice surface speed measured by satellites exceeds its "speed limit", the maximum velocity at which the ice could flow and still be frozen to the rock beneath it. Fourth, they studied imagery from the MODIS (Moderate Resolution Imaging Spectroradiometers) on the NASA Terra and Aqua satellites looking for rugged surface terrain that is usually indicative of ice sliding over a thawed bed. 31)
"Each of these methods has strengths and weaknesses. Considering just one isn't enough. By combining them, we produced the first large-scale assessment of Greenland's basal thermal state," MacGregor said. For each method, MacGregor's team looked for areas where the technique confidently inferred that the bed of Greenland's ice sheet was thawed or frozen. They then looked at the places where these methods agreed and classified these areas as likely thawed or likely frozen. The zones where there was insufficient data or the methods disagreed, they classified as uncertain.
From this synthesis, MacGregor and his colleagues determined that the bed is likely thawed under Greenland's southwestern and northeastern ice drainages, while it's frozen in the interior and west of the ice sheet's central ice divide. For a third of the Greenland ice sheet, there's not enough data available to determine its basal thermal state.
MacGregor said the team's map (Figure 21) is just one step in fully assessing the thermal state of the bottom of Greenland's ice sheet. "I call this the piñata, because it's a first assessment that is bound to get beat up by other groups as techniques improve or new data are introduced. But that still makes our effort essential, because prior to our study, we had little to pick on."
Figure 21: This first-of-a-kind map, showing which parts of the bottom of the Greenland Ice Sheet are likely thawed (red), frozen (blue) or still uncertain (gray), will help scientists better predict how the ice will flow in a warming climate (image credit: NASA Earth Observatory, Jesse Allen)
Study of hazardous thunderstorm intensification over Lake Victoria
• Sept. 22, 2016: An international study has determined that Lake Victoria in East Africa will become a hotspot for hazardous thunderstorms due to climate change using techniques that could improve regional climate and weather forecasts around inland bodies of water in the United States. 32)
Lake Victoria is divided among Uganda, Kenya and Tanzania. With a surface area close to 70,000 km2, it is the biggest lake in Africa. The lake is also a notoriously dangerous place for the 200,000 people who go fishing there at night. The International Red Cross estimates that between 3,000 and 5,000 fishermen per year lose their lives in violent storms on the lake.
Climate scientist Wim Thiery, affiliated with KU Leuven (Catholic University of Leuven, Belgium) and ETH Zürich (Swiss Federal Institute of Technology in Zürich, Switzerland), was able provide scientific evidence for why the storms occur in collaboration with colleague Kristopher Bedka of NASA's Langley Research Center in Hampton, Virginia.
"Thanks to new satellite-based storm detection products developed at NASA, we were able to map the number of hazardous thunderstorms and their locations in East Africa – every 15 minutes for a period ranging from 2005 to 2013," Thiery said. "During the day, most storms rage over the surrounding land, especially the typical afternoon thunderstorms that are caused by local upsurges of warm air. At night, these storms concentrate above Lake Victoria."
The storms are caused by circulation in the atmosphere above Victoria's enormous surface, according to Thiery. Daytime breezes flow outward from the cool water toward the warm land. At night, the opposite happens. Land breezes converge over the lake. Add in evaporation and you end up with storms.
To predict the impact of climate change on this process, Thiery ran climate simulations using an advanced computer model and determined that if the emission of greenhouse gases continues to increase, the extreme amounts of rainfall over Lake Victoria will increase by twice as much as the rainfall over the surrounding land. As a result, the lake will become a hotspot for night-time storms. "We found out that it is crucial to run our climate model at high resolution, and to couple a lake model to our climate model," Thiery said. "Resolving the lakes and getting lake surface temperatures right in the model is crucial to predicting the impact of these lakes on the regional climate and to get better projections of climate change."
The same advanced computer modeling could be useful in predicting future impacts of climate change around inland bodies of water in the U.S. "When performing climate simulations, it is very important to allow the body of water to accurately modify the atmosphere around it and vice versa," said Bedka, a climate scientist at NASA Langley. "Lakes and wide rivers such as Lake Michigan, the Mississippi River and, locally, the James River generate air circulations that can impact where thunderstorms form over the surrounding land. If climate projections do not properly account for these detailed interactions between land and water, they will underestimate their impact."
Properly coupled high-resolution models would be especially useful if these bodies of water were to warm up by a few degrees. That warming would have an impact on regional climate, and without precise modeling that impact could be difficult to predict and mitigate.
In addition, the satellite-based storm detection products that were crucial to the Lake Victoria study could also be useful in the U.S. Bedka and his team at NASA developed methods to automatically identify hazardous thunderstorms in "any satellite image, anytime, anywhere across the world." Lengthy datasets drawn from those methods could be used in the U.S. to help uncover interactions between land and water with a great degree of clarity.
Satellite data analysis: Satellite observations enable the recognition of severe weather by detecting OTs (Overshooting Tops) , that is, dome-like protrusions atop a cumulonimbus anvil induced by intense updrafts. OTs mark the presence of vigorous thunderstorms and are tightly linked to severe weather reports. By applying an OT detection algorithm to Methods (Meteosat Second Generation observations), the team established a new severe thunderstorm climatology for East Africa. The results reveal a marked imprint of Lake Victoria on the diurnal thunderstorm cycle and confirm its status as one of the most convectively active regions on Earth (Figure 22). From 2005 to 2013, 73% of all 1,400,000 OT pixels detected over the lake occurred at night (22:00 to 9:00 UTC), in contrast to the surrounding land where afternoon storms dominate (72% of all 4,200,000 OT pixels during 9:00 to 16:00 UTC). Local evaporation and mesoscale circulation have been identified as key drivers of the present-day diurnal cycle of precipitation over Lake Victoria, but so far it is not known how mean and extreme precipitation over this lake respond to a temperature increase induced by anthropogenic greenhouse gas emissions. To address this question, the team performed a high resolution (~7 km grid spacing), coupled lake–land–atmosphere climate projection for the African Great Lakes region with the regional climate model COSMO/CLM2 (Consortium for Small-scale Modeling/Community Land Model2), and analyzed coarser-scale ensemble projections from CORDEX (Coordinated Regional climate Downscaling Experiment) for the end of the century under a high-emission scenario. 33)
Figure 22: Lake imprint on severe thunderstorm occurrence. (a,b) Satellite-based overshooting tops (OT) detections during 2005–2013 over the Lake Victoria region (red square in the inset panel), from 9:00 to 15:00 UTC and from 00:00 to 9:00 UTC, respectively, as derived from the SEVIRI (Spinning Enhanced Visible and Infrared Imager) of Methods (image credit: Thunderstorm Study Team)
Extreme precipitation projections: The projections show a contrasting change of mean and extreme precipitation over Lake Victoria (Figure 23), with mean precipitation decreasing while the intensity of extreme precipitation increases. Moreover, by the end of the century the increase in extremes (precipitation above the 99th percentile) is 2.4±0.1 times higher over the lake than over its surrounding land in the highresolution projection (1.8±1.0 times in the CORDEX ensemble). Today convection initiates in the eastern part of the lake and intensifies while being advected westwards along the trade winds. In the future, storms are projected to release extreme precipitation more in the eastern part of the lake, leading to an eastward shift of intense precipitation (Figure 23a,b).
Figure 23: Projected end-of-century changes in extreme precipitation over Lake Victoria. (a) Night-time 99th percentile precipitation (P99%,night, 00:00 to 9:00 UTC) and (b) its projected future change from the high-resolution COSMO/CLM2 model. (c,d) 24 h Lake (blue bars) and surrounding land (red bars) binned precipitation change (P bin) from COSMO/CLM2 and the ensemble mean of nine CORDEX-Africa members, respectively (image credit: Thunderstorm Study Team)
The results emphasize a major hazard associated with climate change over East Africa with potential severe human impacts. Lake Victoria directly sustains the livelihood of 30 million people living at its coasts and its fishing industry is a leading natural resource for East African communities. However, given the projected increase in extreme over-lake thunderstorms, the current vulnerability of local fishing communities and their growing exposure driven by rapid urbanization along the lakefront, this lake is likely to remain the most dangerous stretch of water in the world. At the same time, the findings mark an opportunity for developing a satellite-based early warning system for hazardous thunderstorms over Lake Victoria.
