Sea ice

State of Arctic and Antarctic sea ice in 2021

2021

Sea ice
Sea ice

The effects of climate change are not the same in the Arctic and Antarctic. While almost 75% of the Arctic sea ice has disappeared, the state of sea ice in Antarctic seems stable, at least for now.

Last Updated

28 October 2021

Published on

22 October 2021

By Rob Roebeling, Christine Traeger Chatterjee and Hayley Evers-King (EUMETSAT), Thomas Lavergne and Olivier Membrive (OSI SAF), and Ben Loveday (Innoflair)

The effects of climate change are not the same in all parts of the world. The planets average temperature has risen a little more than 1°C since the start of the industrial era. Our planet's polar regions, however, are warming faster than other places (IPCC, 2021). In these regions, temperatures rose about 2°C  in the Arctic and between 1.0 and 1.5°C  in the Antarctic.

Among other impacts, this warming resulted in a decrease of the Arctic sea ice area and ice thickness in both summer and winter, with sea ice becoming younger, thinner and more dynamic. In fact, the current Arctic sea ice area is at its lowest level since at least 1850. The area of late summer Arctic sea ice is now smaller than at any time in at least the past 1,000 years.

In its sixth assessment report, the Intergovernmental Panel on Climate Change (IPCC) of the United Nations states that human influence is very likely the main driver of the global retreat of Arctic sea ice during the last 40 years.

The Antarctic is responding less rapidly to climate change than the Arctic. However, there are large regional differences. The interior of the Antarctic continent and the east Antarctic ice sheet, which were considered unlikely to be affected by climate warming, show a very moderate warming of about 0.1 ± 0.2°C per decade (see Figure 1). Most warming, though, is recorded across the Antarctic Peninsula, where temperatures increased more than 0.3 ± 0.1°C per decade (Bromwich and Nicolas, 2014).

Temperature trends in °C per decade since the 1950s
Figure 1: Temperature trends in °C per decade since the 1950s. The thick black line delineates the area of high confidence (West Antarctica and the Antarctic Peninsula). NS indicates that the area east of the thick black line has confidence levels that are not statistically significant. Within that region, dotted lines indicate areas of medium confidence. Source: NOAA/

Sea ice variability observed from space

Getting a coherent picture of the current sea ice state over the Arctic and the Antarctic requires analysis of long-term climate data records of sea ice parameters, such as sea ice area and sea ice volume. Thanks to their global view and multi-decadal time coverage, satellites provide key information on these parameters. In particular, using brightness temperature measurements from microwave radiometer instruments, radar backscatter signal from scatterometers and synthetic aperture radars (SAR), or using reflected solar light and thermal measurements from imaging satellite instruments.

The measurements from the microwave radiometers, scatterometers, and SARs have the advantage that they are collected day and night in both cloud-free and cloudy conditions. Microwave radiometers and scatterometers have a relatively coarse spatial resolution (coarser than about 12x12 km2) but sub-daily coverage, while SAR has fine spatial resolution (less than 100m) but more irregular coverage.

The big advantage of modern optical imaging satellites is their fine spatial resolution (with sensors like Sentinel-3 OLCI providing global coverage at 0.3x0.3 km2), as it allows observing sea ice edges, as well as the breaking up of sea ice, in much more detail. On the down side, imaging satellites only collect measurements for cloud-free daylight conditions.

EUMETSAT's Ocean and Sea Ice Satellte Application Facility (OSI SAF) provides a unique long-term climate data record of sea ice concentration from 1979 onwards. It derives sea ice concentrations from the Special Sensor Microwave Imager and Sounder (SSMIS) on the DMSP satellites, plus the Advanced Microwave Scanning Radiometer (AMSR) on Aqua. For a more complete picture of the changes in the sea ice, one should also look at changes in sea ice volume (Paul et al., 2018). For this case study, we used the 40 year long-term sea ice volume data record of the American Polar Science Centre (PSC) (Zhang, and Rothrock, 2003). They calculate the volume of the ice from sea ice thickness model-based estimates and sea ice area observations.

Changes in the Arctic

In the Arctic, sea ice cover grows throughout the winter, reaching its maximum in March. As the Sun returns in March and gets stronger during spring and summer, the sea ice starts to melt, reaching its minimum in September.

