Sea Ice Monitoring
Science
Earth Observation
Measurement Types
Sea ice is a key part of the cryosphere, which refers collectively to the portions of the Earth’s surface that are frozen, including snow cover, floating ice, glaciers, ice caps, ice sheets, seasonally frozen ground, and permafrost (depicted in Figure 1). Sea ice forms from frozen seawater floating on the ocean surface in high-latitude regions where there is little sunlight, with the majority forming in the Arctic and Antarctic Oceans.
Related resources
Measurements of sea ice extent, concentration, thickness, and drift are key indicators of climate change, and their change has far-reaching impacts on global weather and climate, ocean circulation, and ship navigation. Satellite observation is a crucial tool for monitoring sea ice extent and thickness, particularly when complemented by airborne and in-situ measurements. 4) 5)
Tracking Sea Ice Trends
Unlike the melting of land ice (such as glaciers, ice sheets, and ice shelves) the melting of sea ice does not contribute to sea level rise, as it floats on the surface of the oceans. 5)
The majority of sea ice forms in Earth’s polar regions. The Arctic is a semi-enclosed ocean, almost completely surrounded by land. The sea ice here is mostly confined to the cold Arctic basin, which consists of thick ridges of ice that melt less in the summer in comparison to Antarctic sea ice. The Antarctic is a land mass surrounded by open ocean. This means that the sea ice can move freely, resulting in higher drift speeds. But Antarctic sea ice tends to have fewer ridges, as it is less confined so fewer collisions happen. Due to the lack of land boundary to the north, the sea ice floats northwards to warmer temperatures where the majority melts over the summer. 3)
Region | Sea Ice Maximum Extent (million square miles) | Month of Sea Ice Maximum | Sea Ice Minimum Extent (million square miles) | Month of Sea Ice Minimum | Sea Ice Thickness (m) | Sea Ice Extent Pattern |
Arctic | ~5.8 (reached in February) | March | ~2.5 (~1/3 of the winter ice becomes multi-year ice) | September | ~2 - 3 | Asymmetric, with ice at some longitudes extending to much lower latitudes than at other longitudes |
Antarctic | ~7 (reached in September) | End of September | ~1 (reached in February) | February | ~1 to 2 | Roughly symmetric around the South Pole |
While Table 1 shows the average characteristics of sea ice in the Arctic and Antarctic, longitudinal measures have shown that both the thickness and extent of Arctic sea ice have changed dramatically since satellite records began in November 1978, which is a clear indication of climate change. The Arctic is warming faster than the rest of Earth, resulting in a massive decline in sea ice (as shown in Figure 3).
Since 2007, the minimum Arctic sea ice extent has been between 1.4 and 1.9 million square miles. While the Arctic sea ice has not yet matched the record-low minimum extent of September 2012 (1.31 million square miles), it is showing a general decline of 13 percent per decade since the beginning of satellite records. This decline means that the amount of ice that survives to become ‘multi-year’ sea ice is also declining (as shown in Figure 4). Multi-year ice tends to be thicker and stronger than new ice. Arctic sea ice therefore has been thinning, with more ice measuring from only 1 to 2 meters as opposed to the 2 to 3 meter average value. 8) 3) 5) 6)
Impacts of Sea Ice Loss
Models of Arctic sea ice loss have allowed for climate projections of a plausible earliest date that the Arctic could be seasonally 'ice-free,' by 2040. The IPCC AR5 (Intergovernmental Panel on Climate Change – 5th Assessment Report) concluded that a nearly ice-free Arctic Ocean in September for at least 5 consecutive years is likely before the middle of the 21st century (as shown in Figure 5). 5)
The decrease in multi-year Arctic sea ice has far-reaching effects, as it plays a key role in regulating the Earth’s climate, weather patterns, and ocean circulation. The loss of sea ice amplifies ocean warming which in turn enhances the sea ice loss due to 'ice-albedo feedback' (as shown in Figure 6). With increased atmospheric and oceanic temperatures, sea ice melts and reveals the ocean surface below. This reduces the albedo of the Arctic surface, as the ocean surface is less reflective than ice, and increases the absorption of solar radiation. This creates an ice-albedo feedback loop, which is strongest during the summer when solar radiation is at its highest. Conversely in the winter, sea ice insulates the ocean from losing heat to the atmosphere, resulting in a warming effect which is diminished with sea ice loss. Hence sea ice continuously plays an important role in preventing ocean temperature extremes through reflection and insulation. This has far-reaching impacts with the reduction in Arctic sea ice extent being linked with changes in the jet stream or the North Atlantic Oscillation. 8) 5) 3)