Climate change already accelerating sea level rise, study finds
• August 10, 2016: Greenhouse gases are already having an accelerating effect on sea level rise, but the impact has so far been masked by the cataclysmic 1991 eruption of Mount Pinatubo in the Philippines, according to a new study led by NCAR (National Center for Atmospheric Research), Boulder, CO. 34) 35)
Driven by both the warming of the oceans and mass loss of the cryosphere,GSML (Global Mean Sea Level) is among the most powerful indicators of a changing climate. Originally estimated from a network of tide gages and restricted to coastlines, it has been derived from satellite measurements over the ice-free oceans since shortly after the launch of the TOPEX/Poseidon satellite in late 1992 with unprecedented accuracy and stability. During a given year, the height of the global seas can now be estimated to within a few millimeters revealing a mean rise of just over 3 mm yr-1. However the holistic nature of sea level has also led to challenges in its interpretation and it can be unclear at times whether its variations arise from changes in ocean heating, cryospheric melting, or the amount of water stored over land. Disentangling these various influences remains an ongoing science objective.
Among the major unanswered questions is why GMSL acceleration has not yet been detected in the altimeter record, given the increasing rates at which glacial and ice sheet melt are estimated to have occurred and as greenhouse gas concentrations have risen. The answer to this question has considerable policy relevance and debates over whether such an expected acceleration should be considered at the local policy level have at times been contentious. GMSL also has the potential to serve as an early indicator of accelerated climate change as it is less sensitive to the large internal variability that characterizes the surface temperature record, as evidenced by the recent so-called hiatus in global warming. Prevailing hypotheses for the lack of observed acceleration have maintained that changes in water storage over land or even drift in the satellite instruments may be to blame. Here it is demonstrated that the environment in which the era began was itself highly anomalous due to the preceding eruption of Mt Pinatubo on June 15, 1991, which cooled the oceans and decreased water storage over land and in the atmosphere. The net effect of these changes was to lower sea level prior to the altimeter era and induce an anomalous rise in the era's early years. It is proposed that this early anomalous rise masked the acceleration that would have otherwise occurred in the broader record. A consequence of this interpretation is that as the altimeter record lengthens, and in the coming decade barring another major volcanic eruption, accelerated rise will likely be detected.
Since late 1992 until the present, the TOPEX/Poseidon (T/P), Jason-1, and Jason-2 satellite altimeter missions have continuously measured sea level changes between ± 66º latitude with a temporal resolution of about 10 days, as shown in Figure 24. With their precision orbit determination, dual-frequency measurements (to remove ionosphere delays) and microwave radiometer (to remove delays due to water vapor in the troposphere), these missions have created an unrivaled 23-year climate data record of sea level change that is now being extended with the successful launch and deployment of the Jason-3 satellite earlier this year. Great care has been taken in calibrating these measurements via overlaps between missions and comparison to tide gauge sea level data. When averaged globally, the record provides an estimate of GMSL with a seasonal mean accuracy of 1–2 mm2.
Over the 23-year time series, it shows that GMSL has been rising at a rate of 3.3 ± 0.4 mm yr-1, but with notable inter-decadal variability.The current best estimate of the rates during the first (1993–2002) and second (2003– 2012) decades of the altimeter era are 3.5 and 2.7 mm yr-1, respectively, though important sources of uncertainty persist and raise caution regarding the record's early years (dashed line, Figure 24). There are several theories to explain this variability, but here we present an additional explanation, with important implications for anticipated near-future acceleration.
Figure 24: The altimeter record with decadal rates of change indicated (image credit: NCAR study team)
Legend to Figure 24: Estimates during the early stages of the record (dashed) are particularly subject to instrument related uncertainty.
Insight from Tide Gages: Prior to satellite altimetry, the primary source of sea level data is from historical tide gage records. These data provide a significantly longer time series relative to that of satellite altimetry, with a few records extending back into the 18th century. Several studies have estimated GMSL from the tide gage record using a variety of techniques. The resulting estimates of the GMSL trend from 1900 to 1990 range from 1.2 mm yr-1 to 1.9 mm yr-1, albeit with significant decadal variability about this long-term trend. Coupled with the higher GMSL trend observed during the satellite altimeter record discussed above, the tide gage record demonstrates unequivocal acceleration since the early 1900 s, with estimates ranging from 0.009 ± 0.002 mm yr-2 to 0.017 ± 0.003 mm yr-2. Based on these same studies, however, the majority of the acceleration arises from a shift that occurs around 1990 when the rate of sea level rise increases to the satellite-measured trend of 3.3 mm yr-1.
While the tide gage record may provide a ballpark estimate for what to expect during the altimeter era, it provides only weak quantitative information regarding what acceleration should be expected. In practice, calculating GMSL from 1900 to the present is a challenging problem based on the spatial and temporal sampling characteristics of available gages. There is little consensus across tide gage studies on the rate and acceleration of GMSL over the past century, thus making it difficult to interpret the altimeter era in a broader context. As an alternative approach to understanding sea level variability, we therefore seek to estimate and remove effects that obscure a possible underlying acceleration from the altimeter record itself. By doing so, we can potentially estimate the acceleration in GMSL directly from altimetry.
The GMSL Influence of the 1991 Mt Pinatubo Eruption: On June 15, 1991 the second largest volcanic eruption of the twentieth century in terms of aerosol radiative forcing began on the Philippine island of Luzon. Estimates of the amount of ash deposited in the stratosphere ranged from 20 to 30 Tg21 (approximately equal to a tenth the mass of all mankind) inducing a cooling of the globe and especially the world's oceans. Model-based estimates of the eruption's cooling effects suggest that the recovery of ocean heat content during the 1990's may have increased sea level rise by as much as 0.5 mm yr-1 on average in the decade following the eruption. The precise temporal evolution of the increase, and the impacts of the eruption on terrestrial and atmospheric water reservoirs remain largely unknown however; as quantifying these impacts is complicated by contemporaneous climate variability that can mask or amplify the response. Moreover, as many of the tools now in place for monitoring climate, such as for example the ARGO network of ocean temperature sensors and the GRACE satellite for estimating terrestrial water storage, were yet to be deployed at the time of the eruption, quantifying its precise climatic effects is nontrivial. However, recent specially designed climate model experiments now provide additional insight. What they reveal is that the eruption had a profound and temporally complex influence on several contributors to sea level and especially the amount of heat stored in the oceans, even when compared to background variations in climate, with major consequences for perceptions of GMSL acceleration during the altimeter era.
The CESM (Community Earth System Model) LE (Large Ensemble) is a 40-member ensemble of state-of-the-art coupled climate simulations spanning the 20th and 21st centuries using estimated historical forcings including volcanic eruptions. The simulated responses of GMSL and its individual contributors to the 1991 eruption of Mt Pinatubo are summarized in Figure 25. Immediately following the eruption, aerosols in the stratosphere blocked sunlight and cooled the surface. Surface temperatures quickly dropped, particularly over land due to its relative lack of thermal inertia. In turn, the atmosphere cooled, reducing the amount of moisture stored within it as water vapor. A cooler surface evaporated less moisture and was less convectively unstable, leading to a subsequent reduction in rainfall globally and disproportionately over land where diminished land water storage and runoff were a consequence of the eruption.
As these terrestrial and atmospheric changes are associated with reductions in their storage of water, their initial influence was to delay by about six months the eruption's main effect on sea level, which was a significant and rapid drop arising from a reduction in ocean heat content (OHC). The short timescale of the terrestrial and atmospheric influences relative to the oceans however limited their persistence, and by the beginning of the altimeter era in 1993, a GMSL drop of 5 to 7 mm from the eruption is estimated to have occurred, due largely to cooling of the oceans. While the LE's estimated OHC deficit is difficult to verify directly, given the large uncertainties and errors inherent in global ocean observations, confidence in the simulated response is bolstered by satellite estimates of the Earth's radiative imbalance, which strongly constrain the magnitude of ocean cooling and agree closely with simulated fluxes. Confidence in the ability of the LE to capture fundamental features of the eruption is therefore high.
