Arctic and Baltic sea ice

Arctic sea ice is declining rapidly. Since 1979, the Arctic has lost, on average, an area of 76,000 km2 of sea ice per year in summer and 32,000 km2 per year in winter. The Arctic summer sea ice area in 2020 was the second lowest ever. Arctic sea ice is also getting younger and thinner. A nearly ice-free Arctic sea in summer is projected to be a rare event for 1.5°C of global warming but the norm for 2.5°C of warming. The maximum sea ice extent in the Baltic Sea shows a decreasing trend since about 1800 and reached its lowest value ever in winter 2019/20. This decreasing trend is projected to continue.

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Over the period 1979-2021, the sea ice area in the Arctic decreased by 32,000 km2 per year in winter (measured in March) and by 76,000 km2 per year in summer (measured in September) (Figure 1, black line). The decrease in summer sea ice is more than 13% per decade. Summer sea ice cover in each of the last 15 years (2007-2021) was lower than in any previous year since satellite measurements began in 1979. Arctic sea ice is also getting much thinner and younger, as less sea ice survives the summer to grow into thicker, multi-year floes. Ice that is at least 4 years old was still widespread in the mid-1980s when it comprised about one third of total ice area in March, but the share of old ice has declined to about 5% since 2011 .

The decline in Arctic sea ice is unprecedented in the last 1,000 years, based on historical reconstructions and paleoclimate evidence. At least half of the observed loss in summer sea ice area can be attributed to anthropogenic greenhouse gas emissions . The rapid changes in Arctic sea ice can trigger complex feedback processes in the global climate system, which can also affect climate and weather extremes in Europe .

All climate models agree that Arctic sea ice will continue to shrink and thin (Figure 1, coloured lines). Projections from the most recent global modelling exercise (CMIP6) suggest that a nearly ice-free Arctic sea will likely be experienced at least once before 2060, even for low emissions scenarios. An ice-free Arctic in late summer is expected to be a rare event for 1.5°C of global warming (relative to pre-industrial levels), a frequent event for 2°C warming, and a permanent feature for warming above 2.5°C . Note that these details are not visible from Figure 1, because the multi-model mean shown there reduces the interannual variability compared to individual model runs. Longer-term simulations suggest that the Arctic could become nearly ice-free year-round during the 22nd century for high emissions scenarios .

An animation from NASA shows the decline in Arctic sea ice from 1984 to 2019.

The maximum winter sea ice extent in the Baltic Sea has mostly displayed a decreasing trend since about 1800. It reached a record low level in winter 2019/2020 (Figure 2). The decrease in sea ice extent appears to have accelerated since the 1980s. The frequency of mild ice winters (defined as having a maximum ice cover of less than 130,000 km2) has increased from seven in 30 years in the period 1950-1979 to 16 years in the period 1992-2021, whereas the frequency of severe ice winters (at least 270,000 km2 of ice) has decreased from 6 years to 1 year during the same periods.

Baltic sea ice extent and thickness are projected to continue to shrink significantly. The best estimate of the decrease in maximum ice extent over the 21st century is 640 km2/year for a medium emissions scenario (RCP4.5) and 1,090 km2/year for a high emissions scenario (RCP8.5). For the latter scenario, largely ice-free winters are projected by the end of the century.

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