Since the start of the industrial era, the Arctic warmed twice as fast as other parts of the world, with average temperatures increasing about 2°C. The warmer temperatures are affecting both Arctic ocean and Arctic land. Satellite observations show that the Arctic experienced large losses of sea ice during the last 40 years (see Figures 2 and 3).

Figure 2: Maximum Arctic sea ice concentration at the end of the winter season (end of March) and minimum Antarctic sea ice concentration at the end of the summer season (end of September). Source: OSI SAF.

Arctic sea ice has predominantly diminished around the time of the ice minimum in September. More specifically, the summer Arctic sea ice area has reduced by approximately 40% since 1979. This is about 2.5Mio km², which corresponds to one fifth of the Northern Hemisphere average sea ice area. The reduction in the area of Arctic sea ice at the end of the winter season is more moderate, approximately 10%. The decline in volume of Arctic sea ice is even stronger, with a decrease of 35% at the end of the winter season, and a decrease of 75% at the end of the summer season (see Figure 3).

Less sea ice is causing fewer days of snow cover each year in the Arctic. Less snow and ice cover has a compounding effect because it exposes darker ground surfaces and ocean waters, which allows more solar radiation to be absorbed. This, in turn, leads to more snow and ice melt. Based on observed trends and model predictions, the IPCC expects that the annual Arctic sea ice area minimum will likely fall below 1 million km2, a decrease of almost 90% since the 1980s, at least once before the middle of the 21st century (IPCC, 2021).

Trend in Arctic aea ice area & volume
Figure 3: Trend in Arctic sea ice area (OSI SAF data) and sea ice volume (Polar Science Centre data) for the months March (maximum) and September (minimum), 1979-2021.

2021 was a normal-to-warm year in the Arctic. Strong sea ice melt during late winter and spring months made the sea ice area decline rapidly, and resulted in record low sea ice areas during these months (see Figure 6). In summer 2021, this strong melt slowed down. Changing weather conditions over the Beaufort Sea caused lower temperatures in the Arctic in July and August. In September melt increased again, resulting in an September ice minimum that was about 20% below the long-term mean.

The sea ice area in most recent years has been substantially lower than the mean ice area observed between 1970 and 2021. The OSI SAF High Latitude Processing Center (assessed 28 September 2021) provides daily updates of the sea ice situation in the Arctic and Antarctic.

Changes in the Antarctic

Similar to the Arctic, the Southern Ocean around the Antarctic continent freezes in winter and melts in summer. However, the Antarctic differs in many ways from the Arctic. Among others, one way it differs is that the Arctic is an ice-covered ocean surrounded by landmasses, whereas Antarctica is a continent surrounded by ocean. Maximum ice area is usually reached around September and minimum ice area around March. Due to these differences, the sea ice area in the Antarctic is smaller than in the Arctic at the end of summer, and larger at the end of winter.

Since the start of the industrial era, the Antarctic had warmed between 1.0 and 1.5°C (Martin et al., 2019). However, this warming, slowed down strongly 40 years ago, and has since been showing at a moderate increase of about 0.2°C in overall temperatures (HadCRUT4 temperature data from the Climatic Research Unit (CRU), assessed on 21 September 2021).

In satellite observations no significant trend in sea ice area and volume occurred during the last 40 years, in both winter and summer. In fact, Antarctic sea ice area and volume remained stable, or even increased slightly, both at the end of summer and the end of the winter (see Figures 4 and 5).

Figure 4: maximum Antarctic sea ice concentration at the end of the winter season (end of September) and minimum Antarctic sea ice concentration at the end of the summer season (end of March). Source: OSI SAF.

However, the observed gains in Antarctic sea ice are almost an order of magnitude smaller than those observed over the Arctic (see Figure 5).

Despite moderate warming, sea ice area and volume over the Antarctic show no significant trend. The latter may be attributed to regionally opposing trends and large internal variability (IPCC, 2021). There are several theories explaining the drives of the 'quasi stable' situation in Antarctic Sea ice. Among others, this may be explained by increased cooling of the Antarctic Sea due to increased melt of the Antarctic glacier (Bintanja et al., 2013) or by changes in wind patterns (Schroeter, et al., 2018).

Trend in Antarctic sea ice area & volume
Figure 5: Trend in Antarctic Sea ice area (OSI SAF data) and sea ice volume (Polar Science Centre data) for the months March (minimum) and September (maximum), 1979-2021.

