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.
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.
Note: As of May 2020, the previously single large EO-Topics file has been split into five files, to make the file handling manageable for all parties concerned, in particular for the user community.
• This article covers the period 2020 + 2021
Index Ranks Rainforests’ Vulnerability to Climate and Human Impacts
• July 26, 2021: A new index shows that the world’s rainforests are responding differently to threats like a warming climate and deforestation. 3)
Scientists from NASA’s Jet Propulsion Laboratory in Southern California and other international research institutions have created a tropical rainforest vulnerability index. It will detect and evaluate the vulnerability of these diverse ecosystems to two main categories of threats: the warming and drying climate, and the consequences of human land use such as deforestation and fragmentation from encroaching roads, agricultural fields, and logging.
The index shows that the world’s three major rainforest areas have different degrees of susceptibility to these threats. The Amazon Basin in South America is extremely vulnerable to both climate change and changes in human land use. The Congo Basin in Africa is undergoing the same warming and drying trends as the Amazon but is more resilient. Most Asian rainforests appear to be suffering more from changes in land use than from the changing climate.
“Rainforests are perhaps the most endangered habitat on Earth – the canary in the climate-change coal mine,” said Sassan Saatchi, a JPL scientist and lead author of the new study published July 23 in the journal OneEarth. 4)
These diverse ecosystems are home to more than half of the planet’s life forms and contain more than half of all the carbon in land vegetation. They serve as a natural brake on the rise of carbon dioxide in the atmosphere from fossil fuel burning because they “breathe in” carbon dioxide and store carbon as they grow.
But in the last century, 15 to 20% of rainforests have been cut down, and another 10% have been degraded. Today’s warmer climate, which has led to increasingly frequent and widespread forest fires, is limiting the forests’ capacity to absorb carbon dioxide as they grow while also increasing the rate at which forests release carbon to the atmosphere as they decay or burn.
Figure 1: A rainforest in Malaysia (image credit: Wikimedia Common)
The National Geographic Society convened a team of scientists and conservationists in 2019 to develop the new index. The index is based on multiple satellite observations and ground-based data from 1982 through 2018, such as Landsat and the Global Precipitation Measurement (GPM) mission, covering climate conditions, land use, and forest characteristics.
When an ecosystem can no longer recover from stress as quickly or as completely as it used to, that’s a sign of its vulnerability. The researchers correlated data on stressors, such as temperature, water availability, and the extent of degradation with data on how well the forests are functioning: the amount of live biomass, the amount of carbon dioxide plants were absorbing, the amount of water the forests transpire into the atmosphere, the intactness of a forest’s biodiversity, and more. The correlations show how different forests have responded to stressors and how vulnerable the forests are now.
The team then used statistical models to extend trends over time, looking for areas with increasing vulnerability and possible tipping points where rainforests will transition into dry forests or grassy plains.
The data from the tropical rainforest vulnerability index provides scientists with an opportunity to perform more in-depth examinations of natural rainforest processes, such as carbon storage and productivity, changes in energy and water cycles, and changes in biodiversity. Those studies will help scientists understand whether there are tipping points and what they are likely to be. The information can also help policy makers who are planning for conservation and forest restoration activities.
NASA Study Finds Tropical Forests’ Ability to Absorb Carbon Dioxide Is Waning
• July 20, 2021: The finding comes out of an effort to map where vegetation is emitting and soaking up carbon dioxide from the atmosphere. 5)
Earth’s trees and plants pull vast amounts of carbon dioxide out of the atmosphere during photosynthesis, incorporating some of that carbon into structures like wood. Areas that absorb more carbon than they emit are called carbon sinks. But plants can also emit the greenhouse gas during processes like respiration, when dead plants decay, or during combustion in the case of fires. Researchers are particularly interested in whether – and how – plants at the scale of an ecosystem like a forest act as sources or sinks in an increasingly warming world.
A recent study led by scientists at NASA’s Jet Propulsion Laboratory in Southern California identified whether vegetated areas like forests and savannas around the world were carbon sources or sinks every year from 2000 to 2019. The research found that over the course of those two decades, living woody plants were responsible for more than 80% of the sources and sinks on land, with soil, leaf litter, and decaying organic matter making up the rest. But they also saw that vegetation retained a far smaller fraction of the carbon than the scientists originally thought.
In addition, the researchers found that the total amount of carbon emitted and absorbed in the tropics was four times larger than in temperate regions and boreal areas (the northernmost forests) combined, but that the ability of tropical forests to absorb massive amounts of carbon has waned in recent years. The decline in this ability is because of large-scale deforestation, habitat degradation, and climate change effects, like more frequent droughts and fires. In fact, the study, published in Science Advances, showed that 90% of the carbon that forests around the world absorb from the atmosphere is offset by the amount of carbon released by such disturbances as deforestation and droughts. 6)
Figure 2: The Amazon rainforest is often called “the lungs of the world.” It produces oxygen and stores billions of tons of carbon every year (image credit: USDA Forest Service photo by Diego Perez)
The scientists created maps of carbon sources and sinks from land-use changes like deforestation, habitat degradation, and forest planting, as well as forest growth. They did so by analyzing data on global vegetation collected from space using instruments such as NASA’s Geoscience Laser Altimeter System (GLAS) on board ICESat and the agency’s Moderate Resolution Imaging Spectroradiometer (MODIS) aboard the Terra and Aqua satellites, respectively. The analysis used a machine-learning algorithm that the researchers first trained using vegetation data gathered on the ground and in the air using laser-scanning instruments.
“A lot of research that has come before hasn’t been spatially explicit – we haven’t had a map of where carbon fluxes were occurring,” said Nancy Harris, research director of the forest program at the World Resources Institute in Washington and one of the study authors.
Other ways of estimating how much carbon is exchanged between vegetated areas and the atmosphere include looking at how many plants or forests are in a particular region and studying land-use changes, combining that information with carbon emission estimates. But those methods have spatial or temporal limitations that the study authors tried to address with their machine-learning method.
Knowing where plants are taking up carbon and where they’re emitting it is important for monitoring how forests and other vegetated regions respond to a changing climate. “The Amazon was considered a substantial carbon sink because of large tracts of pristine forest that soak up carbon dioxide,” said Sassan Saatchi, principal scientist at JPL and the study lead investigator. “However, our results show that overall, the Amazon Basin is becoming almost neutral in terms of carbon balance because deforestation, degradation, and the impacts of warming, frequent droughts, and fires over the past two decades release carbon dioxide to the atmosphere.”
Saatchi and his colleagues developed their analysis so that it’s easier to track changes in vegetated areas based on data collected on the ground as well as remotely. “Our approach is designed to make sure we can systematically balance the global carbon budget every year, and that countries can use the results and methodology for carbon management and their own reporting needs,” he said.
Figure 3: This map shows the change in how much carbon a vegetated area stored or emitted between the years 2000 and 2019. Greener areas absorbed more carbon than they emitted, while more pink and purple regions released more carbon than they stored. One megagram of carbon (MgC) is one metric ton (image credit: NASA/JPL-Caltech)
This budgeting analysis helped the researchers better understand the dynamics of how forests and other vegetated areas around the world were storing the carbon that they’re absorbing from the atmosphere. “Many previous studies found that vegetation around the world absorbs a lot of atmospheric carbon dioxide,” said study lead author Alan Xu, a carbon researcher at JPL and UCLA. “It gives the impression that global forests are growing and getting bigger everywhere, but that’s not the case.”
This study helps to fill in the picture of where and how trees and plants are absorbing or emitting carbon, but there’s more work to be done. The satellite-based carbon maps in this study covered about 39 square miles (100 km2) at a time, but they couldn’t necessarily pick up changes happening on smaller scales. And there was some information about how forests stored and emitted carbon within those maps that wasn’t necessarily accounted for in the researchers’ source-sink calculations. Some of these information gaps should be remedied by higher-resolution carbon maps provided by newer satellites already in orbit, as well as upcoming missions like the NASA-Indian Space Research Organization’s NISAR.
Saatchi is hopeful that having a more systematic and consistent approach to keeping track of which parts of the world are acting as carbon sources or sinks will enable better monitoring across regions and countries. “It could allow countries around the world to use the data as guidance for meeting their national commitments to the Paris Climate Agreement.”
ESA and NASA join forces to understand climate change
• July 13, 2021: Climate change is, arguably, the biggest environmental challenge the global population faces today. To address this major issue, decision-makers not only need accurate information on how our world is changing now, but also predictions on what may happen in the future. A sound knowledge of how Earth behaves as one system is the foundation to all of this – and the pieces of this complex puzzle come largely from satellites orbiting our planet. To ensure that data from Earth-observing satellites are used to their best advantage, further science and, ultimately, bring the most benefit to humankind, ESA and NASA have formed a strategic partnership for Earth science and climate change. 7)
NASA and ESA (European Space Agency) have formed a first-of-its-kind strategic partnership to observe Earth and its changing environment. The global climate is rapidly changing and the demand for accurate, timely, and actionable knowledge is more pressing than ever. Recognizing that climate change is an urgent global challenge, the timing is right for NASA and ESA, as partners in space, to join forces to lead and support a global response to climate change. The partnership is an effort to help address and mitigate climate change through monitoring Earth with combined efforts of both agencies in Earth science observations, research, and applications. 8)
“Climate change is an all-hands-on deck, global challenge that requires action – now,” said NASA Administrator Bill Nelson. “NASA and ESA are leading the way in space, building an unprecedented strategic partnership in Earth science. This agreement will set the standard for future international collaboration, providing the information that is so essential for tackling the challenges posed by climate change and helping to answer and address the most pressing questions in Earth science for the benefit of the United States, Europe, and the world.”
Figure 4: NASA Deputy Administrator Pam Melroy, left, NASA Administrator Bill Nelson, NASA Associate Administrator for International and Interagency Relations Karen Feldstein, and NASA Associate Administrator for Science Thomas Zurbuchen (all left side of image) and ESA (European Space Agency) Director General Josef Aschbacher are seen in a screen capture from a signing ceremony for a new NASA-ESA joint statement of intent aimed at addressing global climate change July 13, 2021 (image credit: NASA)
This is not the first time ESA and NASA have joined forces. For example, ESA and NASA teams worked together on field campaigns in the Arctic to validate their respective CryoSat and ICESat missions. They also work together and with other partners on the recently launched Copernicus Sentinel-6 mission, which is a new mission to extend the long-term record of sea-level rise.
The partnership was formalized through a joint statement of intent, signed Tuesday (13 July), which outlines how the agencies will collaborate to ensure continuity of Earth observations; advance understanding of the Earth System, climate change and application of that knowledge; and collaborate on an open data policy that promotes open sharing of data, information, and knowledge within the scientific community and the wider public.
“Together, NASA and ESA provide most of the world’s Earth science coverage through our Earth-observing satellites,” said Thomas Zurbuchen, NASA associate administrator for science. “This transformative agreement will build on that capability, forging a critical international climate science partnership to tackle the most challenging climate questions in an integrated and strategic way. Not only will NASA and ESA work together to deliver unparalleled Earth science observations, research, and applications, but all of our findings will also be free and open for the benefit of the entire world as we work together to combat and mitigate climate change.”
NASA and ESA have a long and successful history working together to understand climate change. In 2020, NASA, National Oceanic and Atmospheric Administration (NOAA) and European partners, including ESA, launched the Sentinel-6 Michael Freilich satellite, which is collecting the most accurate data yet on global sea level and how oceans are rising in response to climate change. The mission also is collecting data of atmospheric temperature and humidity that will help improve climate models and weather forecasts.
“Without doubt, space is the best vantage point to measure and monitor climate change, but joining forces is also key to tackling this global issue,” said Josef Aschbacher, ESA director general. “This is why today’s agreement between our organizations is so crucial. Timing is also important, particularly as we look to the COP26 climate conference later this year, where we have the chance to further make space an integral part of the solution when it comes to climate-change mitigation.”
In May, NASA announced its Earth System Observatory, which will design a new set of Earth-focused missions to provide key information to guide efforts related to climate change, disaster mitigation, fighting forest fires, and improving real-time agricultural processes. The joint statement of intent complements activities underway for the Earth System Observatory.
Climate adaptation and mitigation efforts cannot succeed without robust climate observations and research. NASA has more than two dozen satellites and instruments observing how the planet is changing and measuring key climate indicators, such as the height of oceans and inland waters, clouds and precipitation, and carbon dioxide.
Study Projects a Surge in Coastal Flooding, Starting in 2030s
• July 7, 2021: In the mid-2030s, every U.S. coast will experience rapidly increasing high-tide floods, when a lunar cycle will amplify rising sea levels caused by climate change. 9)
- High-tide floods – also called nuisance floods or sunny day floods – are already a familiar problem in many cities on the U.S. Atlantic and Gulf coasts. The National Oceanic and Atmospheric Administration (NOAA) reported a total of more than 600 such floods in 2019. Starting in the mid-2030s, however, the alignment of rising sea levels with a lunar cycle will cause coastal cities all around the U.S. to begin a decade of dramatic increases in flood numbers, according to the first study that takes into account all known oceanic and astronomical causes for floods.