Figure 25: Simulated sea level rise contributions during and following the eruption of Mt Pinatubo (image credit: NCAR study team)
Legend to Figure 25: Shown are changes in (A) clear-sky albedo over the tropical oceans (30ºN – 30ºS) as an indicator of the eruption's radiative effects and associated global mean sea level (GMSL) anomalies. In (B) contributions from ocean heat content (OHC) atmospheric water vapor (PW) and terrestrial water storage (TWS) estimated from the LE are shown. The large standard deviation across ensemble members (shaded) highlights the obscuring effect of natural climate variability on the eruption's influence in observations.
Discussion: This assessment of the sea level budget during Mt Pinatubo's 1991 eruption and in the several years thereafter has far reaching implications. First, it suggests that our monitoring of sea level via altimetry began in a highly anomalous environment, one in which OHC had been significantly depressed by the eruption while the offsetting influences of the atmosphere and land surface had largely diminished. As the oceans equilibrated from Pinatubo's initial cooling, the sea level experienced an anomalously rapid rise. It is therefore suggested that the GMSL rise estimated from the past decade is likely to be more representative of the background rate due to climate change than that observed during altimetry's initial decade. The eruption of Pinatubo is therefore also reaffirmed as a contributor to the apparently large shift in the rates of rise between the gauge and altimeter eras, consistent with the findings of previous studies.
The budgets simulated by the LE also have implications for the estimation of acceleration. From them, it is estimated that the rate of rise from 1993–2002 is subject to anomalous contribution of about 5 to 7 mm from the recovery to Pinatubo's oceanic cooling. This contribution therefore likely eclipses the background acceleration inferred from the tide gauge record. Given however that no such major eruption has occurred since 1991, a reasonable expectation is also that an accelerated rate of rise may emerge in the near future, particularly as the influences of climate variability and instrument drift reported in previous studies abate.
An estimate of this near-future GMSL acceleration can be made using LE projections of OHC, TWS, and PW in conjunction with independent estimates of future ice sheet losses (Figures 26 and 27). The estimate suggests that, as discussed above, it is unsurprising that acceleration has yet to be detected given the forced response to Pinatubo and the noise of internal climate variability in both OHC and TWS (shaded regions of Figs 2–4), and potential retrieval biases12. Moreover, the result also demonstrates that as anthropogenic influences continue to increase (as a result of both increasing greenhouse gas concentrations and decreasing anthropogenic aerosol emissions), a detectable acceleration of GMSL rise is likely to emerge as it exceeds the noise of background climate variability (shaded) in the coming years. The main contributor to this acceleration is the accelerated increase in OHC, which is offset somewhat by increasing but secondary influences from atmospheric and terrestrial storage, while a key component of the noise obscuring acceleration is the variability of TWS. Moreover, when the effects of the Mt Pinatubo eruption estimated form the LE are removed (blue), acceleration becomes apparent, even in the present day. The magnitude of the acceleration in the mid-21st century is estimated here to be 0.12 mm yr-2, though this value depends strongly on future ice sheet losses, which are highly uncertain1. Its accurate estimation depends both on the accuracy of altimeter retrievals and our ability to distinguish it from internal variability, which can be pronounced in some years but over the long term becomes increasingly negligible, particularly if acceleration is estimated from the full post-1993 record (red line in Figure 27).
With the launch of Jason-3 earlier this year, it is reasonable to ask what new climate insights the instrument may bring. This analysis concludes that if the lifespan of the instrument is comparable to that of its predecessor, Jason-2, the acceleration suggested in earlier studies will likely emerge from the noise of internal climate variability during its lifetime, barring another major volcanic eruption. Moreover, given the unique strengths of sea level as a stable and holistic measure of climate change, the broader altimeter record is likely to stand as one of the benchmark measures of accelerating changes in the climate system.
Figure 26: Sea level rise associated with ocean heat storage and the sum of all contributions estimated from LE budgets and cryospheric contributions (image credit: NCAR study team)
Legend to Figure 26: Shaded are 1σ (light) and 2σ ranges of internal variability. The mean LE estimate with Pinatubo effects removed is also shown (blue) along with the altimeter record (red).
Figure 27: Estimated acceleration in sea level rise based on the budget from the LE and cryospheric contributions using trend differences 1) between the first and second half of a record beginning in 1993 and ending in a given year (abscissa, red) and 2) for the trailing two decades, based on given end years (abscissa, black), image credit: NCAR study team
Legend to Figure 27: Acceleration with Pinatubo effects removed is also shown (blue). The 2σ spread in simulated acceleration estimates is shown for all contributions (light grey) and the contribution due to TWS alone (dark grey).
Difficulties in making weather measurements in the Arctic have led to underrepresentation of this rapidly warming area in historic temperature records
• July 21, 2016: A new NASA-led study finds that almost one-fifth of the global warming that has occurred in the past 150 years has been missed by historical records due to quirks in how global temperatures were recorded. The study explains why projections of future climate based solely on historical records estimate lower rates of warming than predictions from climate models. 36)
The study applied the quirks in the historical records to climate model output and then performed the same calculations on both the models and the observations to make the first true apples-to-apples comparison of warming rates. With this modification, the models and observations largely agree on expected near-term global warming. The results were published in the journal Nature Climate Change. Mark Richardson of NASA's Jet Propulsion Laboratory, Pasadena, California, is the lead author. 37)
The Arctic is warming faster than the rest of Earth, but there are fewer historic temperature readings from there than from lower latitudes because it is so inaccessible. A data set with fewer Arctic temperature measurements naturally shows less warming than a climate model that fully represents the Arctic. — Because it isn't possible to add more measurements from the past, the researchers instead set up the climate models to mimic the limited coverage in the historical records.
The new study also accounted for two other issues. First, the historical data mix air and water temperatures, whereas model results refer to air temperatures only. This quirk also skews the historical record toward the cool side, because water warms less than air. The final issue is that there was considerably more Arctic sea ice when temperature records began in the 1860s, and early observers recorded air temperatures over nearby land areas for the sea-ice-covered regions. As the ice melted, later observers switched to water temperatures instead. That also pushed down the reported temperature change.
Scientists have known about these quirks for some time, but this is the first study to calculate their impact. "They're quite small on their own, but they add up in the same direction," Richardson said. "We were surprised that they added up to such a big effect."
These quirks hide around 19 percent of global air-temperature warming since the 1860s. That's enough that calculations generated from historical records alone were cooler than about 90 percent of the results from the climate models that the IPCC (Intergovernmental Panel on Climate Change) uses for its authoritative assessment reports. In the apples-to-apples comparison, the historical temperature calculation was close to the middle of the range of calculations from the IPCC's suite of models.
Any research that compares modeled and observed long-term temperature records could suffer from the same problems, Richardson said. "Researchers should be clear about how they use temperature records, to make sure that comparisons are fair. It had seemed like real-world data hinted that future global warming would be a bit less than models said. This mostly disappears in a fair comparison."
Antarctic sea-ice expansion between 2000 and 2014 driven by tropical Pacific decadal climate variability
• July 4, 2016: The recent trend of increasing Antarctic sea ice extent — seemingly at odds with climate model projections — can largely be explained by a natural climate fluctuation, according to a new study led by NCAR (National Center for Atmospheric Research) of Boulder, CO. The study offers evidence that the negative phase of the IPO (Interdecadal Pacific Oscillation), which is characterized by cooler-than-average sea surface temperatures in the tropical eastern Pacific, has created favorable conditions for additional Antarctic sea ice growth since 2000. 38) The findings, published in the journal Nature Geoscience, may resolve a longstanding mystery: Why is Antarctic sea ice expanding when climate change is causing the world to warm? 39)
The study's authors also suggest that sea ice may begin to shrink as the IPO switches to a positive phase. "The climate we experience during any given decade is some combination of naturally occurring variability and the planet's response to increasing greenhouse gases," said NCAR scientist Gerald Meehl, lead author of the study. "It's never all one or the other, but the combination, that is important to understand."