2021 was a normal year in the Antarctic. Overall, the area of sea ice was normal to slightly higher during the entire cycle (see Figure 6). The 2021 maximum sea ice area was about 16Mio km2, which is close to the 40 year record high. The 2021 minimum sea ice area was around the long-term average, with about 2.5Mio km2. More details can be found on the sea ice dashboard of Met Norway (assessed on 21 September 2021).

Changes in annual cycle of Arctic and Antarctic Sea ice area since the 1980s
Figure 6: Changes in annual cycle of Arctic and Antarctic Sea ice area since the 1980s. The black line shows the annual cycle of 2021. This figure is based on data provided by the OSI SAF.

Advances in ice monitoring capabilities

New satellites and algorithm development ensure the continuation and improvement of sea ice monitoring. Among them are the high-resolution visible and thermal imaging radiometers, and Synthetic Aperture Radar Altimeter (SRAL) aboard Sentinel-3. Currently in development, is an Ice Surface Temperature processor that will use data from the Sea and Land Surface Temperature Radiometer (SLSTR) instrument aboard Sentinel-3.

Although these measurements will help to develop a much better understanding of the changes in sea ice parameters, a drawback of all visible and thermal optical measurements is that clouds can contaminate these measurements, and, thus, need to be detected and removed from the products. This is a challenge as it involves removing bright targets (clouds) above a bright surface (sea ice covered by snow).

A further development from Sentinel-3 is the use of the SRAL altimeter to provide freeboard and measurements relating to sea ice leads. The Copernicus Sentinel-6 Michael Freilich satellite, launched in November 2020, will ensure the continuity of sea-level measurements for climate applications. In addition, its high precision altimeter and Synthetic Aperture Radar (SAR) interleaved (open burst) measurement mode, also presents opportunities for investigating marginal ice zone dynamics. Figure 7 shows an early result from Donlon et al. (2021), of a fully focused SAR image over an ice-covered lake in Russia, generated from the Poseidon-4 data collected onboard Sentinel-6.

Comparison images
Figure 7: (a) Sentinel-2B colour composite image of Ozero Nayvak peninsular (64.433 N, −172.3466 W), Russia obtained on 15th August 2020 with annotations of key features. (b) Sentinel-1b interferometric wide-swath C-band SAR image obtained on 29th November 2020. (c) S6-MF fully-focused SAR (FFSAR) image obtained on November 30th 2020 processed by Aresys. The FFSAR image is not geolocated or projected and the white square indicates the approximate position of the Sentinel-6 altimeter image in c.

All of these advances are currently in the development stage, so it will be some time before products are operational. In the meanwhile, you can find out about the latest results from these projects on our science studies pages.

Conclusion

The ongoing reduction in sea ice area and volume impacts the climate system as a whole: ranging from changes in global ocean and atmospheric circulation, which in turn influences weather patterns, to changes in the Earth radiation budget through a decrease of surface albedo, which amplifies the warming. Monitoring changes in sea ice and understanding underlying processes, are crucial for predicting future climate change and its impacts.

This case study provides an update on the sea ice area and volume over the Arctic and Antarctic in 2021. Over the Antarctic, only a slight change in sea ice parameters manifested since the 1980s, staying mostly within +/- 5%. Since the 1980s the bulk of the changes occurred in the Arctic, where the sea ice area reduced fast (up to 40% at the end of summer) and its volume very fast (up to 75% at the end of summer).

2021 was a normal to warm year in the Arctic. The year started with record strong ice melt in winter and spring 2021, which, however, slowed down in summer. In the end the 2021 Arctic Sea ice minimum was 20% below the 40 years mean. The annual minimum sea ice area in the Arctic will likely fall below 1 million km2 at least once before 2050 (IPCC. 2021).

Table 1: Summary of changes in sea ice area and sea ice volume in the Arctic and Antarctic since 1979
Region Sea ice area change since 1979 - winter maximum Sea ice area change since 1979 - summer minimum Sea ice volume change since 1979 - winter maximum Sea ice volume change since 1979 - summer minimum
Arctic -10% -40% -35% -75%
Antarctic +3% +3% +10% +10%

Benefits

This study shows the synergetic use of OSI SAF data to deepen our understanding on the consequences of changes in Arctic and Antarctic sea ice parameters and their impacts on the climate system as a whole, including weather patterns directly affecting populated areas in high- and mid-latitudes.