- Led by the members of the NASA Sea Level Change Science Team from the University of Hawaii, the new study shows that high tides will exceed known flooding thresholds around the country more often. What’s more, the floods will sometimes occur in clusters lasting a month or longer, depending on the positions of the Moon, Earth, and the Sun. When the Moon and Earth line up in specific ways with each other and the Sun, the resulting gravitational pull and the ocean’s corresponding response may leave city dwellers coping with floods every day or two.
- “Low-lying areas near sea level are increasingly at risk and suffering due to the increased flooding, and it will only get worse,” said NASA Administrator Bill Nelson. “The combination of the Moon’s gravitational pull, rising sea levels, and climate change will continue to exacerbate coastal flooding on our coastlines and across the world. NASA’s Sea Level Change Team is providing crucial information so that we can plan, protect, and prevent damage to the environment and people’s livelihoods affected by flooding.”
- “It’s the accumulated effect over time that will have an impact,” said Phil Thompson, an assistant professor at the University of Hawaii and the lead author of the new study, published this month in Nature Climate Change. Thompson pointed out that because high-tide floods involve a small amount of water compared to hurricane storm surges, there’s a tendency to view them as a less significant problem overall. “But if it floods 10 or 15 times a month, a business can’t keep operating with its parking lot under water. People lose their jobs because they can’t get to work. Seeping cesspools become a public health issue.”
Figure 5: High-tide flooding in Honolulu (image credit: Hawaii Sea Grant King Tides Project)
- Why will cities on such widely separated coastlines begin to experience these higher rates of flooding at almost the same time? The main reason is a regular wobble in the Moon’s orbit that takes 18.6 years to complete. There’s nothing new or dangerous about the wobble; it was first reported in 1728. What’s new is how one of the wobble’s effects on the Moon’s gravitational pull – the main cause of Earth’s tides – will combine with rising sea levels resulting from the planet’s warming.
- In half of the Moon’s 18.6-year cycle, Earth’s regular daily tides are suppressed: High tides are lower than normal, and low tides are higher than normal. In the other half of the cycle, tides are amplified: High tides get higher, and low tides get lower. Global sea level rise pushes high tides in only one direction – higher. So half of the 18.6-year lunar cycle counteracts the effect of sea level rise on high tides, and the other half increases the effect.
- The Moon is in the tide-amplifying part of its cycle now. However, along most U.S. coastlines, sea levels have not risen so much that even with this lunar assist, high tides regularly top flooding thresholds. It will be a different story the next time the cycle comes around to amplify tides again, in the mid-2030s. Global sea level rise will have been at work for another decade. The higher seas, amplified by the lunar cycle, will cause a leap in flood numbers on almost all U.S. mainland coastlines, Hawaii, and Guam. Only far northern coastlines, including Alaska’s, will be spared for another decade or longer because these land areas are rising due to long-term geological processes.
- The researchers uncovered these tipping points in flood numbers by studying 89 tide gauge locations in every coastal U.S. state and territory but Alaska. They created a new statistical framework that mapped NOAA’s widely used sea level rise scenarios and flooding thresholds, the number of times those thresholds have been exceeded annually, astronomical cycles, and statistical representations of other processes, such as El Niño events, that are known to affect tides. They projected results to 2080.
- Ben Hamlington of NASA’s Jet Propulsion Laboratory in Southern California is a co-author of the paper and also the leader of NASA’s Sea Level Change Team. He notes that the findings of the new study are a vital resource for coastal urban planners, who may be focused on preparing for extreme events rather than more high-tide floods.
- “From a planning perspective, it’s important to know when we’ll see an increase,” Hamlington said. “Understanding that all your events are clustered in a particular month, or you might have more severe flooding in the second half of a year than the first – that’s useful information.” A high-tide flood tool developed by Thompson already exists on the NASA team’s sea level portal, a resource for decision-makers and the general public. The flood tool will be updated in the near future with the findings from this study. 10)
Ocean Circulation Helps Explain Uncertainties in Climate Change Predictions
• June 16, 2021: Thirty state-of-the-art International Panel on Climate Change (IPCC)–climate models predict dramatically different climates for the Northern Hemisphere, especially Europe. An analysis of the range of responses now reveals that the differences are mostly down to the individual model's simulations of changes to the North Atlantic ocean currents and not only—as normally assumed—atmospheric changes. 11)
The work, by Katinka Bellomo, National Research Council of Italy, Institute of Atmospheric Sciences and Climate, and colleagues is published in Nature Communications and is part of the European science collaboration, TiPES (Tipping Points in the Earth System), coordinated by the University of Copenhagen. 12)
All climate models vary in the details. Variables such as atmospheric pressure, cloud cover, temperature gradients, sea surface temperatures, and many more are tuned to interact slightly differently for every model. This means, the predictions of the many models also vary.
International modelling centers run a coordinated set of climate model simulations, which are then assessed by the IPCC and summarized in a balanced report. But naturally, an uncertainty remains, reflecting the many different details of models.
"We want to understand the differences among these models. Why does one model project an overall global temperature change of two degrees while another one of four degrees? Our objective is to narrow down this inter-model uncertainty. We also want to understand the differences in these models in terms of regional climate change," explains Bellomo.
Two types of climate scenarios
Bellomo and colleagues analyzed simulations from 30 different climate models and found an important difference. There is disagreement among the models on the rate of the decline in the Atlantic meridional overturning circulation (AMOC), a large system of ocean currents in the North Atlantic which overturns surface water to a deep ocean current and plays a crucial role in the distribution of heat from the tropics to the Northern Hemisphere.
"To see how this difference was reflected in projections of the future climate, we grouped together the top 10 models (from a total of 30) in which the AMOC decline is smaller. We then compared the group with the average of the 10 models that have the largest decline," explains Bellomo.
The analysis revealed two distinct types of climate scenarios. In models where the AMOC decline is large, Europe warms only slightly, but wind patterns in Europe and precipitation patterns in the tropics change dramatically. However, in models where the AMOC decline is smaller, the Northern hemisphere heats up considerably, and a well-known pattern of wet-regions-get- wetter, dry-get-dryer emerges.
Tease mechanisms apart
This means, that the uncertainties in predictions of the future climate may to a large degree depend on how climate models predict changes in the overturning circulation in the North Atlantic. Thus, the result challenges the previous understanding of mechanisms controlling climate change over the North Atlantic, in which parameterizations of the atmosphere have been suspected to cause the main part of the uncertainty.
"This is important because it points to the AMOC as one of the largest sources of uncertainties in climate prediction," says Bellomo.
"I am excited about this research because there is so much more that can be done in addition to this. We need to investigate the processes leading to the inter-model differences in the ocean circulation response, the link between the ocean circulation response and precipitation change, and we also need to compare this with near-future projections of climate change," says Bellomo.
The TiPES project, investigating tipping points in the Earth system has received funding from the EU Horizon 2020 Research and Innovation program.
Joint NASA, NOAA Study Finds Earth's Energy Imbalance Has Doubled
• June 15, 2021: Earth's climate is determined by a delicate balance between how much of the Sun's radiative energy is absorbed in the atmosphere and at the surface and how much thermal infrared radiation Earth emits to space. A positive energy imbalance means the Earth system is gaining energy, causing the planet to heat up. The doubling of the energy imbalance is the topic of a recent study, the results of which were published June 15 in Geophysical Research Letters. 13) 14)
- Scientists at NASA and NOAA compared data from two independent measurements. NASA's Clouds and the Earth's Radiant Energy System (CERES) suite of satellite sensors measure how much energy enters and leaves Earth's system. In addition, data from a global array of ocean floats, called Argo, enable an accurate estimate of the rate at which the world’s oceans are heating up. Since approximately 90 percent of the excess energy from an energy imbalance ends up in the ocean, the overall trends of incoming and outgoing radiation should broadly agree with changes in ocean heat content.
- "The two very independent ways of looking at changes in Earth's energy imbalance are in really, really good agreement, and they're both showing this very large trend, which gives us a lot of confidence that what we're seeing is a real phenomenon and not just an instrumental artifact, " said Norman Loeb, lead author for the study and principal investigator for CERES at NASA's Langley Research Center in Hampton, Virginia. "The trends we found were quite alarming in a sense."
Figure 6: Researchers have found that Earth’s energy imbalance approximately doubled during the 14-year period from 2005 to 2019. Click image to animate: Comparison of overlapping one-year estimates at 6-month intervals of net top-of-the-atmosphere annual energy flux from CERES (solid orange line) and an in situ observational estimate of uptake of energy by Earth climate system (solid turquoise line), image credit: NASA/Tim Marvel
- Increases in emissions of greenhouse gases such as carbon dioxide and methane due to human activity trap heat in the atmosphere, capturing outgoing radiation that would otherwise escape into space. The warming drives other changes, such as snow and ice melt, and increased water vapor and cloud changes that can further enhance the warming. Earth’s energy imbalance is the net effect of all these factors. In order to determine the primary factors driving the imbalance, the investigators used a method that looked at changes in clouds, water vapor, combined contributions from trace gases and the output of light from the Sun, surface albedo (the amount of light reflected by the Earth's surface), tiny atmospheric particles called aerosols, and changes in surface and atmospheric temperature distributions.
- The study finds that the doubling of the imbalance is partially the result an increase in greenhouse gases due to human activity, also known as anthropogenic forcing, along with increases in water vapor are trapping more outgoing longwave radiation, further contributing to Earth’s energy imbalance. Additionally, the related decrease in clouds and sea ice lead to more absorption of solar energy.
- The researchers also found that a flip of the Pacific Decadal Oscillation (PDO) from a cool phase to a warm phase likely played a major role in the intensification of the energy imbalance. The PDO is a pattern of Pacific climate variability. Its fingerprint includes a massive wedge of water in the eastern Pacific that goes through cool and warm phases. This naturally occurring internal variability in the Earth system can have far-reaching effects on weather and climate. An intensely warm PDO phase that began around 2014 and continued until 2020 caused a widespread reduction in cloud coverage over the ocean and a corresponding increase in the absorption of solar radiation.
- "It's likely a mix of anthropogenic forcing and internal variability," said Loeb. "And over this period they're both causing warming, which leads to a fairly large change in Earth's energy imbalance. The magnitude of the increase is unprecedented."
- Loeb cautions that the study is only a snapshot relative to long-term climate change, and that it's not possible to predict with any certainty what the coming decades might look like for the balance of Earth's energy budget. The study does conclude, however, that unless the rate of heat uptake subsides, greater changes in climate than are already occurring should be expected.
- "The lengthening and highly complementary records from Argo and CERES have allowed us both to pin down Earth’s energy imbalance with increasing accuracy, and to study its variations and trends with increasing insight, as time goes on." said Gregory Johnson, co-author on the study and physical oceanographer at the National Oceanic and Atmospheric Administration's Pacific Marine Environmental Laboratory in Seattle, Washington. "Observing the magnitude and variations of this energy imbalance are vital to understanding Earth’s changing climate."
The rocky road to accurate sea-level predictions
• June 11, 2021: The type of material present under glaciers has a big impact on how fast they slide towards the ocean. Scientists face a challenging task to acquire data of this under-ice landscape, let alone how to represent it accurately in models of future sea-level rise. “Choosing the wrong equations for the under-ice landscape can have the same effect on the predicted contribution to sea-level rise as a warming of several degrees,” says Henning Åkesson, who led a new published study on Petermann Glacier in Greenland. 15)
Figure 7: Swedish icebreaker Oden at the front of Petermann Ice Shelf in 2019. The new study shows that this ice shelf may break up if ocean warming continues. Photo: Martin Jakobsson)
- Glaciers and ice sheets around the world currently lose more than 700,000 Olympic swimming pools of water every day. Glaciers form by the transformation of snow into ice, which is later melted by the atmosphere in summer, or slides all the way into the sea. With climate change, glaciers are breaking up and drop icebergs into the ocean at an accelerating pace. Exactly how fast depends to a large extent on the bed below all the ice. Glaciers conceal a landscape under the ice covered by rocks, sediments and water. A new study shows that the way we represent this under-ice landscape in computer models means a great deal for our predictions of future sea-level rise. More specifically, how we incorporate the friction between the ground and the ice sliding over it in glacier models is what affects our predictions. This was found by a team of Swedish and American scientists, when they simulated the future of Petermann Glacier, the largest and fastest glacier in northern Greenland.