The loss of Arctic sea ice also affects the salinity and density of the surrounding ocean, which impacts large-scale ocean circulation. As sea ice grows over multiple years, brine rejection occurs - a process that drains the salt out of sea ice. When this ice melts, freshwater is released into the ocean and affecting its density, structure, and circulation. In addition, Arctic sea ice loss affects the lives and customs of Indigenous Peoples, marine life, and surrounding coastal areas. 5) 3)
Antarctic sea ice, on the other hand, has historically exhibited minimal long-term trends with significant year-to-year variability. Since 2014, Antarctic sea ice has exhibited both record-high and record-low extents. Since the record-high Antarctic sea ice extent in 2014, a general trend of decrease began with new record-lows in 2017, 2022, and 2023. The September 2023 record-low winter maximum was around 6.7 million square miles, which was 9% below the average for September from 1991 to 2020. However, NASA NSIDC (National Snow and Ice Data Center) Sea Ice Index data indicates that the general trend of maximum Antarctic sea ice extent from 1979 to 2023 remained on a positive trend, though at a slowing rate. This increase in Antarctic sea ice is however outweighed by the decrease in Arctic sea ice, resulting in net global sea ice loss. 3) 9) 4) 10)
Sea Ice Monitoring Methods
Many in-situ measurement techniques have developed this data record of sea ice extent, thickness, age, and movement. Naval submarines with upward-looking sonars provide the longest record of Arctic sea ice thickness, with declassified submarine sonar data available from as early as 1958. Automated submersibles are used in field campaigns, as well as static upward-looking sonars to measure sea ice thickness at select locations. Aircraft-based altimeters provide an alternative to satellite altimetry, as their measurements of the ice surface typically have much higher spatial resolution than satellites, but with limited coverage. Airborne instruments can use electromagnetic inductance to probe the water underneath sea ice, which is used to determine sea ice thickness when paired with laser altimetry measurements of the ice surface. 11)
Embedding buoys in sea ice floes and tracking them provides measurements of sea ice movement and age. These buoys can carry sensors that measure the contribution of melting from the atmosphere and ocean. Similarly, icebreaker ships can be embedded in ice becoming ‘ice camps’ from which year-round continuous observations can be undertaken. The first such drifting laboratory was named the Surface Heat Budget of the Arctic (SHEBA) mission and operated from September 1997 to October 1998, and more recently, the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) (2017 to 2022). Ice camps are increasingly used to employ new and emerging technologies such as Unmanned Aerial Vehicles (UAVs) and Autonomous Underwater Vehicles (AUVs). 11)
Routine satellite measurements with near-full coverage have been made since 1979. This provides estimates of the age of sea ice, as radars can observe the differences in ice surface roughness (as shown in Figure 8). Multi-year ice has lower salinity (due to brine rejection), causing it to have a lower conductivity which can be detected using passive microwave or SAR (Synthetic Aperture Radar) instruments. Ice drift maps can be constructed from sequential satellite imagery, using microwave and radar instruments. However, these maps can only be used for a main ice pack, as ice floes at the ice edge move too fast to be followed by imagery. 11)
Sea ice extent can be inferred from satellite observations of thermal microwave radiation emitted from the open ocean as compared to those emitted by sea ice. The retrieval errors are greatest in the summer as melt ponds are difficult to distinguish from open water and the technique cannot see thin ice as it does not mask the microwave emission of the underlying ocean. 11)