Study co-authors include Julie Arblaster of NCAR and Monash University in Australia, Cecilia Bitz of the University of Washington, Christine Chung of the Australian Bureau of Meteorology, and NCAR scientist Haiyan Teng. The study was funded by the U.S. Department of Energy and by the National Science Foundation, which sponsors NCAR.
The sea ice surrounding Antarctica has been slowly increasing in area since the satellite record began in 1979. But the rate of increase rose nearly five fold between 2000 and 2014, following the IPO transition to a negative phase in 1999.
The new study finds that when the IPO changes phase, from positive to negative or vice versa, it touches off a chain reaction of climate impacts that may ultimately affect sea ice formation at the bottom of the world.
When the IPO transitions to a negative phase, the sea surface temperatures in the tropical eastern Pacific become somewhat cooler than average when measured over a decade or two. These sea surface temperatures, in turn, change tropical precipitation, which drives large-scale changes to the winds that extend all the way down to Antarctica.
The ultimate impact is a deepening of a low-pressure system off the coast of Antarctica known as the Amundsen Sea Low. Winds generated on the western flank of this system blow sea ice northward, away from Antarctica, helping to enlarge the extent of sea ice coverage.
"Compared to the Arctic, global warming causes only weak Antarctic sea ice loss, which is why the IPO can have such a striking effect in the Antarctic," said Bitz. "There is no comparable natural variability in the Arctic that competes with global warming."
Figure 28: On Sept. 19, 2014, the five-day average of Antarctic sea ice extent exceeded 20 million km2 for the first time since 1979, according to the National Snow and Ice Data Center. The red line shows the average maximum extent from 1979-2014 (image credit: NASA's Scientific Visualization Studio, Cindy Starr)
Sifting through simulations:
To test if these IPO-related impacts were sufficient to cause the growth in sea ice extent observed between 2000 and 2014, the scientists first examined 262 climate simulations created by different modeling groups from around the world.
When all of those simulations are averaged, the natural variability cancels itself out. For example, simulations with a positive IPO offset those with a negative IPO. What remains is the expected impact of human-caused climate change: a decline in Antarctic sea ice extent.
But for this study, the scientists were not interested in the average. Instead, they wanted to find individual members that correctly characterized the natural variability between 2000-2014, including the negative phase of the IPO. The team discovered 10 simulations that met the criteria, and all of them showed an increase in Antarctic sea ice extent across all seasons.
"When all the models are taken together, the natural variability is averaged out, leaving only the shrinking sea ice caused by global warming," Arblaster said. "But the model simulations that happen to sync up with the observed natural variability capture the expansion of the sea ice area. And we were able to trace these changes to the equatorial eastern Pacific in our model experiments."
Scientists suspect that in 2014, the IPO began to change from negative to positive. That would indicate an upcoming period of warmer eastern Pacific Ocean surface temperatures on average, though year-to-year temperatures may go up or down, depending on El Niño/La Niña conditions. Accordingly, the trend of increasing Antarctic sea ice extent may also change in response.
"As the IPO transitions to positive, the increase of Antarctic sea ice extent should slow and perhaps start to show signs of retreat when averaged over the next 10 years or so," Meehl said.
Climate Change May Shift or Shrink Penguin Habitat in Antarctica
• July 13, 2016: For thousands of years, spells of warm weather along the coast of Antarctica helped penguin populations thrive. Milder weather meant more bare-rock locations for the birds to lay their eggs and nurture their chicks, as well as less stress on the parents and newborns and less sea ice to fight through in the harbors. But over the past few decades, some parts of the southern continent have turned too warm—particularly the water—due to global climate change. According to a new study, that warmth is likely to change where Adélies can raise chicks. 40)
The contribution of climate change to shifts in a species' geographic distribution is a critical and often unresolved ecological question. Climate change in Antarctica is asymmetric, with cooling in parts of the continent and warming along the West Antarctic Peninsula (WAP). The Adélie penguin (Pygoscelis adeliae) is a circumpolar meso-predator exposed to the full range of Antarctic climate and is undergoing dramatic population shifts coincident with climate change. 41)
Using observations of penguin populations and of environmental conditions, as well as computer models of projected environmental changes, the team of researchers projected that nearly a third of Adélie penguin colonies around Antarctica (representing about 20 percent of the total population) could be in decline by 2060. By the end of the 21st century, as much as 60 percent of the colonies could be in trouble. On the other hand, some new refuges on other parts of the coast could open up and offset the losses.
"It is only in recent decades that we know Adélie penguins population declines are associated with warming, which suggests that many regions of Antarctica have warmed too much and that further warming is no longer positive for the species," said Megan Cimino.
The maps of Figures 29 and 30 were built from previous observations (in-person counts and satellite studies) of penguin populations. Red dots mark colonies that have been declining in population—most notably along the Antarctic Peninsula, one of the fastest-warming places on Earth. Blue dots show the location of growing colonies, which often matches where temperatures have been stable or somewhat cooler. Yellow dots denote stable penguin populations.
Warmer temperatures can open up land-based habitat for the penguins, while also keeping waterways ice-free and making it less arduous to search for food. But those warmer temperatures can also make the waters less hospitable (too warm; too little plankton) for the krill and fish that penguins eat. On the other hand, some coastlines could become more hospitable in the future—warming enough to create more habitat, but not enough to disrupt the food supply. The Cape Adare region on Ross Sea is one such area.
Figure 29: Adélie penguin breeding colonies and population status across Antarctica (image credit: Penguin Science Team)
Legend to Figure 29: Each colored circle represents a colonies' current population trend. The black dashed line separates West Antarctic Peninsula (WAP) from continental Adélie penguin colonies.
Figure 30: This Figure shows the output of "habitat suitability models" in which the science team plugged in data on past conditions around penguin colonies while projecting various scenarios in future water temperatures and sea ice concentration. Shades of red indicate areas where habitat is likely to be less suitable for raising penguin chicks as the world warms; blue areas are likely to become more favorable (image credit: NASA Earth Observatory, Joshua Stevens)
El Niño conditions in 2015 and early 2016 altered rainfall patterns around the world
• July 5, 2016: In the Amazon basin, El Niño reduced rainfall during the wet season, leaving the region drier at the start of the 2016 dry season than any year since 2002. "Severe drought conditions at the start of the dry season have set the stage for extreme fire risk in 2016 across the southern Amazon," said Doug Morton, an Earth scientist at NASA/GSFC (Goddard Space Flight Center) and a co-creator of the fire forecast. The wildfire risk for July to October now exceeds the fire risk in 2005 and 2010—drought years when wildfires burned large swaths of the rainforest. 42)
- The Amazon fire forecast analyzes the relationship between certain climate observations and active fire detections from NASA satellites to predict fire season severity. Developed in 2011 by scientists at the University of California, Irvine (UC Irvine) and NASA, the forecast model is focused particularly on the link between sea surface temperatures and fire activity. Warmer sea surface temperatures in the tropical Pacific (as observed during an El Niño) and Atlantic oceans shift rainfall away from the Amazon region, increasing the risk of fire during dry months.
- The forecast team also tracks changes in water storage during the dry season. The maps of Figure 31 show the accumulated deficit in the rainfall input to surface and underground water storage in 2016 and other recent drought years, as reported by the GPCC (Global Precipitation Climatology Centre). The accumulated deficit is measured from August through May; for example, August 2015 to May 2016 sets the stage for the 2016 dry season. Shades of red depict areas where rainfall has been below normal, while blues were above normal.
- For 2016, El Niño-driven conditions are far drier than in 2005 and 2010—the last years when the region experienced drought. The prediction team has developed a web tool to track the evolution of the fire season in near-real time by Amazon region. Estimated fire emissions from each region are updated daily based on active fire detections—made by the MODIS (Moderate Resolution Imaging Spectroradiometer) instrument on NASA's Terra satellite—and data from previous years in the Global Fire Emissions Database. So far, the region has seen more fires through June 2016 than in previous years, another indicator of a potentially tough season.
- "When trees have less moisture to draw upon at the beginning of the dry season, they become more vulnerable to fire and evaporate less water into the atmosphere," said UC-Irvine scientist Jim Randerson, who collaborated with UC-Irvine scientist Yang Chen on building the forecast model. "This puts millions of trees under stress and lowers humidity across the region, allowing fires to grow bigger than they normally would."