Figure 8: The 20 km-wide Petermann Fjord with the ice shelf in the far distance (Photo: Martin Jakobsson)
- Petermann is one of the few glaciers in the northern hemisphere with a remaining ice tongue, a type of floating glacier extension otherwise mainly found in Antarctica, where they are called ice shelves. These floating extensions have been found to be exposed to warm subsurface water flowing from the open ocean towards the glaciers. This happens both in Antarctica and in many fjords around Greenland, including the Petermann Fjord. “Petermann lost 40% of its floating ice tongue over the last decade. It still has a 45 km tongue, but we found that a slightly warmer ocean than today would lead to its break up, and trigger a retreat of the glacier”, says Henning Åkesson, a postdoctoral researcher at Stockholm University who led the study. 16)
- Many glaciers in Greenland and Antarctica flow towards the ocean much faster than they did a few decades ago, and therefore contribute more to global sea-level rise. Scientists have therefore mobilized great efforts into learning what is going on in these environments. This has spurred new insights into the landscape under glaciers and the shape of the seafloor where they drain. We now also know much more about what happens to the ice when glaciers meet the sea.
- Still, the remote polar regions are notoriously difficult to study because of sea ice, icebergs, and often harsh weather. The under-ice landscape is a particular challenge because, frankly, it is hard to measure something covered by a kilometer of ice on top. Even in areas of known under-ice topography, describing its physical properties using mathematical equations is difficult. Computer models are therefore still somewhat in the dark when it comes to how to represent things like sediments, rocks, ponds and rivers under glaciers in the equations that describe ice flow. These equations are ultimately the foundation of the models used by the IPCC to estimate how fast glaciers flow and how much sea levels will rise under future climate warming.
- “Choosing the wrong equations for the under-ice landscape can have the same effect on the contribution to sea-level rise as a warming of several degrees”, Åkesson says. “In fact, predicted sea-level rise for this Greenland glacier can quadruple depending on how we represent friction under the ice. We still don’t know which way is the best, but our study illustrates that ice-sheet models still need to progress in this respect, in order to improve our estimates of mass loss from Earth’s polar ice sheets.”
Local Lockdowns Brought Fast Global Ozone Reductions, NASA Finds
• June 09, 2021: When lockdowns during the coronavirus pandemic cut local nitrogen oxide emissions, the effect on ozone pollution was global and unexpectedly rapid. 17)
- As the coronavirus pandemic slowed global commerce to a crawl in early 2020, emissions of nitrogen oxides (NOx) – which create ozone, a danger to human health and to climate – decreased 15% globally, with local reductions as high as 50%, according to a study led by scientists at NASA’s Jet Propulsion Laboratory in Southern California. As a result of the lower NOx emissions, by June 2020, global ozone levels had dropped to a level that policymakers thought would take at least 15 years to reach by conventional means, such as regulations.
Figure 9: As the coronavirus pandemic slowed global commerce to a crawl in early 2020, emissions of nitrogen oxides (NOx) – which create ozone, a danger to human health and to climate – decreased 15% globally with local reductions as high as 50%, according to a study led by scientists at NASA's Jet Propulsion Laboratory (video credit: NASA's Goddard Space Flight Center, Scientific Visualization Studio)
- The study shows that innovative technologies and other solutions intended to decrease NOx locally have the potential to rapidly improve air quality and climate globally. It published today in Science Advances. 18)
- Ozone protects us from destructive solar radiation when it’s high above Earth in the stratosphere. Closer to the ground, though, it has other lasting impacts. Ozone at the surface was estimated to cause 365,000 deaths globally in 2019 by damaging the lungs of vulnerable people, such as young children and those with asthma. Similarly, it damages the breathing systems of plants – their ability to photosynthesize – reducing plant growth and crop yields. And at the top of the troposphere, it’s a potent greenhouse gas, increasing global temperatures.
- When the world went into lockdown, scientists had an unprecedented opportunity to study how human activity interacts with natural Earth system processes at regional and global scales. A team of international researchers led by JPL scientist Kazuyuki Miyazaki used this opportunity to research the two main oxides of nitrogen: nitrogen oxide and nitrogen dioxide, collectively called NOx. They charted the chain of events from reduced fossil fuel burning during lockdowns to reduced local NOx emissions and finally to reduced global tropospheric ozone pollution. The more stringent the lockdown a nation imposed, the greater the reduction in emissions. For example, China’s stay-at-home orders in early February 2020 produced a 50% drop in NOx emissions in some cities within a few weeks; most U.S. states achieved a 25% drop later in the spring.
- The total result of the reduced NOx emissions was a 2% drop in global ozone – half the amount that the most aggressive NOx emission controls considered by the Intergovernmental Panel on Climate Change, the authoritative body of international experts on climate, were expected to produce over a 30-year period.
- Ozone reductions from the reduced NOx emissions quickly spread both around the globe and from the surface upward more than 6 miles (10 kilometers). “I was really surprised at how large the impact on global ozone was,” said JPL scientist Jessica Neu, a co-author of the new study. “We expected more of a local response at the surface.”
- he reactions that transform NOx into ozone require sunlight and depend on many additional factors, such as weather and what other chemicals are in the air. These factors interact in so many ways that, in some circumstances, reducing NOx emissions actually increases ozone. So researchers can’t understand or predict ozone concentrations from NOx emissions data alone. That requires a more thorough analysis, like this study.
- The researchers used measurements of NOx, ozone, and other atmospheric gases from five NASA and ESA (European Space Agency) Earth-observing satellites. They fed the multiple satellite observations into four numerical models of atmospheric chemical reactions and weather, using a data analysis system developed at JPL. They found that the changes in the models’ atmospheres matched the satellite observations well and reproduced known increases and decreases in emissions as regions went into and out of lockdowns. These findings indicate that both NOx emissions and global ozone will climb again as the world economy revs back up.
- “I was very happy that our analysis system was able to capture the detailed changes in emissions across the world,” said Miyazaki. “The challenging and unprecedented nature of this work is a testament to improvements in satellite monitoring in service of societal needs.” This new capability of combining multiple types of satellite observations and models is already unlocking new understanding of Earth’s atmosphere and how it is changing.
- The research team also included scientists from the Japan Agency for Marine-Earth Science and Technology in Yokohama, Nagoya University in Japan, and the Royal Netherlands Meteorological Institute in De Bilt.
Satellites Show How Earth’s Water Cycle Is Ramping Up as Climate Warms
• May 27, 2021: NASA scientists have studied 17 years of gravity observations of our planet to understand how the global water cycle is changing. 19)
Figure 10: This image shows a forest giving off moisture into the air, or transpiring. When combined with moisture that evaporates from the land, both processes drive evapotranspiration, a key branch of the water cycle. As the climate warms, these processes are expected to intensify (image credit: © Acarapi / Adobe Stock)
The rate at which plants and the land surface release moisture into the air has increased on a global scale between 2003 and 2019. These processes are collectively known as evapotranspiration, and a new NASA study has calculated its increase by using observations from gravity satellites.
By gauging the mass change of water between the oceans and the continents, the researchers determined that evapotranspiration’s rate of increase is up to two times higher than previous estimates. This is important because evapotranspiration represents a critical branch of the global water cycle – a cycle that creates the conditions for life on land.
While it is known that a warming climate should increase the rate of evapotranspiration, accurate global measurements have, until now, been elusive.
“Our study found that evapotranspiration has increased by about 10% since 2003, which is more than previously estimated, and is mostly due to warming temperatures,” said Madeleine Pascolini-Campbell, a postdoctoral researcher at NASA’s Jet Propulsion Laboratory in Southern California, who led the study. “We hope that this information about the water cycle will help to better inform the development and validation of climate models.” 20)
But how does the rate of evapotranspiration affect the global water cycle? As moisture from the oceans circulates through the atmosphere, a portion falls as precipitation over the continents. Some of this water goes into rivers as runoff, and some seeps into soils. The remaining water evaporates from the land and transpires from plants back into the air.
Finding that evapotranspiration is increasing at a faster rate than previously known has implications for understanding how climate change could impact Earth in the future. As the world warms, evapotranspiration will accelerate, speeding up the drying of land and vegetation. Weather patterns can also be affected: Increased evaporation from land can create droughts in some regions. This is a symptom of a warming world that can have major consequences for ecosystems and human societies as stress on surface and groundwater supplies increases.
“Images of melting glaciers and shrinking ice sheets are a palpable way for us to understand the impacts of global warming,” said Pascolini-Campbell. “But dramatic changes are also happening to other key components of our planet’s water cycle that aren’t so visible, such as when water evaporates from the land before it can enter the rivers as runoff.”
The Gravity of Water
To get a global estimate of how evapotranspiration is changing, researchers found a new way to leverage data collected by the pair of Gravity Recovery and Climate Experiment (GRACE) satellites that operated from 2002 to 2017, and the successor pair, GRACE Follow-On, that launched in 2018. The GRACE mission was launched by NASA and the German Aerospace Center (DLR), and GRACE-FO is a partnership between NASA and German Research Centre for Geosciences (GFZ).
Because water has mass and therefore contributes to the Earth’s gravity signal, these spacecraft are exquisitely sensitive to the movement of water around the world, from tracking changes in ice sheets to water stored on land to variations in ocean mass. Seeing an opportunity, the researchers studied the 17-year dataset from GRACE and GRACE-FO to see if it was possible to tease out the gravitational signal associated with the movement of water by evapotranspiration.
“With the combined record of GRACE and GRACE-FO, we now have a long-enough observational record to be able to monitor these critical signs of global change,” said J. T. Reager, a JPL scientist and an investigator on the study. “When the gravity signal decreases, it means the land is losing water. Some of that loss is through rivers flowing back into the oceans, but the rest of it goes up into the atmosphere as evapotranspiration.”
By subtracting all the water mass outputs from the inputs over land and then calculating the residual mass of water, the researchers were able to estimate the rate of evapotranspiration. They did this by subtracting independent estimates of global river discharge (in other words, the rate of water flowing through rivers to the ocean) and GRACE and GRACE-FO satellite data (that reveal the local changes in water mass on and in the ground) from global precipitation measurements to find out the mass of water being lost to the atmosphere.
Due to observational and measurement challenges, global estimates of evapotranspiration are typically approximated using models or by taking measurements from individual locations and then scaling those measurements up. But these methods can be prone to error. By measuring global mass changes using gravity satellite observations, however, the researchers were able to get a more precise estimate for the rate of global evapotranspiration.
Using this method, they found that evapotranspiration increased from 405 mm (about 16 inches) per year in 2003 to 444 mm (about 17.5 inches) per year in 2019. That represents an upward trend of 2.30 mm (about 0.1 inches) per year –a 10% increase – with a corresponding uncertainty of 0.5 mm (0.02 inches) per year, or 2%.
“For years, we’ve been looking for a way to measure gross changes in the global water cycle, and finally we’ve found it,” said Reager. “The magnitude of the evapotranspiration increases really surprised us: This is a sizable signal indicating our planet’s water cycle is changing.”
These results add to a growing body of research about our planet’s water cycle while also underlining the importance of continuity for Earth observations. Continuous satellite observations by satellites with a global view of water mass changes provide the long record necessary to observe the changing planet over the decades. These observations also help scientists track year-to-year variability in the water cycle caused by climate change and natural cycles.
What a Glacial River Reveals About the Greenland Ice Sheet
• April 5, 2021: With data from a 2016 expedition, scientists supported by NASA are shedding more light into the complex processes under the Greenland Ice Sheet that control how fast its glaciers slide toward the ocean and contribute to sea level rise. 21)
On the surface of the ice sheet, bottomless sinkholes called moulins can funnel meltwater into the base of the ice. As that water reaches the ice sheet’s underlying bed, it can make the ice detach slightly and flow more rapidly.
Glaciers that slide faster can eventually lead to the ice sheet melting a bit faster than expected, also increasing the amount of ice calved into the ocean. With a vast surface area roughly the size of Mexico, Greenland’s melting ice is the largest contributor to global sea level rise.
In a new study, published April 5 in Geophysical Research Letters, the authors concluded that the one important factor influencing the speed of a sliding glacier in southwest Greenland was how quickly water pressure changed within cavities at the base of the ice where meltwater met bedrock. 22)
Figure 11: At the fringes of the Greenland Ice Sheet, where glaciers are constantly melting, water rushes everywhere through an intricate system of lakes and streams that branch out like slip and slide shoots of super chilled, bright turquoise water. Some of that water eventually cascades straight into the surrounding land and ocean through channels and cracks. Some of it thunders off into sinkhole-like structures on the ice called moulins. Rumbling 24 hours a day, these holes swallow water from the surface and funnel it to the bedrock at the base of the ice (image credit: Laurence C. Smith)
“Even if the cavities are small, as long as the pressure is ramping up very fast, they will make the ice slide faster,” said Dr. Laurence C. Smith, a professor of environmental studies and Earth, environmental, and planetary sciences at Brown University in Providence, Rhode Island.