Satellite altimetry measures the difference in height between the ocean surface and the sea ice surface, providing a measurement of sea ice ‘freeboard’ thickness - the depth of ice above the waterline (as shown in Figure 9). However, sea ice is often covered with snow, which laser altimeters cannot penetrate. Radar altimeters however can see through the snow to the ice surface, as well as through clouds (unlike laser altimeters). Altimeters are limited to measurements directly under the satellite so cannot provide complete global coverage, and also struggle to measure the ice surface while it melts. Therefore, both types of altimeter are more suited to measurements of ice freeboard during the winter when there is no surface melting and lower cloud coverage. 11)
Example Products
Sea Ice Extent and Concentration
Sea ice concentration is defined as the percentage of sea ice coverage over a particular area. This is typically a local measurement that can be extended to sea ice extent, which is defined as the total area of the ocean that contains sea ice with a concentration above 15%. Sea ice concentration can be displayed through ice charts (as shown in Figure 10), which are delineations of data from various sources, such as on-shore, satellite, and aircraft observations. 11) 4) 5)
Sea Ice Thickness
Radar and laser altimetry data can be used to measure the freeboard, which is the height of sea ice above the sea surface. The ‘draft,’ which is the depth of sea ice below the surface of the surrounding ocean, is measured using electromagnetic inductance. Electrical currents are generated in the water under the sea ice through inductance, and detected by the satellite. Combining both of these measurements can provide a measurement of sea ice thickness. 11)
Sea Ice Drift
Ice drift maps (as shown in Figure 12), showing how sea ice moves as a result of ocean currents and winds, are constructed from sequential satellite images taken every one to three days, either by passive microwave radiometers or high resolution SAR. While these techniques are effective in tracking main ice packs, ice floes on the edges move too rapidly to be followed by imagery. Embedded buoys with fitted GPS provide higher frequency measurements of ice drift, as they transmit position data every few minutes. 11)
Related Missions
Copernicus: Sentinel-1
ESA’s Copernicus Sentinel-1 mission is a two-satellite constellation: Sentinel-1A and Sentinel-1B. 1A was launched in April 2014 and 1B in April 2016. Both Sentinel-1 satellites are identical and have the C-SAR (C-band Synthetic Aperture Radar) onboard to monitor sea and land ice. After Sentinel-1B experienced an anomaly which rendered it unable to deliver radar data, the full capability of the constellation was restored with the launch of Sentinel-1C in December 2024.
Copernicus: Sentinel-3
Sentinel-3 (S3) is a dual constellation of ESA radar imaging satellites supported by EUMETSAT (European Organisation for the Exploitation of Meteorological Satellites). Both S3A, launched in February 2016, and S3B, launched in April 2018, are part of Copernicus, the European Union’s Earth observation programme managed by the European Commission (COM). Sentinel-3 aims to measure Earth's oceans, land, ice and atmosphere to monitor and understand large-scale global dynamics. Sea ice is monitored using the SAR Radar Altimeter (SRAL).
CryoSat
Launched in October 2005, ESA’s CryoSat was a next-generation radar altimetry mission which aimed to observe the polar regions. CryoSat was lost in a launch failure in October 2005, prompting the launch of CryoSat-2 in April 2010. CryoSat-2 is an ESA Earth Explorer mission that monitors changes in polar ice sheets using a radar altimeter named SIRAL (SAR Interferometer Radar Altimeter), which monitors land and sea ice sheet thickness intending to explain the connection between the melting of polar ice and the rise in sea levels.
CRISTAL (Copernicus Polar Ice and Snow Topography Altimeter)
The Copernicus Polar Ice and Snow Topography Altimeter (CRISTAL) mission, also referred to as Sentinel-9, is being developed by ESA to measure and monitor sea-ice thickness, overlying snow depth, and ice-sheet elevations. CRISTAL will carry an Interferometric Radar altimeter for Ice and Snow (IRIS) and a Microwave Radiometer (MWR). IRIS will be used to measure the thickness of sea ice and the snow that covers it, as well as the height of the world’s ice sheets and glaciers.