- NASA and UC-Irvine scientists have worked in recent years with South American officials and scientists to make them aware of the forecast. Liana Anderson, a from Brazil's National Center for Monitoring and Early Warning of Natural Disasters, said "fire forecasts three to six months before peak fire activity are important to identify areas with higher fire probability for integrated planning."
- The fire forecast team is also working to improve wildfire forecasts in other regions of the world. The scientists recently identified nine regions outside the Amazon where fire season risk could be forecast three- to six months ahead of peak fire activity. Randerson said it may be possible to build seasonal fire forecasts for much of Central America and for many countries in Southeast Asia.
Figure 31: Fire forecast maps for the Amazon region. The maps show the accumulated deficit in the rainfall input to surface and underground water storage in 2016 and other recent drought years, as reported by the GPCC (image credit: NASA Earth Observatory images by Joshua Stevens, using data courtesy of Yang Chen, University of California Irvine, and the GPCC)
Sea Ice Differences at Earth's Poles
May 2016: Why has the sea ice cover surrounding Antarctica been increasing slightly, in sharp contrast to the drastic loss of sea ice occurring in the Arctic Ocean? A new NASA-led study finds the geology of Antarctica and the Southern Ocean are responsible. 43)
A NASA/NOAA/university team led by Son Nghiem of NASA/JPL (Jet Propulsion Laboratory), Pasadena, California, used satellite radar, sea surface temperature, land form and bathymetry (ocean depth) data to study the physical processes and properties affecting Antarctic sea ice. They found that two persistent geological factors — the topography of Antarctica and the depth of the ocean surrounding it — are influencing winds and ocean currents, respectively, to drive the formation and evolution of Antarctica's sea ice cover and help sustain it. "Our study provides strong evidence that the behavior of Antarctic sea ice is entirely consistent with the geophysical characteristics found in the southern polar region, which differ sharply from those present in the Arctic," said Nghiem.
The Antarctic sea ice cover is dominated by first-year (seasonal) sea ice. Each year, the sea ice reaches its maximum extent around the frozen continent in September and retreats to about 17 percent of that extent in February. Since the late 1970s, its extent has been relatively stable, increasing just slightly; however, regional differences are observed.
Over the years, scientists have floated various hypotheses to explain the behavior of Antarctic sea ice, particularly in light of observed global temperature increases. Are changes in the ozone hole involved? Could fresh meltwater from Antarctic ice shelves be making the ocean surface less salty and more conducive to ice formation, since salt inhibits freezing? Are increases in the strength of Antarctic winds causing the ice to thicken? Something is protecting Antarctic sea ice, but a definitive answer has remained elusive.
To tackle this cryospheric conundrum, Nghiem and his team adopted a novel approach. They analyzed radar data from NASA's QuikSCAT satellite from 1999 to 2009 to trace the paths of Antarctic sea ice movements and map its different types. They focused on the 2008 growth season, a year of exceptional seasonal variability in Antarctic sea ice coverage.
Their analyses revealed that as sea ice forms and builds up early in the sea ice growth season, it gets pushed offshore and northward by winds, forming a protective shield of older, thicker ice that circulates around the continent. The persistent winds, which flow downslope off the continent and are shaped by Antarctica's topography, pile ice up against the massive ice shield, enhancing its thickness. This band of ice, which varies in width from roughly 100 to 1,000 km, encapsulates and protects younger, thinner ice in the ice pack behind it from being reduced by winds and waves.
The team also used QuikSCAT radar data to classify the different types of Antarctic sea ice. Older, thicker sea ice returns a stronger radar signal than younger, thinner ice does. They found the sea ice within the protective shield was older and rougher (due to longer exposure to wind and waves), and thicker (due to more ice growth and snow accumulation). As the sea ice cover expands and ice drifts away from the continent, areas of open water form behind it on the sea surface, creating "ice factories" conducive to rapid sea ice growth.
To address the question of how the Southern Ocean maintains this great sea ice shield, the team combined sea surface temperature data from multiple satellites with a recently available bathymetric chart of the depth of the world's oceans. Sea surface temperature data reveal that at the peak of ice growth season, the boundary of the ice shield remains behind a -1º Celsius temperature line surrounding Antarctica. This temperature line corresponds with the southern Antarctic Circumpolar Current front, a boundary that separates the circulation of cold and warm waters around Antarctica. The team theorized that the location of this front follows the underwater bathymetry.
When they plotted the bathymetric data against the ocean temperatures, the pieces fit together like a jigsaw puzzle. Pronounced seafloor features strongly guide the ocean current and correspond closely with observed regional Antarctic sea ice patterns. For example, the current stays near Bouvet Island, located 1,600 km from the nearest land, where three tectonic plates join to form seafloor ridges. Off the coast of East Antarctica, the -1º Celsius sea surface temperature lines closely bundle together as they cross the Kerguelen Plateau (a submerged microcontinent that broke out of the ancient Gondwana supercontinent), through a deep channel called the Fawn Trough. But those lines spread apart over adjacent deep ocean basins, where seafloor features are not pronounced. Off the West Antarctica coast, the deep, smooth seafloor loses its grip over the current, allowing sea ice extent to decrease and resulting in large year-to-year variations.
The study results are published in the journal Remote Sensing of Environment. 44) Other participating institutions include the Joint Institute for Regional Earth System Science and Engineering at UCLA; the Applied Physics Laboratory at the University of Washington in Seattle; and the U.S. National/Naval Ice Center, NOAA Satellite Operations Facility in Suitland, Maryland. Additional funding was provided by the National Science Foundation.
Figure 32: Location of the southern Antarctic Circumpolar Current front (white contour), with -1º Celsius sea surface temperature lines (black contours) on Sept. 22 each year from 2002-2009, plotted against a chart of the depth of the Southern Ocean around Antarctica (image credit: NASA/JPL-Caltech)
Figure 33: The edge of the protective ice shield is determined by the boundary of the surface temperature being -1ºC; the white cross is Bouvet Island (image credit: NASA/JPL- Caltech)
Methane and Carbon Dioxide on the rise in the period 2003-2014
• May 2016: Satellite readings show that atmospheric methane and carbon dioxide are continuing to increase despite global efforts to reduce emissions. Methane concentrations were somewhat constant until 2007, but since then have increased at about 0.3% per year, whereas global carbon dioxide levels continue to rise at about 0.5% per year. The reason for this recent methane increase is not fully understood, but scientists attribute it to several sources such as agriculture and fossil fuels.45)
The data also show seasonal fluctuations, such as higher concentrations of methane in India and China during August and September. This is because wetlands and rice paddies are a major source of methane and emissions are largest if it is warm and humid. - Other regions such as the Tropics, the USA and parts of Russia experience similar seasonal changes.
Carbon dioxide shows similar seasonal fluctuations, albeit with a maximum concentration earlier in the season at northern latitudes. This is due to the regular uptake and release of carbon dioxide by the growing and decay of terrestrial vegetation: photosynthesis, respiration and decay of organic matter. Overall, carbon dioxide has shown a steady increase over the past decade despite global efforts to reduce emissions.
Currently, plants take up about 25% of the carbon dioxide we are emitting and, without this, atmospheric carbon dioxide levels and related consequences would be much larger," said Michael Buchwitz from the IUP (Institute of Environmental Physics) of the University of Bremen in Germany. "However, we do not know how plants will respond to a changing climate. Our understanding of the ‘land carbon sink' is limited. A goal of the satellite carbon dioxide observations is to close related knowledge gaps, which will lead to improved climate prediction."
The upcoming Sentinel-5P mission(launch scheduled for October 2016) for Europe's Copernicus program is set to continue data collection on methane and other components of atmospheric chemistry by scanning the whole globe every day. "For the future, Sentinel-5P will be very important, in particular because of its very dense, high-resolution observations of atmospheric methane, which have the potential to detect and quantify the emissions of important methane emission hot spots such as oil and gas fields," noted Michael Buchwitz, who also leads the Greenhouse Gases project under ESA's Climate Change Initiative.