It’s the first time observations directly from field research show how changes in the volume of water under the Greenland Ice Sheet drive the flow velocities of a glacier.
The findings contradict a long-held view about ice sliding velocities and water stored under a glacier known as steady-state basal sliding law, which has helped scientists predict how fast ice sheets will slide based on the total volume of water underneath the ice.
Figure 12: Five years after a NASA-funded field study returned to to set up camp once again in the melt zone of the Greenland Ice Sheet, a new study adds to the rich findings from this innovative project. We look back on this bold undertaking, which featured helicopters, floating drifters plunging into holes in the ice, and all-night shifts operating a sonic boogie board under endless daylight. Scientist Larry Smith, at the time with UCLA and now with Brown University, takes us back to the challenges on the ice and the important findings made with the hard-won data (video credits: NASA's Goddard Space Flight Center/Scientific Visualization Studio; Additional field footage courtesy UCLA)
Dr. Lauren Andrews, a glaciologist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, likes to explain the interactions between surface meltwater, basal ice, and the bedrock, as tires that slide very rapidly on a wet road because of hydroplaning.
“If you have a rapid perturbation of water going into the subglacial system, you overwhelm the system, and so you create essentially a layer of water at the interface that's not contained in channels or cavities anymore,” Andrews said.
It's not the actual volume in water that drives ice velocity, she explained, but the speed with which it builds up at a bedrock ice interface. For slow increases in water the subglacial system has time to evolve to accommodate the same amount of water.
Until recently, the lack of data directly from the ground had made it difficult for scientists to probe the interactions that speed up glaciers in Greenland. One of the trickiest aspects preventing scientists from fully understanding ice sliding dynamics is the need to pair measurements of the flow of meltwater into a glacier with observations of the motion of the ice at the surface.
The research team set camp on Russell Glacier near Kangerlussuaq, Greenland, and studied a glacial river named to honor the late NASA researcher Alberto Behar. By comparing GPS measurements of the motion of ice at the surface with the amount of meltwater discharging into a vertical shaft in the glacier, known as a moulin, as well as meltwater exiting the glacier’s edge, the team identified changes in water stored under the ice that corresponded with small accelerations in the ice at the surface. Past research on small alpine glaciers guided the design of the study.
“There's not a direct one-to-one relationship between the melting on the top and the meltwater exiting the ice sheet because the water is going through goodness knows what down below,” Smith said.
The new findings will be valuable for satellites such as the upcoming NISAR satellite mission, a joint Earth-observing mission between NASA and the Indian Space Research Organization (ISRO), which will measure changes in ice surface velocity with unprecedented resolution for the entire Greenland and Antarctic ice sheets, said Thorsten Markus, Cryospheric Science program manager at NASA. Projected to launch no earlier than 2022, NISAR may also enable further studies of ice surface velocities at much larger scales.
Eventually, combining satellite observations with data acquired from the ground can help scientists as they consider adjusting their models to represent the hydrology at the base of ice sheets more accurately.
Integrating new data in models is a gradual process, but Smith hopes the new findings can improve how climate models predict the pace of future sea level rise from Greenland’s ice in the face of climate change.
“The only tools that we have to predict the future are models,” Smith said. “We have remote sensing, and we have field campaigns, so if we can use both to improve our modeling capability, we'll be better able to adapt and mitigate sea level rise and climate change.”
The fieldwork is one of many projects NASA has supported over the last two decades to interpret satellite observations and study the Greenland Ice Sheet using local field data.
Satellite observations prove crucial in new climate science report
• February 2, 2021: With impacts from climate change threatening major disruption to society in the coming years, leading scientists have released a compilation of the 10 most important insights on climate to help inform collective action on the ongoing climate crisis, in which satellites have played a crucial role in aiding scientific understanding. 23)
The report, presented last week to Patricia Espinosa, Executive Secretary of the United Nations Framework Convention on Climate Change (UNFCCC), summarizes the most important results over the past 12 months within the field of climate science.
These findings range from improved models that underline the need for aggressive emission cuts in order to meet the Paris Agreement, to the impact of climate change on our mental health. The report also points out a number of growing risk factors, including the risk of weakening carbon uptake by land ecosystems, as well as significant emissions from abrupt thaw of permafrost.
The report was prepared by a consortium of 57 leading researchers from 21 countries as a partnership of Future Earth, the Earth League, and the World Climate Research Program (WCRP) and has been published annually since 2017.
Sophie Hebden, Future Earth liaison seconded to ESA’s Climate Office, commented, “This report is unique in providing a particularly broad view of the latest, most important climate research insights we should all be aware of. It highlights the current state of knowledge on the environmental risks posed by climate change, its impacts on society and human well-being, as well as the opportunities emerging from research into climate change economics and governance.”
Emissions of greenhouse gases from permafrost are likely to be worse than expected because of abrupt thaw processes, which are not yet included in global climate models. These abrupt thaw effects could as much as double the emissions from permafrost thaw under moderate and high emission scenarios (see Figure 27).
Satellite observations are fundamental in monitoring abrupt thaw of permafrost, which is visible as surfaces collapse across the Arctic landscape, producing slumps, gullies and wetland areas that are expected to substantially increase carbon emissions from permafrost this century as climate warms. Permafrost is one of the ‘essential climate variables’ defined by the Global Climate Observing System.
Through ESA’s Climate Change Initiative, research teams have compiled climate data records of permafrost temperature and extent over decades, as well as snow cover, that can be used to determine trends and understand more about how permafrost is changing and how it fits into the climate system.
In December 2020, ESA-funded researchers developed and released the longest satellite-derived permafrost record currently available. Long-term satellite-derived records, such as these, are a key tool to evaluate and improve global climate models and confidence in the predictions of both future emissions and change.
The new report also details the uptake of carbon by land ecosystems which is being eroded by human-driven land-use change, particularly in the tropics. Other factors, such as water stress and permafrost thaw, could further impede the land sink.
Several knowledge gaps exist regarding how climate change will affect the ability of land-based ecosystems to absorb greenhouse gases from the atmosphere. Better quantification of land-use change is therefore key for a better understanding of the natural land sink.
ESA is making significant contributions in monitoring the storage of carbon on land through its Climate Change Initiative (CCI) Biomass project which provides global maps of above-ground biomass.
As well as this, ESA’s CCI Land Cover Project releases annual maps of land cover types worldwide, helpful in tracking changes. These, combined with other CCI datasets such as land surface temperature and fire, can be used in climate and vegetation models, enabling researchers to improve their understanding of the ability of the land to store carbon.
Figure 13: Biomass: quantifying carbon. Satellite data was used to create a map of above-ground Biomass for 2017-18. The new map uses optical, lidar and radar data acquired in 2017 and 2018 from multiple Earth observation satellites, and is the first to integrate multiple acquisitions from the Copernicus Sentinel-1 mission and Japan’s ALOS mission (image credit: biomass_cci project funded under ESA's Climate Change Initiative)
Clement Albergel, Climate Applications Scientist at ESA’s Climate Office, commented, “Assuming a dynamic equilibrium, fire has a negligible net effect on the long-term carbon budget because fire-induced carbon loss is eventually compensated by subsequent vegetation growth as the ecosystem recovers. However, shifts in fire regimes could result in long-term carbon loss, or gain, if they lead to differing states of carbon pools.”
“By using satellite-derived products, such as burned areas and land cover changes from ESA’s CCI Fire and Land Cover projects, and combining these data into both fire emission and dynamic global vegetation models, allows us to determine land-use change carbon dioxide emissions.”
ESA’s Climate Change Initiative provides a wide range of stable, long-term, satellite-based essential climate variable data products derived from multiple satellite datasets, through international collaboration – not only key to understanding the changes taking place through climate change, but essential for climate policy.
Warming Seas Are Accelerating Greenland’s Glacier Retreat
• January 25, 2021: Scientists with NASA’s Oceans Melting Greenland (OMG) mission are probing deep below the island’s warming coastal waters to help us better predict the rising seas of the future. 24)
Greenland’s melting glaciers, which plunge into Arctic waters via steep-sided inlets, or fjords, are among the main contributors to global sea level rise in response to climate change. Gaining a better understanding of how warming ocean water affects these glaciers will help improve predictions of their fate. Such predictions could in turn be used by communities around the world to better prepare for flooding and mitigate coastal ecosystem damage.
But researchers have long lacked measurements of the depths of the fjords along Greenland’s craggy coast. Without this information, it’s extremely difficult to arrive at a precise assessment of how much ocean water is being allowed into the fjords and how that affects glacier melt. By measuring their fjords one by one, a new study published in Science Advances has quantified, for the first time, how the warming coastal waters are impacting Greenland’s glaciers.
Figure 14: To measure water depth and salinity, the OMG project dropped probes by plane into fjords along Greenland's coast. Shown here is one such fjord in which a glacier is undercut by warming water (image credit: NASA/JPL-Caltech)
For the past five years, scientists with the Oceans Melting Greenland (OMG) mission have been studying these marine-terminating glaciers from the air and by ship. They found that of the 226 glaciers surveyed, 74 in deep fjords accounted for nearly half of the total ice loss (as previously monitored by satellites) from Greenland between 1992 and 2017. These glaciers exhibited the most undercutting, which is when a layer of warm, salty water at the bottom of a fjord melts the base of a glacier, causing the ice above to break apart. In contrast, the 51 glaciers that extend into shallow fjords or onto shallow ridges experienced the least undercutting and contributed only 15% of the total ice loss.
“I was surprised by how lopsided these findings were. The biggest and deepest glaciers are undercut much faster than the smaller glaciers in shallow water,” said lead author Michael Wood, a post-doctoral researcher at NASA’s Jet Propulsion Laboratory in Southern California, who began this research as a doctoral student at the University of California, Irvine. “In other words, the biggest glaciers are the most sensitive to the warming waters, and those are the ones really driving Greenland’s ice loss.”
In the case of Greenland’s glaciers, the bigger they are, the faster they melt. And the culprit is the depth of the fjord they occupy: Deeper fjords allow in more warm ocean water than shallow fjords, hastening the undercutting process.
Undercutting and Calving
Greenland is home to one of Earth’s only two ice sheets. The ice there is over 2 miles (3 kilometers) thick in places. At the edges of Greenland, the vast glaciers extending from the ice sheet travel slowly down valleys like icy conveyor belts, which pour into the fjords and then melt or break off (or calve) as icebergs. The ice is replenished by snowfall that is compressed over time into the ice pack.
If the ice sheet were in balance, the amount of snow accumulating on the top would roughly equal the ice lost from melt, evaporation, and calving. But previous observations have shown that the ice sheet has been out of balance since the 1990’s: Melt has accelerated and calving has increased. In other words, the rate of ice being lost to the ocean is exceeding the supply from the ice sheet. This is causing the ice sheet to shrink and the glaciers to retreat toward land.
The main cause of such glacier retreat is the process of undercutting, which is driven by two factors: the amount of meltwater flowing from the glacier and the warm layer of salty water at the base of the fjord. During the summer months, increasing air temperatures heat the glacier’s surface, creating pools of meltwater. These pools leak through the ice and flow from the glacier in rivers below the surface. As the meltwater flows into the sea, it encounters the warmer salty water at the bottom of the fjord.
Glacial meltwater doesn’t contain salt, so it is less dense than saltwater and thus rises as a plume. The plume drags the warmer ocean water into contact with the glacier’s base. The amount of undercutting depends on the depth of the fjord, the warmth of the ocean water, and the amount of meltwater flowing out from beneath the glacier. As the climate warms, the amount of meltwater will increase and the ocean temperature will rise, two factors that boost the undercutting process.
Figure 15: The presence of warm ocean water and meltwater flowing off the glaciers that plunge into Greenland’s fjords combine to cause melting of the glaciers from below – a process known as undercutting – that causes pieces to break off as icebergs. OMG Principal Investigator Josh Willis explains how the process works in this animation (video credit: NASA/JPL-Caltech)
These findings suggest that climate models may underestimate glacial ice loss by at least a factor of two if they don’t account for undercutting by a warm ocean.
The study also lends insight into why many of Greenland’s glaciers never recovered after an abrupt warming of ocean water between 1998 and 2007, in which ocean temperature increased by nearly 2 degrees Celsius. Although ocean warming paused between 2008 and 2017, the glaciers had already experienced such extreme undercutting in the previous decade that they continued to retreat at an accelerated rate.
“We have known for well over a decade that the warmer ocean plays a major role in the evolution of Greenland glaciers,” said OMG Deputy Principal Investigator Eric Rignot of UCI and JPL, which manages the mission. “But for the first time, we have been able to quantify the undercutting effect and demonstrate its dominant impact on the glacier retreat over the past 20 years.”