Data used

  • Satellite-based climate data records of sea ice concentration, derived from passive microwave sensor observations (SMMR, DMSP/SSM/I, DMSP/SSMIS), over the period 1979-2020 by the OSI SAF that are cited as:
    • EUMETSAT Ocean and Sea Ice Satellite Application Facility, Global Sea Ice Concentration Climate Data Record v2.1 - Multimission, over the period 1979-2015, OSI-450, doi: 10.15770/EUM_SAF_OSI_0008, data extracted from OSI SAF FTP server, accessed 5 May 2021.
    • EUMETSAT Ocean and Sea Ice Satellite Application Facility, Interim Global Sea Ice Concentration Climate Data Record v2.1 - Multimission, over the period 2016-onwards, OSI-430-b, doi: 10.15770/EUM_SAF_OSI_NRT_2008 (minted but not published on 7 October 2021), data extracted from OSI SAF FTP server, accessed 5 May 2021.
  • A satellite and model based climate data record of Sea Ice Volume from the Pan-Arctic Ice Ocean Modeling and Assimilation System (PIOMAS), Zhang and Rothrock, 2003, of the Polar Science Centre (PSC)
  • A satellite-based data record of Sea Ice Concentration from OLCI on Sentinel-3.

References

IPCC, 2021: Summary for Policymakers. In: Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [Masson Delmotte, V., P. Zhai, A. Pirani, S. L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M. I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T. K. Maycock, T. Waterfield, O. Yelekçi, R. Yu and B. Zhou (eds.)]. Cambridge University Press. In Press.

Donlon, C.J., Cullen, R., Giulicchi, L., Vuilleumier, P., Francis, C.R., Kuschnerus, M., Simpson, W., Bouridah, A., Caleno, M., Bertoni, R. and Rancaño, J., 2021. The Copernicus Sentinel-6 mission: Enhanced continuity of satellite sea level measurements from space. Remote Sensing of Environment, 258, p.112395. https://doi.org/10.1016/j.rse.2021.112395

Claire L. Parkinson, 2019: A 40-y record reveals gradual Antarctic sea ice increases followed by decreases at rates far exceeding the rates seen in the Arctic, PNAS, https://doi.org/10.1073/pnas.1906556116

Schroeter, S., Hobbs, W., Bindoff, N. L., Massom, R., & Matear, R. (2018). Drivers of Antarctic sea ice volume change in CMIP5 models. Journal of Geophysical Research: Oceans, 123, 7914–7938. https://doi.org/10.1029/2018JC014177

Paul, S., Hendricks, S., Ricker, R., Kern, S., and Rinne, E., 2018: Empirical parametrization of Envisat freeboard retrieval of Arctic and Antarctic sea ice based on CryoSat-2: progress in the ESA Climate Change Initiative, The Cryosphere, 12, 2437–2460, https://doi.org/10.5194/tc-12-2437-2018

Kern, S.; Ozsoy-Çiçek, B.; Worby, A.P. , 2016: Antarctic Sea-Ice Thickness Retrieval from ICESat: Inter-Comparison of Different Approaches. Remote Sens., 8, 538. https://doi.org/10.3390/rs8070538

Nicolas, J.P., Bromwich, D.H. (2014). New reconstruction of Antarctic near-surface temperatures: Multidecadal trends and reliability of global reanalyse. Journal of Climate, 27, 8070–8093. doi:10.1175/JCLI-D-13-00733.1.

Schweiger, A., R. Lindsay, J. Zhang, M. Steele, H. Stern, 2011: Uncertainty in modeled arctic sea ice volume, J. Geophys. Res., doi:10.1029/2011JC007084

Bintanja, R., van Oldenborgh, G. J., Drijfhout, S. S., Wouters, B. & Katsman, 2013: C. A. Important role for ocean warming and increased ice-shelf melt in Antarctic sea-ice expansion. Nat. Geosci. 6, 376–379 (2013). https://doi.org/10.1038/ngeo1767

Zhang, J. and Rothrock, D.A. , 2003: Modeling global sea ice with a thickness and enthalpy distribution model in generalized curvilinear coordinates. Monthly Weather Review, 131, 845–861. https://doi.org/10.1175/1520-0493(2003)