The newly released ‘Climate Research Data Package No. 3' covers more than one decade (2003–14) of atmospheric data products used to get information on the sources and sinks of carbon dioxide and methane. 46) The atmospheric data products are available through the GHG-CCI website.47)
GHG-CCI (Green House Gas-Climate Change Initiative): Carbon dioxide (CO2) and methane (CH4) are the two most important anthropogenic greenhouse gases (GHGs). Satellite observations combined with modelling helps to improve our knowledge on CO2 and CH4 sources and sinks as required for better climate prediction. GHG-CCI aims at delivering the high quality satellite retrievals needed for this application.
Figure 34: The maps show atmospheric levels of methane from 2003 to 2005 and 2008 to 2010, showing increased concentrations in the latter dataset (in red), image credit:
Figure 35: Carbon dioxide observed by SCIAMACHY on Envisat and by TANSO-FTS on GOSAT (image credit: GHG-CCI, IUP, Univ. Bremen/SRON/JPL/ESA/DLR)
The timescales of global surface-ocean connectivity
• April 2016: The billions of single-celled marine organisms known as phytoplankton can drift from one region of the world's oceans to almost any other place on the globe in less than a decade, Princeton University researchers have found.- Unfortunately, the same principle can apply to plastic debris, radioactive particles and virtually any other man-made flotsam and jetsam that litter our seas, the researchers found. Pollution can thus become a problem far from where it originated within just a few years. 48) 49)
The finding that objects can move around the globe in just 10 years suggests that ocean biodiversity may be more resilient to climate change than previously thought, according to a study published this week in the journal Nature Communications. Phytoplankton form the basis of the marine food chain, and their rapid spread could enable them to quickly repopulate areas where warming seas or ocean acidification have decimated them.
"Our study shows that the ocean is quite efficient in moving things around," said Bror Fredrik Jönsson, an associate research scholar in Princeton's Department of Geosciences, who conducted the study with co-author James R. Watson, a former Princeton postdoctoral researcher who is now a researcher at Stockholm University. "This comes as a surprise to a lot of people, and in fact we spent about two years confirming this work to make sure we got it right," Jönsson said.
One of the strengths of the model is its approach of following phytoplankton wherever they go throughout the world rather than focusing on their behavior in one region, Jönsson said. Because most marine organisms are mobile, this particle-tracking approach can yield new insights compared to the approach of studying one area of ocean.
The resulting model works for objects that have no ability to control their movement such as phytoplankton, bacteria and man-made debris. Organisms that can control their movement even a small amount — such as zooplankton, which can control their vertical position in water — are not accounted for in the model. Nor does the model apply to objects such as boats that protrude above the water and can be pushed by surface winds.
The team applied a computer algorithm to calculate the fastest route an object can travel via ocean currents between various points on the globe. Most previous studies looked only at movement of phytoplankton within regions. The resulting database, Jönsson said, is analogous to a mileage chart one would find on a roadmap or atlas showing the distance between two cities, except that Jönsson and Watson are indicating the speed of travel between different points.
The researchers confirmed that the travel times calculated by their model were similar to the time it took real objects accidentally dumped into the ocean to be carried by currents. For instance, 29,000 rubber ducks and other plastic bath toys toppled off a Chinese freighter in 1992 and have since been tracked as a method of understanding ocean currents. A similar utility has stemmed from the "Great Shoe Spill of 1990" when more than 60,000 Nike athletic shoes plunged into the ocean near Alaska and have been riding the currents off the Pacific Northwest ever since.
The researchers' model also matched the amount of time it took radioactive particles to reach the West Coast of the United States from Japan's Fukushima I Nuclear Power Plant, which released large amounts of radioactive materials into the Pacific Ocean following heavy damage from a tsunami in March 2011. The actual travel time of the materials was 3.6 years; the model calculated it would take 3.5 years.
Figure 36: Princeton University researchers found that ocean currents can carry objects to almost any place on the globe in less than a decade, faster than previously thought. The model above shows how phytoplankton traveling on ocean currents would spread over a three-year period. The researchers "released" thousands of particles representing phytoplankton and garbage from a starting point (green) stretching north to south from Greenland to the Antarctic Peninsula. The colors to the left indicate low (blue) or high (red) concentration of particles. Over time, the particles spiral out to reach the North and South Pacific, Europe, Africa and the Indian Ocean. (Animation by Bror Jönsson, Department of Geosciences)
To create the model, Jönsson and Watson obtained surface-current data from a database of modeled global surface currents developed at the MIT (Massachusetts Institute of Technology) and housed at NASA's Jet Propulsion Laboratory in California. Into this virtual world, they released thousands of particles that represented phytoplankton and then ran simulations multiple times, comparing past and present runs for accuracy and making tweaks to improve the model. They eventually tracked more than 50 billion positions of particles, which is just a fraction of the actual number of phytoplankton in the ocean.
Because phytoplankton mainly reproduce asexually — meaning that one organism alone can produce offspring — only one individual needs to reach a new area to colonize it. This fact led the team to look at the shortest time it takes to get around the world rather than the average time. "The rule for our phytoplankton was 'drive at fast as possible,'" Jönsson said.
To cut down the computing resources needed to track the particles, the researchers calculated the fastest way to get from one place to another using a shortcut commonly employed by smartphone apps and in-car navigation systems. The method, called "Dijkstra's algorithm" after the late Dutch computer scientist Edsger Dijkstra who developed it in the 1950s, calculates how to get from A to C if you know the route from A to B and B to C. "Dijkstra's algorithm is a way of optimizing for the shortest path between two positions when you have a network of possible locations, and we used it to find pathways when there was no direct link from one region to another," Watson said.
Although each step in the pathway from one region to another may be unlikely, the fact that a single phytoplankton organism, which lives only a few weeks, can give rise to millions of offspring means that even unlikely paths will have some followers.
Professor of Marine Sciences Per Jonsson at the University of Gothenburg Center for Sea and Society in Sweden said that the analysis offers a new perspective on global connectivity. "This is the first attempt to identify time scales of connectivity and possible dispersal barriers for plankton across all oceans," said Jonsson, who had no role in the research and is not related to study author Bror Jönsson. "The general message is that all parts of the ocean surface are connected on surprisingly short time scales.
"This implies that regional declines in plankton fitness due to climate change may be buffered by relatively rapid immigration coupled with community sorting or evolutionary change," Jonsson continued. "The authors also offer a practical and predictive tool for a range of studies regarding global ocean dispersal, including the spread of contaminants and marine litter."
The paper, "The timescales of global surface-ocean connectivity," was published online in-advance-of-print April 19 in the journal Nature Communications. The work was funded in part by the National Science Foundation, NASA and the Nippon Foundation-University of British Columbia's Nereus Program.
Evidence for ice-free summers 6-10 million years ago in the late Miocene central Arctic Ocean
• April 2016: An international team of scientists led by the AWI (Alfred Wegener Institute, Helmholtz Center for Polar and Marine Research) in Bremerhaven, Germany have managed to open a new window into the climate history of the Arctic Ocean. Using unique sediment samples from the Lomonosov Ridge, the researchers found that six to ten million years ago the central Arctic was completely ice-free during summer and sea-surface temperature reached values of 4 -9º Celsius. In spring, autumn and winter, however, the ocean was covered by sea ice of variable extent, the scientists explain in the current issue of the journal Nature Communications. These new findings from the Arctic region provide new benchmarks for ground-truthing global climate reconstructions and modelling.50) 51)
Although the permanently to seasonally ice-covered Arctic Ocean is a unique and sensitive component in the Earth's climate system, the knowledge of its long-term climate history remains very limited due to the restricted number of pre-Quaternary sedimentary records. During Polarstern Expedition PS87/2014. "The Arctic sea ice is a very critical and sensitive component in the global climate system. It is therefore important to better understand the processes controlling present and past changes in sea ice. In this context, one of our expedition's aims was to recover long sediment cores from the central Arctic, that can be used to reconstruct the history of the ocean's sea ice cover throughout the past 50 million years. Until recently, only a very few cores representing such old sediments were available, and, thus, our knowledge of the Arctic climate and sea ice cover several millions of year ago is still very limited," explained Rüdiger Stein, AWI geologist, expedition leader and lead author of the study.