Into the Icy Depths
Now in its sixth year, the OMG mission has carried out the mammoth task of measuring ocean temperature and salinity around the entire coast of Greenland. Each summer since 2016, the team has spent several weeks dropping between 250 and 300 probes from an aircraft to measure how water temperature and salinity change with depth while mapping the depth of otherwise-inaccessible fjords.
This data complements other surveys of the region – including OMG measurements via boat (which began in 2015) and observational data collected from the Landsat satellites from NASA and the U.S. Geologic Survey – and builds on a growing body of glacier research on ice-ocean interactions. During this time, the OMG team has been able to gain a detailed view of how quickly the 226 glaciers studied are melting and which are retreating the fastest.
OMG is planning its campaign for the summer of 2021. The team hopes that the ongoing measurements of ocean conditions will be invaluable for refining predictions of ice loss, ultimately helping the world prepare for a future of rising oceans.
“When the ocean speaks, the Greenland Ice Sheet listens,” said OMG Principal Investigator Josh Willis, also of JPL. “This gang of 74 glaciers in deep fjords is really feeling the influence of the ocean; it’s discoveries like these that will eventually help us predict how fast the ice will shrink. And that’s a critical tool for both this generation and the next.”
Our world is losing ice at record rate
• January 25, 2021: A research team – the first to carry out a survey of global ice loss using satellite data – has discovered that the rate at which ice is disappearing across the planet is speeding up. The findings also reveal that 28 trillion tons of ice was lost between 1994 and 2017 – equivalent to a sheet of ice 100 meters thick covering the whole of the UK. 25)
A paper, published today in The Cryosphere, describes how a team of researchers led by the University of Leeds in the UK used information from ESA’s ERS, Envisat and CryoSat satellites as well as the Copernicus Sentinel-1 and Sentinel-2 missions to find that the rate at which Earth has lost ice has increased markedly within the past three decades, from 0.8 trillion tons per year in the 1990s to 1.3 trillion tons per year by 2017. 26)
Figure 16: To put this into perspective, one trillion tonnes of ice can be thought of as a cube of ice measuring 10 x 10 x 10 km, which would be taller than Mount Everest – illustrated here as a cube of ice over New York (image credit: Planetary Visions)
The research shows that overall, there has been a 65% increase in the rate of ice loss over the 23-year survey. This has been driven mainly by steep rises in losses from the polar ice sheets in Antarctica and Greenland.
Ice melt from ice sheets and glaciers raises sea levels, increases the risk of flooding in coastal communities, which has severe consequences for society, the economy and the environment.
Lead author Thomas Slater, a research fellow at Leeds’ CPOM (Centre for Polar Observation and Modelling), said, “Although every region we studied lost ice, losses from the Antarctic and Greenland ice sheets have accelerated the most.
“The ice sheets are now following the worst-case climate warming scenarios set out by the IPCC (Intergovernmental Panel on Climate Change). Sea-level rise on this scale will have very serious impacts on coastal communities this century.”
The study is the first of its kind to examine all the ice that is disappearing on Earth, using satellite observations.
The survey covers 215,000 mountain glaciers spread around the planet, the polar ice sheets in Greenland and Antarctica, the ice shelves floating around Antarctica, and sea ice drifting in the Arctic and Southern Oceans.
Figure 17: A research team – the first to carry out a survey of global ice loss using satellite data – has discovered that the rate at which ice is disappearing across the planet is speeding up. The findings also reveal that 28 trillion tonnes of ice was lost between 1994 and 2017 (video credit: CPOM)
Dr Slater added, “Over the past three decades there’s been a huge international effort to understand what’s happening to individual components in Earth’s ice system. This has been revolutionized by satellites as they allow us to routinely monitor the vast and inhospitable regions where ice can be found.”
The increase in ice loss has been triggered by warming of the atmosphere and oceans, which have warmed by 0.26°C and 0.12°C per decade since 1980, respectively.
During the survey period, there was a loss of 7.6 trillion tons of Arctic sea ice and a loss of 6.5 trillion tons from Antarctic ices shelves, both of which float on the polar oceans.
Isobel Lawrence, also a research fellow at the CPOM (Centre for Polar Observation and Modelling), said, “Sea-ice loss doesn't contribute directly to sea-level rise, but it does have an indirect influence. One of the key roles of Arctic sea ice is to reflect solar radiation back into space, which helps keep the Arctic cool.
“As the sea ice shrinks, more solar energy is being absorbed by the oceans and atmosphere, causing the Arctic to warm faster than anywhere else on the planet.”
Half of all losses were from ice on land – including 6.1 trillion tons from mountain glaciers, 3.8 trillion tons from the Greenland ice sheet, and 2.5 trillion tons from the Antarctic ice sheet. These losses have raised global sea levels by 35 mm.
It is estimated that for every centimeter of sea-level rise, approximately a million people in low-lying regions are in danger of being displaced.
Despite storing only 1% of Earth's total ice volume, glaciers have contributed to almost a quarter of the global ice losses over the study period, with all glacier regions around the world losing ice.
Report co-author and PhD researcher Inès Otosaka, also from the CPOM, said, “As well as contributing to global mean sea-level rise, mountain glaciers are also critical as a freshwater resource for local communities.
“The retreat of glaciers around the world is therefore of crucial importance at both local and global scales.”
ESA’s Mark Drinkwater added, “The tap to the vast global icy reservoir has been well and truly opened by global warming. Continuity in satellite data is the key to predicting future ice losses, and to assist in mitigating the threats posed by sea-level rise, shrinking high mountain glaciers and further climate feedbacks. The Copernicus Expansion missions, CRISTAL, CIMR and ROSE-L have been designed to fill the gaps in current Sentinel capabilities for comprehensive monitoring of changes in the global ice cover.”
Figure 18: Ice lost between 1994–2017 (image credit: CPOM)
Lake heatwaves to increase due to climate change
• January 21, 2021: Lake heatwaves – periods of extreme warm surface water temperature in lakes – may become hotter and longer by the end of the 21st century, according to a new study published in Nature, increasing the link between climate change and extreme events. 27) 28)
The modelling study, which is validated using satellite observation records generated by ESA’s Climate Change Initiative, shows that under a high greenhouse gas emissions scenario, the average duration of lake heatwaves could increase by around three months on average, with some lakes reaching a permanent heatwave state.
The increasing frequency of marine and land heatwaves has been linked to global warming in previous studies. However, less is known about lake heatwaves and how they will be affected by global warming.
Figure 19: Lake heatwave projections. Historical and future projections of global lake heatwave strength for three different greenhouse gas emission scenarios: a is RCP (Representative Concentration Pathway) 2.6, b is RCP 6.0 and c is RCP 8.5 [image credit: Woolway et al., (2021)]
Iestyn Woolway, a research fellow with ESA’s Climate Office, and colleagues modelled the impact of heatwaves on 702 lakes all over the globe from 1901 to 2099 in this latest study. They show, for the first time, that heatwaves frequently occur in lakes, and that they are very sensitive to climatic variations.
Under future climate change, their multi-model projections demonstrate that lake heatwaves will become progressively worse as the century progresses, particularly under a high greenhouse gas emissions scenario. The average temperature of lake heatwaves is likely to increase from approximately 3.7ºC to 5.4ºC, while the average duration will increase from around a week to more than three months by the end of the 21st century.
Under the most conservative emissions scenario, the average increases in temperature and duration are around 4.0°C and one month, respectively. The authors found that heatwaves would be longer lasting but less intense in deeper lakes.
As lakes warm over the 21st century, heatwaves will extend across all seasons and some lakes will reach a permanent heatwave state, the authors suggest.
Iestyn Woolway commented, “Agencies from around the world recently reported that 2020 was part of the warmest decade on record, and climate projections suggest that this warming will continue. Unless climate change is mitigated, our projections suggest that lake heatwaves will become increasingly severe this century, threatening lake biodiversity and pushing ecosystems to the limits of their resilience.”
Given their ability to monitor lake properties on a global scale over multiple decades, satellites observations prove crucial in this study, with the authors using a long-term lakes satellite dataset generated by the ESA Climate Change Initiative (CCI).
Initially released last year, the CCI dataset provides information for 250 globally distributed lakes worldwide over the period 1995—2019. In addition to surface temperature, information on four other lake variables are also available, including daily observations for lake level, water extent, ice cover and reflectance, which help support studies related to lakes and climate.
The next version of the dataset, anticipated for release later this year, will increase the number of lakes for which data is provided to around 2000, to further support studies aimed at understanding the response of lakes to climate change.
ESA’s Climate Change Initiative is a research programme that merges multiple sources of satellite data, to create long time series for 21 key aspects of the climate, known as Essential Climate Variables, including lakes.
Figure 20: The percentage of studied lakes which are projected to experience a permanent heatwave state during the 21st century (image credit: ESA, Woolway et al., (2021))
2020 Tied for Warmest Year on Record, NASA Analysis Shows
• January 14, 2021: Earth’s global average surface temperature in 2020 tied with 2016 as the warmest year on record, according to an analysis by NASA. 29)
Continuing the planet’s long-term warming trend, the year’s globally averaged temperature was 1.84 degrees Fahrenheit (1.02 degrees Celsius) warmer than the baseline 1951-1980 mean, according to scientists at NASA’s Goddard Institute for Space Studies (GISS) in New York. 2020 edged out 2016 by a very small amount, within the margin of error of the analysis, making the years effectively tied for the warmest year on record.
“The last seven years have been the warmest seven years on record, typifying the ongoing and dramatic warming trend,” said GISS Director Gavin Schmidt. “Whether one year is a record or not is not really that important – the important things are long-term trends. With these trends, and as the human impact on the climate increases, we have to expect that records will continue to be broken.”
Figure 21: Globally, 2020 was the hottest year on record, effectively tying 2016, the previous record. Overall, Earth’s average temperature has risen more than 2 degrees Fahrenheit since the 1880s. Temperatures are increasing due to human activities, specifically emissions of greenhouse gases, like carbon dioxide and methane (video credits: NASA’s Scientific Visualization Studio/Lori Perkins/Kathryn Mersmann)
A Warming, Changing World
Tracking global temperature trends provides a critical indicator of the impact of human activities – specifically, greenhouse gas emissions – on our planet. Earth's average temperature has risen more than 2 degrees Fahrenheit (1.2 degrees Celsius) since the late 19th century.
Rising temperatures are causing phenomena such as loss of sea ice and ice sheet mass, sea level rise, longer and more intense heat waves, and shifts in plant and animal habitats. Understanding such long-term climate trends is essential for the safety and quality of human life, allowing humans to adapt to the changing environment in ways such as planting different crops, managing our water resources and preparing for extreme weather events.
Ranking the Records
A separate, independent analysis by the National Oceanic and Atmospheric Administration (NOAA) concluded that 2020 was the second-warmest year in their record, behind 2016. NOAA scientists use much of the same raw temperature data in their analysis, but have a different baseline period (1901-2000) and methodology. Unlike NASA, NOAA also does not infer temperatures in polar regions lacking observations, which accounts for much of the difference between NASA and NOAA records.
Like all scientific data, these temperature findings contain a small amount of uncertainty – in this case, mainly due to changes in weather station locations and temperature measurement methods over time. The GISS temperature analysis (GISTEMP) is accurate to within 0.1 degrees Fahrenheit with a 95 percent confidence level for the most recent period.
Beyond a Global, Annual Average
While the long-term trend of warming continues, a variety of events and factors contribute to any particular year’s average temperature. Two separate events changed the amount of sunlight reaching the Earth’s surface. The Australian bush fires during the first half of the year burned 46 million acres of land, releasing smoke and other particles more than 18 miles high in the atmosphere, blocking sunlight and likely cooling the atmosphere slightly. In contrast, global shutdowns related to the ongoing coronavirus (COVID-19) pandemic reduced particulate air pollution in many areas, allowing more sunlight to reach the surface and producing a small but potentially significant warming effect. These shutdowns also appear to have reduced the amount of carbon dioxide (CO2) emissions last year, but overall CO2 concentrations continued to increase, and since warming is related to cumulative emissions, the overall amount of avoided warming will be minimal.
The largest source of year-to-year variability in global temperatures typically comes from the El Nino-Southern Oscillation (ENSO), a naturally occurring cycle of heat exchange between the ocean and atmosphere. While the year has ended in a negative (cool) phase of ENSO, it started in a slightly positive (warm) phase, which marginally increased the average overall temperature. The cooling influence from the negative phase is expected to have a larger influence on 2021 than 2020.
“The previous record warm year, 2016, received a significant boost from a strong El Nino. The lack of a similar assist from El Nino this year is evidence that the background climate continues to warm due to greenhouse gases,” Schmidt said.
