How is sea ice in the Arctic and Antarctic changing?

Sea ice is the frozen portion of the polar oceans11 Sea ice is distinct from icebergs, which ultimately come from land-based ice sheets., covering roughly 3 to 6% of the Earth's surface area at any given time (Comiso, 2010). The Earth hosts major sea ice systems at both poles: one in the Arctic Ocean and another in the Southern Ocean around Antarctica, which is referred to here simply as Antarctic sea ice. Ice forms in the local autumn/winter and melts in local spring/summer, driven by exchanges of heat between the sea ice, ocean, and atmosphere. Sea ice is typically bright – snow-covered ice can reflect away more than 80% of incoming solar radiation. As sea ice melts, it exposes the darker (less reflective) water beneath it, and the planet's surface becomes darker and warmer overall. By this mechanism, changes in sea ice can amplify changes in the climate system (e.g. warming leads to melt, exposing darker surfaces, which leads to additional absorption of incoming sunlight and therefore more melt). In addition to its direct relevance for climate, sea ice has a large impact on ocean biology and serves as a home and/or hunting ground for some large animals, such as polar bears in the Arctic Ocean and emperor penguins around Antarctica. Since 1979, satellite surveys have provided reliable, continuous, year-round measurements of sea ice extent. This long, unbroken record shows that sea ice is constantly changing, with variations over seasons, years and decades. Arctic sea ice typically used to cover an area of about 7 million km2 (roughly equivalent to the size of Australia) in summer and an area of about 16 million km2 in late winter. Antarctic sea ice covers an area of roughly 3 million km2 in the summer and grows to 17–20 million km2 in the winter. Arctic sea ice typically reaches its smallest extent each September. Over the satellite record (1979–present), September Arctic sea ice extent decreased by about 13% per decade, relative to the 1981–2010 average (https://nsidc.org/arcticseaicenews/) and now typically covers 4–5 million km2. Most years this century have featured exceptionally low sea ice extents; the only years this century whose September sea ice extent has not been less than any recorded in the twentieth century satellite record are 2000, 2001 and 2003. Between 1980 and 2008, the average winter sea ice thickness within the Arctic basin decreased (likely between 1.3m and 2.3m; Vaughan et al., 2013), and since 1979, September Arctic sea ice volume has decreased by roughly 3% per decade (http://psc.apl.uw.edu/research/projects/arctic-sea-ice-volume-anomaly/). In contrast to the Arctic, summertime Antarctic sea ice extent does not feature a statistically significant trend over the satellite record, although there are strong regional variations (i.e. more sea ice in some areas and less in others). Research suggests that this lack of significant change in ice extent may have been due to changes in the winds. Stronger cold winds from Antarctica can (a) push sea ice further northwards, exposing more open water near the coasts, where new sea ice can form, and (b) encourage sea ice formation by lowering air temperatures. However, 2016 and 2017 saw some of the lowest annual mean Antarctic sea ice extents on record; in 2017, the annual mean Antarctic sea ice extent was the lowest it has been since satellite observations began in the 1970s. These low sea ice extents have been associated with variations in large-scale atmospheric circulation, but more research on this topic is needed before further conclusions can be drawn (Schlosser et al., 2018). Arctic and Antarctic sea ice are physically similar22 On average, Arctic sea ice is thicker and less snow-covered than Antarctic sea ice., but they exist in different land/ocean/atmosphere configurations. A large portion of Arctic sea ice cover sits in a nearly closed basin, with relatively restricted movement and only small openings to the other ocean basins. Antarctic sea ice, however, is influenced by distinct climate regimes in the Atlantic, Pacific and Indian oceans and by winds from the Antarctic continent. The Arctic and Antarctic are not, therefore, expected to respond to climate change in the same way. Sea ice is influenced by a combination of human-driven (e.g. greenhouse gases, ozone depletion) and natural factors (e.g. the configuration of the continents). Climate model studies project declining Arctic and Antarctic sea ice extents in the twenty-first century as global mean surface temperature rises, although there is more confidence in Arctic projections than in Antarctic projections (Collins et al., 2013). As described in a recent IPCC special report, there is high confidence that the probability of a sea-ice-free summertime Arctic Ocean (with less than 1 million km2) is substantially higher at 2 degC of warming compared with 1.5 degC (IPCC, 2018). Under 1.5 degC of warming, it is very likely that the Arctic summer will be sea-ice-free for one summer in 100 years, while under 2 degC of warming, it is very likely that the Arctic summer will be sea-ice-free for one summer in 10 years. We would like to thank Paul Holland, David Vaughan, John Turner and Andrew Shepherd for comments that greatly improved the quality of this work.