The AWI researchers discovered an ideal place for recovering the sediment cores on the western slope of the Lomonosov Ridge, a large undersea mountain range in the central Arctic. "This slope must have experienced gigantic recurring landslides in the past, which resulted in the exhumation of more than 500 m thick ancient sediment and rock formations. We were also surprised about the wide-spread occurrence of these slide scars, which extend over a length of more than 300 km, almost from the North Pole to the southern end of the ridge on the Siberian side," said Rüdiger Stein.
Within a two-day coring action, Stein and his team took 18 sediment cores from this narrow area on Lomonosov Ridge on board the Polarstern research vessel. Although the recovered sediment cores were only four to eight meter long, one of them turned out to be precisely one of those climate archives that the scientists had been looking for a long time. "With the help of certain microfossils, so-called dinoflagellates, we were able to unambiguously establish that the lower part of this core consists of approximately six to eight million-year-old sediments, thereby tracing its geological history back to the late Miocene. With the help of so-called ‘climate indicators or proxies', this gave us the unique opportunity to reconstruct the climate conditions in the central Arctic Ocean for a time period for which only very vague and contradictory information was available," said Rüdiger Stein.
Some scientists were of the opinion that the central Arctic Ocean was already covered with dense sea ice all year round six to ten million years ago – roughly to the same extent as today. The new research findings contradict this assumption. "Our data clearly indicate that six to ten million years ago, the North Pole and the entire central Arctic Ocean must in fact have been ice-free in the summer," explained Rüdiger Stein.
There is a general consensus that the polar regions—and in particular the Arctic Ocean and surrounding areas—are at present, and were over historic and geologic time scales, subject to rapid and dramatic environmental changes. Owing to complex feedback processes, collectively known as ‘polar amplification', the Arctic is both a contributor to climate change and a region that will be most affected by global warming. Despite the importance of the Arctic Ocean in the global climate system, this permanently to seasonally ice-covered region (Figure 37) is one of the last major physiographic provinces on Earth, whose climate history and its transition from early Cenozoic Greenhouse to late Cenozoic Icehouse conditions remain still poorly known. Only one drill site recovered from the central Arctic Ocean during the ACEX (Arctic Coring Expedition)—the IODP (Integrated Ocean Drilling Program) Expedition 302 in 2004—gives some insight into the early Cenozoic climate (Ref. 51).
Concerning recent climate change, the most prominent example is the dramatic decrease of the extent and thickness of the Arctic sea-ice cover the last decades, a decrease that seems to be by far more rapid than predicted by climate models. The scientific community recognized this drastic change with major concern as the Arctic sea ice is a critical component in the global climate system, which contributes to changes in the Earth's albedo, primary productivity and deep-water formation, a driving mechanism for global thermohaline circulation. The causes of these recent changes, that is, natural versus anthropogenic forcings, and their relevance within the global climate system, however, are subject of intense scientific and societal debate. Thus, understanding the processes controlling Arctic sea-ice variability is of overall interest and significance. In this context, records of past climate and sea-ice conditions going beyond instrumental records and representing times of different boundary conditions are of major value: such records can be used to assess the sensitivity of the Earth‘s climate system to changes of different forcing parameters, for example, level of CO2, and to test the reliability of climate models by evaluating their simulations for conditions very different from the modern climate. This type of records giving detailed information about past Arctic sea-ice conditions are still very rare, especially due to the lack of precise proxies for sea-ice reconstructions.
Here, the project applies the new sea-ice biomarker approach together with alkenone-based SSTs (Sea Surface Temperatures) to sediment cores most recently recovered during Polarstern Expedition PS87, to reconstruct upper Miocene Arctic Ocean sea-ice and SST conditions. The proxy data are combined with climate model simulations using a coupled atmosphere-ocean general circulation model with focus on seasonal changes in the high northern latitudes. Based on our new proxy records, we demonstrate that only a seasonal sea-ice cover has been predominant in the central Arctic Ocean during (most of) the Late Miocene time interval. Furthermore, our combined data/modelling approach seems to indicate either relatively high atmospheric CO2 concentrations and/or an overly weak sensitivity of the model to simulate the magnitude of warm polar temperatures in the late Miocene. These new findings from the Arctic region provide new benchmarks for ground-truthing global climate reconstructions and modelling.
Figure 37: Modern Arctic Ocean SST (Sea Surface Temperature) and sea-ice conditions (image credit: AWI Research Team)
Legend to Figure 37: (a) Map of modern August SST with locations of ODP Site 907, ODP Site 910 and IODP Expedition 302 (ACEX) Site, as well as Core PS87/106 (August SST data—average of 1955–2012—from World Ocean Atlas;). (b) Map of summer sea-ice concentration—average of 1988–2007—with locations of studied sites (black dots). Maps a and b produced with Ocean Data View software (source: https://odv.awi.de/). (c) Generalized scheme illustrating (1) sea surface conditions and respective (spring/summer) productivities of ice algae and phytoplankton, and (2) sedimentary contents of IP25, terrigenous biomarkers and IRD, phytoplankton-derived biomarkers and PIP25 index for different settings in the modern Arctic Ocean. Owing to ice melting and related nutrient and sediment release, a stable ice-edge situation is characterized by high concentrations of IP25 and phytoplankton biomarkers, but also by high concentrations of terrigenous biomarkers and IRD. Modern situations at the locations of ACEX and PS87/106, ODP Site 910 (and close-by ODP sites 911 and 912) and ODP Site 907 are indicated.
Large submarine slide scars along Lomonosov Ridge. Polarstern Expedition PS87 was scheduled for August–September 2014 to explore the Lomonosov Ridge area (Figure 38) with the objective of collecting seismic data and sediment cores to reconstruct the short- and long-term climate history and the tectonic evolution of the central Arctic Ocean. More than 3,000 km of high-quality MCS (Multi-Channel Seismic) profiles and B10,000 km of high-quality multibeam bathymetry and sub-bottom sediment-echosounding (PARASOUND) profiles were acquired along the ship's track and numerous sediment cores were recovered (Figure 38).
One major finding of the expedition was the discovery of numerous submarine slide scars that occurred on both sides of the crest of Lomonosov Ridge over a distance of ca. 350 km between 81º07' N and 84º 14' N in water depths from B800 to 1,500 m (Figure 39a). Single scars are up to several km wide and long, and their head walls are 100-500 m high. Swath-bathymetry data indicate that different processes probably triggered slope failures, that various processes of sediment evacuation took place, and that failures occurred at various times. Slide scars were earlier described from a restricted area on Lomonosov Ridge near 88ºN18. However, the wide lateral distribution of mass wasting as presented here is a new discovery.
On top of the southern Lomonosov Ridge in areas between the slide scars, the team discovered SE–NW oriented, streamlined landforms over distances of 4100 km at water depths between 800 and 1,000 m (Figure 39a). These features are interpreted to be glacial lineations that formed beneath grounded ice. Similar unidirectional bed forms have also been identified further east on the East Siberian continental margin where they were related to large and coherent ice masses ESCIS (East Siberian Chukchi Ice Sheet). The lineations identified in this study are similar to those on the East Siberian continental margin with respect to their orthogonal orientations to the proposed center of the former ESCIS.
At the present state of knowledge, the most plausible glacial scenario is a larger than originally proposed ESCIS including an ice shelf extending into the Arctic Ocean, which formed an ice rise on the Southern Lomonosov Ridge over areas presently shallower than 1,000 m. The load and erosional behavior of this ice rise that probably occurred during extended Quaternary glaciations, for example, during MIS 6 (Marine Isotope Stage 6), may have caused physical conditions that triggered the landslides on this part of the Lomonosov Ridge. A MIS 6 age of this erosional event is in line with the proposed age of a major glaciation with extended ice sheets/shelves in Eurasia and East Siberia.