The 2020 GISS values represent surface temperatures averaged over both the whole globe and the entire year. Local weather plays a role in regional temperature variations, so not every region on Earth experiences similar amounts of warming even in a record year. According to NOAA, parts of the continental United States experienced record high temperatures in 2020, while others did not.
In the long term, parts of the globe are also warming faster than others. Earth’s warming trends are most pronounced in the Arctic, which the GISTEMP analysis shows is warming more than three times as fast as the rest of the globe over the past 30 years, according to Schmidt. The loss of Arctic sea ice – whose annual minimum area is declining by about 13 percent per decade – makes the region less reflective, meaning more sunlight is absorbed by the oceans and temperatures rise further still. This phenomenon, known as Arctic amplification, is driving further sea ice loss, ice sheet melt and sea level rise, more intense Arctic fire seasons, and permafrost melt.
Land, Sea, Air and Space
NASA’s analysis incorporates surface temperature measurements from more than 26,000 weather stations and thousands of ship- and buoy-based observations of sea surface temperatures. 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 if not taken into account. The result of these calculations is an estimate of the global average temperature difference from a baseline period of 1951 to 1980.
Figure 22: This plot shows yearly temperature anomalies from 1880 to 2019, with respect to the 1951-1980 mean, as recorded by NASA, NOAA, the Berkeley Earth research group, and the Met Office Hadley Centre (UK). Though there are minor variations from year to year, all five temperature records show peaks and valleys in sync with each other. All show rapid warming in the past few decades, and all show the past decade has been the warmest (image credits: NASA GISS/Gavin Schmidt)
NASA measures Earth's vital signs from land, air, and space with a fleet of satellites, as well as airborne and ground-based observation campaigns. The satellite surface temperature record from the Atmospheric Infrared Sounder (AIRS) instrument aboard NASA’s Aura satellite confirms the GISTEMP results of the past seven years being the warmest on record. Satellite measurements of air temperature, sea surface temperature, and sea levels, as well as other space-based observations, also reflect a warming, changing world. The agency develops new ways to observe and study Earth's interconnected natural systems with long-term data records and computer analysis tools to better see how our planet is changing. NASA shares this unique knowledge with the global community and works with institutions in the United States and around the world that contribute to understanding and protecting our home planet.
NASA’s full surface temperature data set – and the complete methodology used to make the temperature calculation – are available at: https://data.giss.nasa.gov/gistemp
Shrinking Margins of Greenland
• January 2, 2021: A recent study of Greenland’s ice sheet found that glaciers are retreating in nearly every sector of the island, while also undergoing other physical changes. Some of those changes are causing the rerouting of freshwater rivers beneath the ice. 30)
In a study led by Twila Moon of the National Snow and Ice Data Center, researchers took a detailed look at physical changes to 225 of Greenland’s ocean-terminating glaciers—narrow fingers of ice that flow from the ice sheet interior to the ocean. They found that none of those glaciers has substantially advanced since the year 2000, and 200 of them have retreated. 31)
About 80 percent of Greenland is blanketed by an ice sheet, also known as a continental glacier, that reaches a thickness of up to 3 kilometers (2 miles). As glaciers flow toward the sea, they are usually replenished by new snowfall on the interior of the ice sheet that gets compacted into ice. Multiple studies have shown that the balance between glacier melting and replenishment is changing, as is the rate of iceberg calving. Due to rising air and ocean temperatures, the ice sheet is losing mass at an accelerating rate and additional meltwater is flowing into the sea.
Figure 23: At least 200 of the island’s coastal glaciers have retreated over the past 20 years. This map shows measurements of ice velocity across Greenland as measured by satellites. The data were compiled through the Inter-mission Time Series of Land Ice Velocity and Elevation project (ITS_LIVE), which brings together observations of glaciers collected by multiple Landsat satellites between 1985 and 2015 into a single dataset open to scientists and the public [image credit: NASA Earth Observatory image by Joshua Stevens, using Landsat data from the U.S. Geological Survey and the ITS_LIVE project at NASA/JPL-Caltech, and the General Bathymetric Chart of the Oceans (GEBCO). Story by Calla Cofield, Jet Propulsion Laboratory, with Mike Carlowicz]
“The coastal environment in Greenland is undergoing a major transformation,” said Alex Gardner, a snow and ice scientist at NASA’s Jet Propulsion Laboratory and co-author of the study. “We are already seeing new sections of the ocean and fjords opening up as the ice sheet retreats, and now we have evidence of changes to these freshwater flows. So losing ice is not just about changing sea level, it’s also about reshaping Greenland’s coastline and altering the coastal ecology.”
Though the findings by Moon, Gardner, and colleagues are in line with other Greenland observations, the new survey captures a trend that has not been apparent in previous work. As individual glaciers retreat, they are also changing in ways that are likely rerouting freshwater flows under the ice. For example, glaciers change in thickness not only as warmer air melts ice off of their surfaces, but also as their flow speed changes. Both scenarios can lead to changes in the distribution of pressure beneath the ice. This, in turn, can change the path of subglacial rivers, since water will always take the path of least resistance (lowest pressure).
Citing previous studies on the ecology of Greenland, the authors note that freshwater rivers under the ice sheet deliver nutrients to bays, deltas, and fjords around Greenland. In addition, the under-ice rivers enter the ocean where the ice and bedrock meet, which is often well below the ocean’s surface. The relatively buoyant freshwater rises, carrying nutrient-rich deep ocean water to the surface, where the nutrients can be consumed by phytoplankton. Research has shown that glacial meltwater rivers directly affect the productivity of phytoplankton, which serve as a foundation of the marine food chain. Combined with the opening of new fjords and sections of ocean as glaciers and ice shelves retreat, these changes amount to a transformation of the local environment.
“The speed of ice loss in Greenland is stunning,” said Moon. “As the ice sheet edge responds to rapid ice loss, the character and behavior of the system as a whole are changing, with the potential to influence ecosystems and people who depend on them.”
Figure 24: An aerial view of the Greenland ice sheet taken in September 1992. New research finds ice loss has accelerated significantly over the past two decades, transforming the shape of the ice sheet edge and therefore coastal Greenland (image credit: Hannes Grobe, Alfred Wegener Institute for Polar and Marine Research (Own work), CC BY-SA 2.5) 32)
Water Limitations in the Tropics Offset Carbon Uptake from Arctic Greening
• December 18, 2020: More plants and longer growing seasons in the northern latitudes have converted parts of Alaska, Canada and Siberia to deeper shades of green. Some studies translate this Arctic greening to a greater global carbon uptake. But new research shows that as Earth’s climate is changing, increased carbon absorption by plants in the Arctic is being offset by a corresponding decline in the tropics. 33)
"This is a new look at where we can expect carbon uptake to go in the future,” said scientist Rolf Reichle with the Global Modeling and Assimilation Office (GMAO) at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.
Reichle is one of the authors of a study, published Dec. 17 in AGU Advances, which combines satellite observations over 35 years from the National Oceanic and Atmospheric Administration (NOAA’s) Advanced Very High Resolution Radiometer (AVHRR) with computer models, including water limitation data from NASA’s Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2). 34)
Together, these provide a more accurate estimate of global "primary productivity" – a measure of how well plants convert carbon dioxide and sunlight to energy and oxygen via photosynthesis, for the time span between 1982 to 2016.
Arctic gains and tropical losses
Plant productivity in the frigid Arctic landscape is limited by the lengthy periods of cold. As temperatures warm, the plants in these regions have been able to grow more densely and extend their growing season, leading to an overall increase in photosynthetic activity, and subsequently greater carbon absorption in the region over the 35-year time span.
However, buildup of atmospheric carbon concentrations has had several other rippling effects. Notably, as carbon has increased, global temperatures have risen, and the atmosphere in the tropics (where plant productivity is limited by the availability of water) has become drier. Recent increases in drought and tree mortality in the Amazon rainforest are one example of this, and productivity and carbon absorption over land near the equator have gone down over the same time period as Arctic greening has occurred, canceling out any net effect on global productivity.
Figure 25: A map of the world shows the changes in global gross primary productivity (GPP), an indicator of carbon uptake, from 1982–2016. Each dot indicates a region with a statistically significant trend (image credit: NASA/Nima Madani)
Adding Satellites to Productivity Models
Previous model estimates suggested that the increasing productivity of plants in the Arctic could partially compensate for human activities which release atmospheric carbon, like the burning of fossil fuels. But these estimates relied on models that calculate plant productivity based on the assumption that they photosynthesize (convert carbon and light) at a given efficiency rate.
In reality, many factors can affect plants’ productivity. Including satellite records like those from AVHRR provide scientists with consistent measurements of the global photosynthetic plant cover, and can help account for variable events such as pest outbreaks and deforestation that previous models do not capture. These can impact the global vegetation cover and productivity.
“There have been other studies that focused on plant productivity at global scales,” said Nima Madani from NASA’s Jet Propulsion Laboratory, (JPL) Pasadena, California, and lead author of the study, which also includes scientists from the University of Montana. “But we used an improved remote sensing model to have a better insight into changes in ecosystem productivity.” This model uses an enhanced light use efficiency algorithm, which combines multiple satellites’ observations of photosynthetic plant cover and variables such as surface meteorology.
“The satellite observations are critical especially in regions where our field observations are limited, and that’s the beauty of the satellites,” Madani said. “That’s why we are trying to use satellite remote sensing data as much as possible in our work.”
It was only recently that the satellite records began to show these emerging trends in shifting productivity. According to Reichle, “The modelling and the observations together, what we call data assimilation, is what really is needed.” The satellite observations train the models, while the models can help depict Earth system connections such as the opposing productivity trends observed in the Arctic and tropics.
Brown Is the New Green
The satellite data also revealed that water limitations and decline in productivity are not confined to the tropics. Recent observations show that the Arctic’s greening trend is weakening, with some regions already experiencing browning.
“I don’t expect that we have to wait another 35 years to see water limitations becoming a factor in the Arctic as well,” said Reichle. We can expect that the increasing air temperatures will reduce the carbon uptake capacity in the Arctic and boreal biomes in the future. Madani says Arctic boreal zones in the high latitudes that once contained ecosystems constrained by temperature are now evolving into zones limited by water availability like the tropics.
These ongoing shifts in productivity patterns across the globe could affect numerous plants and animals, altering entire ecosystems. That can impact food sources and habitats for various species, including endangered wildlife, and human populations.
The data produced from this study are publicly accessible at: https://doi.org/10.3334/ORNLDAAC/1789
NASA Finds What a Glacier’s Slope Reveals About Greenland Ice Sheet Thinning
• December 18, 2020: As glaciers flow outward from the Greenland Ice Sheet, what lies beneath them offers clues to their role in future ice thinning and sea-level rise contribution. 35)
Outlet glaciers are rivers of ice flowing within the cracks of the bedrock and draining into the surrounding sea. They retreat and start to thin as climate warms, and this thinning works its way toward the center of the ice sheet. Now, by looking at the bed topography beneath the ice, scientists have a better understanding of which glaciers could have a significant impact on the Greenland Ice Sheet’s contribution to sea-level rise in coming years. They found that some glaciers flowing over gentler slopes could have a greater impact than previously thought. The gentle slopes allow thinning to spread from the edge of the ice sheet far into the interior, whereas glaciers with steep drops in their bed topographies limit how far into the interior thinning can spread.
The research, which was published December 11th in Geophysical Research Letters, analyzed 141 outlet glaciers on the Greenland Ice Sheet to predict how far into the interior thinning may spread along their flow lines, starting from the ocean edge. 36)
“What we discovered is some glaciers flow over these steep drops in the bed, and some don’t,” said lead author Denis Felikson with NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and the Universities Space Research Association (USRA). “For the glaciers that do have that steep drop in the bed, thinning can’t make its way past those drops.” Borrowing a term from geomorphology – the study of Earth’s physical features – they coined these steep drop features “knickpoints.”
When a river flows over a knickpoint, it often results in a waterfall or a lake. But for glaciers, steep is a relative term which in reality translates to just about three degrees of incline. “It’s not like the ice is going over a cliff,” said Felikson. “But in terms of glacier dynamics, they are very steep – an order of magnitude more steep than a typical bed that the ice flows over.”
The researchers were able to identify these “steep” changes in topography using digital elevation models of the ice sheet bed and surface topography. Surface topography came from the Greenland Ice Mapping Project, created using NASA’s Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) instrument that flies aboard NASA’s Terra satellite, in conjunction with data from NASA’s Ice, Cloud, and land Elevation Satellite (ICESat) mission. The bed topography digital elevation model, known as the BedMachine data set, is a high-resolution model of the bed beneath the Greenland Ice Sheet, created using data from NASA’s Operation IceBridge airborne surveys of polar ice.
“This bed topography data set was critical to us doing our work,” Felikson said. “And it is thanks to NASA remote sensing, namely the Operation IceBridge surveys, that we were able to do this.” Using the remote sensing data, scientists were able to compare topography measures to produce a single metric along a glacier’s flow line. This helped them identify a break point between the upstream and downstream parts of the glacial ice.
