Cryosphere
- The ice sheets and ice shelves of Antarctica and Greenland are losing ice due to a warmer climate, which is contributing to global sea level rise.
- There has been an abrupt decrease in Antarctic sea-ice extent since 2015, after a small increase over the period from 1979 to 2014.
The cryosphere is the part of Earth’s surface characterised by frozen water. The cryosphere includes the ice sheets (glacial ice that has accumulated from precipitation over land) and ice shelves (floating sheets of ice formed from glacial ice sheets). Together the ice sheets of Antarctica and Greenland contain about 99% of the Earth’s fresh water, which is the equivalent of over 60 metres of sea level rise.
Ice shelves around Antarctica help stabilise the ice sheet there by restricting the flow of glacial ice from the continent to the ocean. Warm ocean water penetrating below the ice shelves of the West Antarctic ice sheet, along with increased iceberg calving, is now destabilising several glaciers, increasing the Antarctic contribution to sea level rise. Surface melt, particularly over Greenland and the Antarctic Peninsula, is also contributing to sea level rise. Partially offsetting this is increased precipitation (snowfall) over Antarctica, due to increased evaporation of moisture from nearby oceans as a result of reduced sea-ice extent. Over the last few decades, the Amundsen Sea sector contributed most to the net mass loss of the Antarctic ice sheet. Between January and November 2023, the net mass loss over the Antarctic ice sheet is estimated to be about 170 Gt. This is in contrast to the net mass gain observed in 2022 which was the highest on record (since 1980), driven by enhanced snowfall.
Unlike the continental ice sheets, changes in sea ice shelves have a negligible direct impact on sea level, though sea ice influences the rate of regional climate warming and ocean/atmosphere moisture fluxes. Since the commencement of satellite monitoring of sea ice in the late 1970s, Arctic sea-ice cover has consistently decreased, whereas the Antarctic has shown a more complex pattern of changes. Overall, Antarctic sea-ice extent increased slightly from 1979 to 2014, but with substantial regional and seasonal variations. The largest daily recorded wintertime sea ice extent since satellite monitoring began, of approximately 20.2 million km2, was in September 2014. Since 2014, there has been a marked, abrupt and relatively persistent decrease in net sea ice extent, which in early 2022 dropped below 2.0 million km2 for the first time since satellite observations began. Extraordinarily low net Antarctic sea-ice extents occurred throughout 2023, with new record low observations in 7 months. Unusually, negative anomalies in sea-ice extent almost surrounded the continent, with only the Bellingshausen and Amundsen Seas showing positive ice extent anomalies. Regional negative anomalies were coincident with above average upper ocean and surface temperatures.
Regionally the trend in sea-ice cover has been variable. Statistically significant trends over the 1979−2023 period show reduced sea-ice duration, by as much as 4 days per year, to the west of the Antarctic Peninsula, offshore of West Antarctica and within the Bellingshausen Sea. These are in contrast to increased sea-ice duration within the western Ross Sea and the southern region of the Weddell Sea, although these increases are smaller, only locally reaching 1 to 2 days per year.
The overall increase in Antarctic sea-ice extent from 1979 to 2014 has largely been attributed to changes in westerly wind strength, whereas the marked decrease since 2015 has been attributed to a combination of atmospheric and oceanic anomalies. The primary influence on low sea-ice growth in recent years, particularly in 2023 and 2024, has been abnormally warm subsurface temperatures in the Southern Ocean, with additional impacts from anomalies caused by large-scale weather patterns across the region.
Reduced Antarctic sea-ice coverage and growth can have significant impact on the global climate, including changes in the ocean circulation. Increased glacial melt has been shown to slow the sinking of dense cold water around the Antarctic margin, causing warming and deoxygenation of the deep ocean. Slowing of the ocean overturning circulation (the network of ocean currents that circles the globe and connects the upper and lower layers of the ocean) would impact climate by reducing how much heat and carbon the ocean can absorb from the atmosphere and transfer to the deep ocean. Slowing of the overturning would also reduce oxygen levels in the deep ocean and the cycling of nutrients and carbon between the upper and lower layers of the ocean.
Further, the reduced presence of sea ice around the continental edge removes the barrier between ocean swell and waves and the ice shelves, potentially destabilising ice shelves and inducing sudden collapse. Since 2022 there has been anomalously high Antarctic coastal exposure (regions of coastline not protected by a sea-ice buffer), with 154 days of record high coastal exposure during 2023.