Figure 38: Cruise track and multibeam bathymetric survey of Polarstern Expedition PS87 (AWI Research Team)
Legend to Figure 38: (a) PS87 cruise track (blue line). AB (Amundsen Basin); AR (Alpha Ridge); GR (Gakkel Ridge); LR (Lomonosov Ridge); MB (Makarov Basin); MR (Mendeleev Ridge); NB (Nansen Basin). Orange circles indicate coring stations, the red asterisk indicates the location of the ACEX Site and the green circle indicates the North Pole (Polarstern reached on 26 August 2014 at 10:23 UTC). (b) Track lines of multibeam bathymetric survey. Color bar indicates water depth in meters (m WD). LR-01A, LR-02A and LORI-05B indicate locations of proposed IODP drill sites51. Areas of Figure 39 a,b are indicated.
Figure 39: Bathymetric and acoustic/seismic profiling records from southern Lomonosov Ridge (AWI Research Team)
Legend to Figure 39: (a) Three-dimensional (3D) image of the swath bathymetry of southern Lomonosov Ridge, showing major slide scars and escarpments, streamlined SE–NW oriented glacial lineations formed beneath grounded ice sheets/streams and transects 1 and 2 with locations of sediment cores. (b) PARASOUND profile across Transect 1 with locations of coring stations. (c) Processed multi-channel seismic profile AWI-20140311 across Transect 1, showing prominent reflectors and age assignments based on correlation between regional seismic lines and well data.
Exhumation of Miocene to lower Quaternary sediments. Sediment removal from the steep slopes of the escarpments exposed older, normally more deeply buried deposits at/near the present seafloor, allowing retrieval of older sediments by gravity coring from Polarstern, rather than expensive drilling. The PARASOUND and MCS profiles suggest that these sedimentary sections are composed of Eocene, Oligocene, Miocene, Pliocene and Quaternary strata (Figure 39 b,c). The age control for the stratigraphic units is based on links of seismic lines to drill site data from the Chukchi Shelf, the ACEX drill site on central Lomonosov Ridge and onshore geology from the New Siberian Islands. After evaluation of the multibeam bathymetry and PARASOUND data, the team selected two transects across the steep western slope of the Lomonosov Ridge for an extensive sediment coring program (Figure 39 a, Transect 1 and Transect 2). In total, 16 sediment cores were recovered from water depths between 900 m (top of Lomonosov Ridge) and 1,500 m (bottom of Lomonosov Ridge), Figure 39 a,b.
Whereas most of the sediment cores of Transect 1 are composed of predominantly upper Quaternary (post slide) deposits, some contain prominent unconformities with lower/pre-Quaternary sediments underneath. At these unconformities, a 50-80 m thick overburden has been removed, as demonstrated in compaction experiments (see ‘Sediment load and compaction experiments'). Unfortunately, the microfossil assemblages (that is,palynomorphs and agglutinated benthic foraminifers) do not allow a precise age determination of the sediments underlying the Quaternary near-surface deposits in most of these cores. The predominance of Quaternary sediments in the cores of Transect 1, however, seems to be supported by the biomarker composition determined in selected cores. Close to zero concentrations or the absence of specific biomarkers indicative for phytoplankton and sea-ice algae productivity point to surface-water conditions similar to those of the central Arctic Ocean during late Quaternary times, characterized by a thick perennial sea-ice cover and SSTs (Sea Surface Temperatures) << 0ºC.
The only core providing a clear indication that old sediments are cropping out near the seafloor is Core PS87/106. In this core recovered at the lower slope at Transect 2 (Figure 39 a), a sharp unconformity or hiatus occurs at 370 cmbsf. This unconformity is characterized by a marked change in color, an abrupt increase in WBD (Wet-Bulk Density); related to enhanced sediment consolidation underneath the hiatus), a significant drop in magnetic susceptibility (MS) and a major change in the biomarker composition, and it separates this interval from the overlying young and unconsolidated (upper Quaternary) sediments (Figure 40).
Figure 40: Proxy evidence from Core PS87/106 for late Miocene Arctic Ocean climate conditions (image credit: AWI Research Team)
Legend to Figure 40: (a) LSI (Line-Scan Image), (b) WBD, (c) MS, (d) alkenone-based SST, (e) concentrations of alkenones, (f) dinosterol and (g) brassicasterol as proxy for primary productivity, (h) concentrations of terrigenous sterols (sum of campesterol and b-sitosterol), (i) concentrations of sea-ice proxy IP25 and (j) sea-ice index PdinoIP25. Purple and open triangles indicate the presence or absence of alkenones, respectively. Green line marks depth of the hiatus. Below the hiatus, sediments are overconsolidated. Zero or near-zero concentrations of IP25 and phytoplankton biomarkers are indicative for a closed (spring) sea-ice cover (PdinoIP25 is indetermable and set to ‘1'; see Figure 37). PIP25 values are used to identify low (0–20%), common (20–50%), ice-edge (50–70%) and extended (>70%) sea-ice concentrations (see Figure 37). The section below the hiatus represents about one cold (‘glacial')–warm (‘interglacial') cycle. S1 and S2 indicate two scenarios of different sea-ice concentrations (that is, maximum and transitional/stable ice edge, respectively) within this climate cycle. The interval between the two dashed horizontal lines marks period with minimum sea-ice cover. Using mean sedimentation rates of ~3.2 cm ky-1 (as calculated independently from close-by gravity cores and seismic data), the duration of the climate cycle is ~40 ky.
Late Miocene SST and sea-ice records. The biomarker data of Core PS87/106 suggest significantly different late Miocene paleoenvironmental conditions in comparison with those predominant during Quaternary times (Figure 40). In the upper Miocene sediments, elevated concentrations of alkenones and alkenone-derived SST between 4 and 7ºC (or even 6–9ºC in case other calibrations are used indicate relatively warm, open-water and productive paleoenvironmental conditions in the central Arctic Ocean during the summer season. This is also supported by SST values of >>0 C determined in some samples from the ACEX Site.
The team's results reveal for the first time the occurrence of the biomarker sea-ice proxy IP25 in sediments as old as late Miocene. This proxy was developed by Belt et al. and was before our study only found in Quaternary and Pliocene sediments. The presence of IP25 in the PS87/106 sediments is indicative for the presence of (spring) sea ice in the late Miocene central Arctic Ocean (Figure 37). In comparison with IP25 values from the Arctic Ocean surface sediments and sediment trap data, the absolute IP25 concentrations ranging between 0.05 and 0.15 µg gOC-1 (Figure 40) are more than one order of magnitude lower. These differences are caused by an early degradation of biomarkers that already starts in the water column and reaches its maximum in the uppermost cm of the sediments. On the other hand, both IP25 and phytoplankton biomarker concentrations determined in Core PS87/106 are in the same range than those determined in early-mid Holocene Arctic sediments.
Using the ‘PIP25 Index' as a more semi-quantitative proxy of paleo-sea-ice cover (Figure 1), the team's data from Core PS87/106 point to a variable spring sea-ice coverage of ~20–70% in the lower part and ~100% in the upper part of the sequence (Figure 40). The combination of IP25 and SST data indicates that the central Arctic Ocean must have been relatively warm and ice-free during summer throughout the time interval recovered in the sedimentary section of Core PS87/106 and variable sea ice must have existed during spring when daylight conditions allowed sea-ice algae production (Figures 37 and 41). Furthermore, this implies the presence of an extended sea-ice cover during the dark, cold winter season. These new data clearly support that periods with only a seasonal sea-ice coverage must have occurred in the central Arctic Ocean during most of the late Miocene (Figure 41).
Figure 41: Schematic illustrations of the seasonal sea-ice cycle during the late Miocene (image credit: AWI Research Team)
Legend to Figure 41: The seasonal sea-ice cycle and related principal processes controlling productivity and carbon flux at location of central Arctic Ocean Core PS87/106 during the late Miocene are shown for two different scenarios. (a) Scenario 2 (‘warmer/transitional situation') =extended period of spring sea-ice algae productivity and increased IP25 and phytoplankton biomarker fluxes. (b) Scenario 1 (‘cold situation') =restricted period of late spring sea-ice algae productivity and very reduced fluxes (almost to zero) of IP25 and spring phytoplankton biomarkers; Figure 37). MIZ (Marginal Ice Zone), that is, ice-edge situation. The dark period, height of the sun and changing thickness of snow and ice over the year, as well as phytoplankton, zooplankton and sea-ice productivity are shown. IP25 values for the different seasons are indicated.