Ice below the knickpoint is susceptible to thinning from the glacier’s edge. But the thinning does not extend beyond this point upstream, so the interior of the ice sheet is not impacted.
Of all the glaciers observed, a majority (65 percent) had discernable knickpoints. Especially steep knickpoints are prevalent in the more mountainous regions of Greenland, where several of the biggest and fastest moving glaciers also show knickpoints that are relatively close to the coast. By sheer size alone these glaciers could contribute significantly to ice sheet thinning and melt, but because their knickpoints are near the coast, thinning is not expected to spread far inland.
Figure 26: GIF image showing the potential distances over which thinning can spread into Greenland’s interior. Glaciers in regions of higher elevation, tend to pervade less inland than those in regions of lower elevation (image credit: Denis Felikson)
However, glaciers that flow through gentle topography are found to either have gradual knickpoints, or no knickpoint at all. Such glaciers are of interest, and concern, because even those that are smaller in size have the potential to let thinning expand hundreds of kilometers inland, eroding the heart of the ice sheet.
“They could be impactful in terms of sea level rise, not because they are big and deep, but because they have access to more ice that they can eat away,” said Felikson. “It will take them a lot longer to respond, but over the long term they could end up contributing just as much to sea level rise, maybe, as the big glaciers.”
Over the gentle topography of the northwest coast of Greenland, nine of twelve neighboring glaciers are predicted to thin more than 250 km (155.3 miles) into the interior of the ice sheet, over a ~140 km (86.9 mile) wide region. The northwest sector of the ice sheet is also the only region experiencing an ongoing increase in ice discharge over the last couple decades, and Felikson predicts that it will continue to do so given the characteristics of these glaciers.
This work was started at the University of Texas as part of Felikson’s dissertation and has continued throughout his time at NASA Goddard. The origins of knickpoints and their implications for long-term thinning, as well as Greenland’s overall contribution to sea level rise, remain the basis for future research.
The data used in this study is available at: https://zenodo.org/record/4284759
Long-term permafrost record details Arctic thaw
• December 16, 2020: Frozen Arctic soils are set to release vast amounts of greenhouse gases to the atmosphere as they continue to thaw in coming decades. Despite concerns that this will fuel future global warming, the scale and speed of this important climate process remain uncertain. To help address this knowledge gap, ESA-funded researchers have developed and released a new permafrost dataset – the longest, satellite-derived permafrost record currently available. 37)
Covering 18 million km2, northern hemisphere permafrost areas have been warming since the 1980s, according to the Intergovernmental Panel on Climate Change’s latest report on oceans and cryosphere. The total carbon released each year may rival present-day emissions from all EU countries by the end of century – and are expected to amplify future climate change.
Figure 27: This animation shows the permafrost extent from 1997-2018.Frozen Arctic soils are set to release vast amounts of greenhouse gases to the atmosphere as they continue to thaw in coming decades. Despite concerns that this will fuel future global warming, the scale and speed of this important climate process remain uncertain. To help address this knowledge gap, ESA-funded researchers have developed and released a new permafrost dataset - the longest, satellite-derived permafrost record currently available [image credit: ESA (data source: Permafrost CCI, Obu, J. et al. 2020)]
The new 21-year satellite-derived record details the annual changes to the northern hemisphere permafrost soils from 1997—2018. This is the longest satellite permafrost record currently available, and extends the time-series by seven years.
Long-term satellite-derived records such as these are a key tool to evaluate and improve global climate models and confidence in the predictions of both future emissions and change.
Permafrost cannot be directly observed from space. Instead, the research team, led by Annett Bartsch from B.geos, combine global satellite data products for land surface temperature and land cover with in situ measurements and the ERA5 climate reanalysis to generate a picture of the permafrost ground conditions.
The resulting one-kilometer resolution dataset provides permafrost ground temperatures at 1 m, 2 m, 5 m and 10 m of the ‘active layer’ – the depth to which the top layer of soil thaws during the summer and freezes again during the autumn. The team also derive and provide permafrost extent data, a standard parameter used for a variety of related applications.
Figure 28: This animation shows the average subsurface temperatures from 1997-2018. Frozen Arctic soils are set to release vast amounts of greenhouse gases to the atmosphere as they continue to thaw in coming decades. Despite concerns that this will fuel future global warming, the scale and speed of this important climate process remain uncertain. To help address this knowledge gap, ESA-funded researchers have developed and released a new permafrost dataset – the longest, satellite-derived permafrost record currently available [ESA (data source: Permafrost CCI, Obu, J. et al. 2020)]
Although currently short of the three-decade minimum required to identify a climate signal, the 21-year record shows interesting trends, according to Dr Bartsch who points to rising ground temperatures, and greater variability along coastal areas and at high arctic latitudes.
“Average ground temperatures are rising at a rate of one degree Celsius per decade in the record,” explains Dr Bartsch, adding that, “Wider temperature variation can be observed along the coasts of east Russia and northwest Canada bordering the Chukchi sea – where rates of coastal erosion are some of the highest in the world, and are, in part, exacerbated by permafrost thaw conditions.”
An unusually warm summer in 2020 in northern Russia, led to ground conditions becoming unstable, contributing to a major diesel oil leak at a facility near the town of Norilsk. The incident threatened to pollute the Arctic Ocean and highlights some of the consequences of changing permafrost.
Figure 29: MAGT (Mean Annual Ground Temperature) at 2m depth for 2003-2017 at coastal Arctic locations in Canada and Russia. Vertical dashed lines indicate years with PALSAR acquisitions (image credit: Obu et al., 2019a)
“Although ground temperatures remained close to zero degrees, on-going slow seasonal ground ice melt and a deepening of the active layer can be observed in the data,” explains, Dr Bartsch’s colleague, Prof Westermann of the University of Oslo and the developer of the satellite-retrieval scheme.
The research-quality dataset is freely available from ESA’s CCI (Climate Change Initiative) Open Data Portal along with a suite of research-quality global, satellite data sets for Essential Climate Variables.
Moving forwards, the permafrost project team is working to integrate snow extent observations into their model to supplement or replace modelled snow data, and develop Arctic-specific land cover maps that will for example help to further improve represent soil and ground temperature further.
Greenland's Retreating Glaciers Could Impact Local Ecology
• October 27, 2020: A new study of Greenland's shrinking ice sheet reveals that many of the island's glaciers are not only retreating, but are also undergoing other physical changes. Some of those changes are causing the rerouting of freshwater rivers beneath the glaciers, where it meets the bedrock. These rivers carry nutrients into the ocean, so this reconfiguring has the potential to impact the local ecology as well as the human communities that depend on it. 38)
"The coastal environment in Greenland is undergoing a major transformation," said Alex Gardner, a research scientist at NASA's Jet Propulsion Laboratory and co-author of the study. "We are already seeing new sections of the ocean and fjords opening up as the ice sheet retreats, and now we have evidence of changes to these freshwater flows. So losing ice is not just about changing sea level, it's also about reshaping Greenland's coastline and altering the coastal ecology."
About 80% of Greenland is blanketed by an ice sheet, also known as a continental glacier, that reaches a thickness of up to 2.1 miles (3.4 km). Multiple studies have shown that the melting ice sheet is losing mass at an accelerating rate due to rising atmosphere and ocean temperatures, and that the additional meltwater is flowing into the sea.
Figure 30: Greenland appears in this image created using data from the ITS_LIVE project, hosted at NASA's Jet Propulsion Laboratory. The coloring around the coast of the arctic island shows the speed of outlet glaciers flowing into the ocean (image credit: NASA/JPL-Caltech/USGS)
This study, published on Oct. 27 in the Journal of Geophysical Research: Earth's Surface, provides a detailed look at physical changes to 225 of Greenland's ocean-terminating glaciers, which are narrow fingers of ice that flow from the ice sheet interior out into the ocean. The data used in the paper was compiled as part of a project based at JPL called Inter-mission Time Series of Land Ice Velocity and Elevation, or ITS_LIVE, which brings together observations of glaciers around the globe - collected by multiple satellites between 1985 and 2015 - into a single dataset open to scientists and the public. The satellites are all part of the Landsat program, which has sent a total of seven spacecraft into orbit to study Earth's surface since 1972. Managed by NASA and the U.S. Geological Survey, Landsat data reveals both natural and human-caused changes to Earth's surface, and is used by land managers and policymakers to make decisions about Earth's changing environment and natural resources. 39)
Figure 31: Glacier flow is imperceptible to the human eye, but this animation shows glaciers in Asia moving over a span of 11 years, from 1991 to 2002. The animation is composed of false-color images from Landsat 5 and 7 spacecraft. Moving ice is gray and blue; brighter blues are changing snow and ice cover (image credit: NASA/JPL-Caltech/USGS/Earth Observatory)
Advancing and Retreating
As glaciers flow toward the sea - albeit too slowly to be perceptible to the eye - they are replenished by new snowfall on the interior of the ice sheet that gets compacted into ice. Some glaciers extend past the coastline and can break off as icebergs. Due to rising atmospheric and ocean temperatures, the balance between glacier melting and replenishment, as well as iceberg calving, is changing. Over time, a glacier's front may naturally advance or retreat, but the new research shows that none of the 225 ocean-terminating glaciers surveyed has substantially advanced since 2000, while 200 have retreated.
Although this is in line with other Greenland findings, the new survey captures a trend that hasn't been apparent in previous work: As individual glaciers retreat, they are also changing in ways that are likely rerouting freshwater flows under the ice. For example, glaciers change in thickness not only as warmer air melts ice off their surfaces, but also as their flow speed changes in response to the ice front advancing or retreating.
Both scenarios were observed in the new study, and both can lead to changes in the distribution of pressure beneath the ice; scientists can infer these pressure changes based on changes in thickness analyzed in the study. This, in turn, can change the path of a subglacial river, since water will always take the path of least resistance, flowing in the direction of lowest pressure.
Citing previous studies on the ecology of Greenland, the authors note that freshwater rivers under the ice sheet deliver nutrients (such as nitrogen, phosphorus, iron, and silica) to bays, deltas, and fjords around Greenland. In addition, the under-ice rivers enter the ocean where the ice and bedrock meet, which is often well below the ocean's surface. The relatively buoyant fresh water rises, carrying nutrient-rich deep ocean water to the surface, where the nutrients can be consumed by phytoplankton. Research has shown that glacial meltwater rivers directly impact the productivity of phytoplankton - meaning the amount of biomass they produce - which serves as a foundation of the marine food chain. Combined with the opening up of new fjords and sections of ocean as glaciers retreat, these changes amount to a transformation of the local environment.
"The speed of ice loss in Greenland is stunning," said Twila Moon, deputy lead scientist of the National Snow and Ice Data Center and lead author on the study. "As the ice sheet edge responds to rapid ice loss, the character and behavior of the system as a whole are changing, with the potential to influence ecosystems and people who depend on them."
The changes described in the new study seem to depend on the unique features of its environment, such as the slope of the land that the glacier flows down, the properties of the ocean water that touch the glacier, as well as the glacier's interaction with neighboring glaciers. That suggests scientists would need detailed knowledge not only of the glacier itself, but also of the glacier's unique environment in order to predict how it will respond to continued ice loss.
"It makes modeling glacial evolution far more complex when we're trying to anticipate how these systems will evolve both in the short term and two or three decades out," Gardner said. "It's going to be more challenging than we previously thought, but we now have a better understanding of the processes driving the variety of responses, which will help us make better ice sheet models."
Space for Climate
• October 22, 2020: The scientific evidence of global climate change is irrefutable. The consequences of a warming climate are far-reaching – affecting fresh water resources, global food production, sea level and triggering an increase in extreme-weather events. 40)
Figure 32: In order to tackle climate change, scientists and governments need reliable data in order to understand how our planet is changing. ESA is a world-leader in Earth observation and remains dedicated to developing cutting-edge spaceborne technology to further understand the planet, improve daily lives, support effect policy-making for a more sustainable future, and benefit businesses and the economy (video credit: ESA)
Figure 33: To tackle climate change, a global perspective is needed and this can be provided by satellites. Their data is key if we want to prepare ourselves for the consequences of climate change. While our Earth Explorers gather data to understand how our planet works and understand the impact that climate change and human activity are having on the planet, the European Union’s Copernicus Sentinels provide systematic data for environmental services that help adapt to and mitigate change (video credit: ESA)
Figure 34: This image of Paris was captured by Sentinel-2A on 15 July 2015. The satellite carries an innovative high-resolution multispectral imager with 13 spectral bands for new perspective of our land and vegetation. It will provide information, for example, for agricultural practices and to help manage food security (image credit: Copernicus Sentinel data (2015)/ESA)
Climate change is the paramount environmental issue of our time, and the greatest challenge is obtaining a detailed understanding of the complex variables involved. It includes health and safety, food production, security, economic and other aspects of our lives. 41)
On 30 November to 11 December 2015 the world’s attention will be firmly on Paris in France as leaders meet for the COP21 climate conference to set the tone for the health of our planet for decades to come.