ICESat (Ice, Cloud and Land Elevation Satellite)
ICESat is a NASA mission within the ESE (Earth Science Enterprise) programme. It was operational from January 2003 to August 2010, within which it monitored the mass balance of the polar ice sheets and their contributions to global sea level change using GLAS (Geoscience Laser Altimeter System). Launched in September 2018, ICESat-2 is a follow-up mission to ICESat that aims to continue ICESat’s measurements using ATLAS (Advanced Topographic Laser Altimeter System). It will measure the polar ice sheet mass balance to determine contributions to sea level changes and ocean circulation, as well as to determine the seasonal cycle and topographic character of ice sheet changes and estimate sea ice thickness.
Moderate Resolution Imaging Spectroradiometer (MODIS)
Developed by NASA’s Goddard Space Flight Centre, the MODIS (Moderate-Resolution Imaging Spectroradiometer) instrument is onboard NASA's Aqua (EOS/PM-1) and Terra (EOS/AM-1) satellites, allowing the collection of extensive data on Earth’s atmosphere, land surface, ocean and cryosphere. It provides daily global coverage of sea ice extent and ice surface temperature. Launched May 2002 and December 1999, Aqua and Terra are joint missions within NASA's ESE (Earth Science Enterprise) programme which both carry MODIS sensors. MODIS has now been replaced by the Visible Infrared Imaging Radiometer Suite (VIIRS).
VIIRS (Visible Infrared Imaging Radiometer Suite)
The VIIRS (Visible Infrared Imaging Radiometer Suite) contributes to improved weather forecasting by tracking long-term data on sea ice, snow cover, and sea ice temperature and collecting visible and IR images of the land, atmosphere, cryosphere, and oceans. VIIRS is flying onboard NASA and NOAA’s Suomi NPP, NOAA-20, and NOAA-21 satellite missions. NOAA-20 and NOAA-21, launched November 2017 and November 2022 respectively, are part of the Joint Polar Satellite System (JPSS) programme of NOAA and NASA. The Suomi National Polar-orbiting Partnership (Suomi NPP), operated by NASA and the National Oceanic and Atmospheric Administration (NOAA), is a weather satellite that was launched in October 2011.
References
1) “What is the Cryosphere?,” NSIDC, URL: https://nsidc.org/learn/what-cryosphere
2) “Cryosphere glossary,” NSIDC, URL: https://nsidc.org/learn/cryosphere-glossary
3) “Sea Ice,” NSIDC, URL: https://nsidc.org/learn/parts-cryosphere/sea-ice
4) “Sea Ice Today,” NSIDC, URL: https://nsidc.org/sea-ice-today/studying-sea-ice
5) “Sea ice in the climate system,” Met Office, URL: https://www.metoffice.gov.uk/research/climate/cryosphere-oceans/sea-ice/index
6) “Sea ice: an overview,” Met Office, URL: https://www.metoffice.gov.uk/research/climate/cryosphere-oceans/sea-ice/overview
7) “Sea Ice,” NASA, URL: https://earthobservatory.nasa.gov/features/SeaIce
8) “Sea Ice Today,” NSIDC, URL: https://nsidc.org/sea-ice-today
9) “Antarctic Sea Ice Reaches Another Record Low,” NASA, URL: https://earthobservatory.nasa.gov/images/151093/antarctic-sea-ice-reaches-another-record-low
10) J. C. Comiso, C. L. Parkinson, R. Gersten, A. C. Bliss, and T. Markus (2024), “Current State of Sea Ice Cover,” NASA, URL: https://earth.gsfc.nasa.gov/cryo/data/current-state-sea-ice-cover
11) “How we measure sea ice,” Met Office, URL: https://www.metoffice.gov.uk/research/climate/cryosphere-oceans/sea-ice/measure