Satellites play a critical role in providing essential information – from mapping ice in the polar regions to monitoring deforestation and urban growth – so that informed decisions can be made.
By using Earth observation techniques from space, we can monitor global environmental change not possible with other techniques.
The observations provide unique information that greatly assist in the understanding and management of climate change. Space delivers data with regular, uniform and global coverage, and reliable assessments of trends over time for specific variables. It also observes remote regions possible that are under-sampled by conventional networks.
Earth observation has not only revolutionized the way we perceive our planet, but it has also changed the way we comprehend our profound impact on the environment. Current satellite missions are building a long-term archive of essential data for local and international policy and planning.
How can different types of missions, instruments and data be used to study changes of our atmosphere, land, oceans and ice?
To respond to the need for climate-quality satellite data, ESA set up the Climate Change Initiative.
The aim is to realize the full potential of the long-term global Earth observation archives that ESA, together with its member states, has established over the last 30 years, as a significant and timely contribution to the ECV (Essential Climate Variables) databases required by the UNFCCC (UN Framework Convention on Climate Change).
The goal is to provide stable, long-term, satellite-based ECV data products for climate researchers. The ECVs will be derived from multiple satellite datasets, through international collaboration, and will include specific information on the errors and uncertainties of the dataset.
ESA’s Climate Change Initiative is making full use of Europe’s Earth observation satellites to exploit robust long-term global records of ECVs, such as greenhouse-gas concentrations, sea-ice extent and thickness, and sea-surface temperature and salinity.
NASA Supercomputing Study Breaks Ground for Tree Mapping, Carbon Research
• October 19, 2020: Scientists from NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and international collaborators demonstrated a new method for mapping the location and size of trees growing outside of forests, discovering billions of trees in arid and semi-arid regions and laying the groundwork for more accurate global measurement of carbon storage on land. 42)
Figure 35: Scientists from NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and international collaborators demonstrated a new method for mapping the location and size of trees growing outside of forests, discovering surprisingly high numbers of trees in semi-arid regions and laying the groundwork for more accurate global measurement of carbon storage on land (video credit: NASA/GSFC, Scientific Visualization Studio)
- Using powerful supercomputers and machine learning algorithms, the team mapped the crown diameter – the width of a tree when viewed from above – of more than 1.8 billion trees across an area of more than 500,000 square miles, or 1,300,000 km2. The team mapped how tree crown diameter, coverage, and density varied depending on rainfall and land use.
- Mapping non-forest trees at this level of detail would take months or years with traditional analysis methods, the team said, compared to a few weeks for this study. The use of very high-resolution imagery and powerful artificial intelligence represents a technology breakthrough for mapping and measuring these trees. This study is intended to be the first in a series of papers whose goal is not only to map non-forest trees across a wide area, but also to calculate how much carbon they store – vital information for understanding the Earth’s carbon cycle and how it is changing over time. 43)
Measuring carbon in trees
- Carbon is one of the primary building blocks for all life on Earth, and this element circulates among the land, atmosphere, and oceans via the carbon cycle. Some natural processes and human activities release carbon into the atmosphere, while other processes draw it out of the atmosphere and store it on land or in the ocean. Trees and other green vegetation are carbon “sinks,” meaning they use carbon for growth and store it out of the atmosphere in their trunks, branches, leaves and roots. Human activities, like burning trees and fossil fuels or clearing forested land, release carbon into the atmosphere as carbon dioxide, and rising concentrations of atmospheric carbon dioxide are a main cause of climate change.
- Conservation experts working to mitigate climate change and other environmental threats have targeted deforestation for years, but these efforts do not always include trees that grow outside forests, said Compton Tucker, senior biospheric scientist in the Earth Sciences Division at NASA Goddard. Not only could these trees be significant carbon sinks, but they also contribute to the ecosystems and economies of nearby human, animal and plant populations. However, many current methods for studying trees’ carbon content only include forests, not trees that grow individually or in small clusters.
- Tucker and his NASA colleagues, together with an international team, used commercial satellite images from DigitalGlobe, which were high-resolution enough to spot individual trees and measure their crown size. The images came from the commercial QuickBird-2, GeoEye-1, WorldView-2, and WorldView-3 satellites. The team focused on the dryland regions – areas that receive less precipitation than what evaporates from plants each year – including the arid south side of the Sahara Desert, that stretches through the semi-arid Sahel Zone and into the humid sub-tropics of West Africa. By studying a variety of landscapes from few trees to nearly forested conditions, the team trained their computing algorithms to recognize trees across diverse terrain types, from deserts in the north to tree savannas in the south.
Figure 36: The team focused on the dryland regions of West Africa, including the arid south side of the Sahara Desert, stretching through the semi-arid Sahel Zone and into the humid sub-tropics. By studying a variety of landscapes from few trees to nearly forested conditions, the team trained their computing algorithms to recognize trees across diverse terrain types, from deserts in the north to tree savannas in the south [image credits: NASA's Scientific Visualization Studio; Blue Marble data is courtesy of Reto Stockli (NASA/GSFC)]
Learning on the job
- The team ran a powerful computing algorithm called a fully convolutional neural network (“deep learning”) on the University of Illinois’ Blue Waters, one of the world’s fastest supercomputers. The team trained the model by manually marking nearly 90,000 individual trees across a variety of terrain, then allowing it to “learn” which shapes and shadows indicated the presence of trees.
- The process of coding the training data took more than a year, said Martin Brandt, an assistant professor of geography at the University of Copenhagen and the study’s lead author. Brandt marked all 89,899 trees by himself and helped supervise training and running the model. Ankit Kariryaa of the University of Bremen led the development of the deep learning computer processing.
- “In one kilometer of terrain, say it’s a desert, many times there are no trees, but the program wants to find a tree,” Brandt said. “It will find a stone, and think it’s a tree. Further south, it will find houses that look like trees. It sounds easy, you’d think – there’s a tree, why shouldn’t the model know it’s a tree? But the challenges come with this level of detail. The more detail there is, the more challenges come.”
- Establishing an accurate count of trees in this area provides vital information for researchers, policymakers and conservationists. Additionally, measuring how tree size and density vary by rainfall – with wetter and more populated regions supporting more and larger trees – provides important data for on-the-ground conservation efforts.
- “There are important ecological processes, not only inside, but outside forests too,” said Jesse Meyer, a programmer at NASA Goddard who led the processing on Blue Waters. “For preservation, restoration, climate change, and other purposes, data like these are very important to establish a baseline. In a year or two or ten, the study could be repeated with new data and compared to data from today, to see if efforts to revitalize and reduce deforestation are effective or not. It has quite practical implications.”
- After gauging the program’s accuracy by comparing it to both manually coded data and field data from the region, the team ran the program across the full study area. The neural network identified more than 1.8 billion trees – surprising numbers for a region often assumed to support little vegetation, said Meyer and Tucker.
- “Future papers in the series will build on the foundation of counting trees, extend the areas studied, and look ways to calculate their carbon content,” said Tucker. NASA missions like GEDI (Global Ecosystem Dynamics Investigation), and ICESat-2 (Ice, Cloud, and Land Elevation Satellite-2), are already collecting data that will be used to measure the height and biomass of forests. In the future, combining these data sources with the power of artificial intelligence could open up new research possibilities.
- “Our objective is to see how much carbon is in isolated trees in the vast arid and semi-arid portions of the world,” Tucker said. “Then we need to understand the mechanism which drives carbon storage in arid and semi-arid areas. Perhaps this information can be utilized to store more carbon in vegetation by taking more carbon dioxide out of the atmosphere.”
- “From a carbon cycle perspective, these dry areas are not well mapped, in terms of what density of trees and carbon is there,” Brandt said. “It’s a white area on maps. These dry areas are basically masked out. This is because normal satellites just don’t see the trees – they see a forest, but if the tree is isolated, they can’t see it. Now we’re on the way to filling these white spots on the maps. And that’s quite exciting.”
Figure 37: An astronaut aboard the International Space Station (ISS) took this oblique photograph that shows most of the West African country of Guinea-Bissau, along with neighboring Guinea, The Gambia and Senegal, and the southern part of Mauritania. This scene stretches from the green forest vegetation and wet climates of the Atlantic coast to the almost vegetation-less landscapes of the Sahara Desert (image credit: NASA)
Prior Weather Linked to Rapid Intensification of Hurricanes Near Landfall
• October 15, 2020: New study results show that ocean heat waves can provide enough fuel for hurricanes to gain momentum as they approach land. 44)
Although most hurricanes tend to weaken as they approach land, some rapidly increase in strength just prior to landfall - a phenomenon that is both dangerous and hard to forecast. As the climate continues to warm, the number of storms that fall into the latter category is likely to increase, presenting a stark reality for communities in their paths. Because current weather models can't accurately predict this sudden intensification, communities preparing for a lesser storm often don't have time to respond to the arrival of a much stronger one or to the magnitude of destruction it is likely to leave behind.
Figure 38: Hurricane Michael was captured from the International Space Station on Oct. 10, 2018, after the storm made landfall as a Category 4 hurricane over the Florida Panhandle. The National Hurricane Center reported maximum sustained winds near 145 mph (233 kph) with the potential to bring dangerous storm surge and heavy rains to the Florida Panhandle (image credit: NASA)
The good news? The results of a new study published in September in Nature Communications identify pre-storm conditions that can contribute to this rapid intensification - an important step in improving our ability to forecast it. 45)
"We analyzed the events that led up to Hurricane Michael in 2018 and found that the storm was preceded by a marine heat wave, an area of the coastal ocean water that had become abnormally warm," said Severine Fournier, a NASA Jet Propulsion Laboratory scientist and a co-author of the study. "Marine heat waves like this one can form in areas that have experienced back-to-back severe weather events in a short period of time."
In October 2018, Hurricane Michael intensified from a Category 2 to a Category 5 storm the day before it made landfall in the Florida Panhandle. Michael is the most intense storm on record to hit the area, having left some $25 billion in damage in its wake. Using a combination of data gathered from weather buoys and satellites, the science team behind the study examined ocean conditions before, during, and after the hurricane.
Figure 39: This map of the Gulf of Mexico shows areas with unusually high sea surface temperatures before Hurricane Michael. The area from land down to the green line, and the small, enclosed areas below the green line experienced an extreme ocean heat wave in this period. The smaller circles show the path of Tropical Storm Gordon (TS), which preceded Michael; larger, darker circles show Michael's track and intensification. The legend's first four icons mark data stations (image credit: NASA/JPL-Caltech/University of South Alabama/DISL)
About a month before the hurricane arrived, Tropical Storm Gordon moved through the Gulf of Mexico. Under normal circumstances, a tropical storm or hurricane - Gordon, in this case - mixes the ocean water over which it travels, bringing up the cold water that is deeper in the water column to the surface and pushing the warm surface water down toward the bottom. This newly present colder water at the surface typically causes the storm to weaken.
But Tropical Storm Gordon was immediately followed by a severe atmospheric heat wave during which the warm air heated the cooler ocean water that had recently been brought to the surface. This, combined with the warm water that Gordon had pushed down through the water column, ultimately produced plenty of warm-water fuel for an incoming hurricane.
"In that situation, basically the whole water column was made up of warm water," said Fournier. "So when the second storm - Hurricane Michael - moved in, the water it brought up during mixing was warm just like the surface water being pushed down. Hurricanes feed off the heat of the ocean, so this sequence of weather events created conditions that were ideal for hurricane intensification."
Although the study focuses in-depth on Hurricane Michael, the scientists note that the pattern of weather events leading up to a major storm - and the resulting storm intensification - doesn't appear to be unique to Michael.
"Both Hurricane Laura and Hurricane Sally, which impacted the U.S. Gulf Coast in 2020, appeared to have similar setups to Michael, with both storms being preceded by smaller storms [Hurricane Hanna and Hurricane Marco, respectively]," said lead author Brian Dzwonkowski of the University of South Alabama/Dauphin Island Sea Lab. "Combined with warmer-than-average summer conditions in the region, this pre-storm setup of the oceanic environment likely contributed to those intensifications prior to landfall as well."
NASA scientists have been tackling the question of what causes hurricanes to intensify rapidly just before landfall from multiple angles. Another recent study led by JPL's Hui Su found that other factors, including the rainfall rate inside a hurricane, are also good indicators that can help forecast if and how much a hurricane is likely to intensify in the hours that follow. Both studies bring us closer to understanding and being better able to forecast rapid intensification of hurricanes near landfall.