In recent decades, hundreds of glaciers draining the Antarctic Peninsula have undergone systematic and progressive change. These changes are widely attributed to rapid increases in regional surface air temperature as well as changing ocean currents.
These glacial losses have offset minor gains in Antarctic ice accrued through additional snowfall in recent years. Both trends, though in opposite directions, are consistent with climate change projections.
The mass of the Antarctic ice-sheet decreased on average by about 100 gigatons a year from 2002 to the beginning of 2016, corresponding to 9 percent of the global mean sea-level rise.
Six large West-Antarctic glaciers have recently picked up speed. The retreat and ultimately final collapse of the entire West Antarctic ice sheet, contributing 23 feet of sea level rise, has probably become ‘unstoppable’. The Antarctic ice sheet as a whole stores the equivalent of approximately 190 feet (58 meters) of global mean sea level rise.
A continent-wide survey published in April 2017 found extensive drainages of meltwater flowing over parts of Antarctica's ice during the summer season. These features were thought to be confined mainly to Antarctica’s fastest-warming, mostly northerly regions. Future warming could significantly magnify their influence on sea level.
The major uncertainty regarding catastrophic sea level rise due to Antarctic ice loss is when (not if) sea levels will increase dramatically, and whether catastrophic sea level rise could arrive this century.
A related uncertainty is whether continued greenhouse gas emissions could advance the advent of catastrophic sea level rise into this century.
The large range of projected sea level rise is primarily due to a lack of knowledge about the rate of positive feedbacks.
- Over the long-term, paleoclimate records indicate that greenhouse gas emissions in the 20th and 21st centuries could cause 95 to 180 feet (29 to 55 meters) of sea level rise, depending on future emissions levels.
Antarctica is warming
Continent-wide, Antarctica has witnessed a positive warming trend over the last 50 years.Satellite and weather station data report that Antarctica has warmed at a rate of about 0.22°F (0.12°C) per decade since 1957 (see image to the right), for a total average temperature rise of 1°F (0.5°C).
However, not every region has responded in the same way. The Antarctica Peninsula has experienced the strongest warming, followed by West Antarctica, while East Antarctica and the continental interior have at times shown cooling trends. In detailing these trends, researchers note that ocean currents deliver heat to the Antarctic Peninsula and coastal regions. In the interior and eastern regions, on the other hand, reduced ozone coverage alters air currents, increases winds, and thereby diverts warm air.
When it comes to sea ice, the story is similarly complicated. In spite of warming temperatures, sea ice extent in some areas of Antarctica has increased. Research suggests this is due to reduced mixing between warm and cool layers in the ocean that ordinarily speeds the melting of ice. The previously mentioned wind patterns induced by ozone depletion may also play a role. However, neither sea ice loss nor sea ice formation make a significant change in sea levels, exactly as melting ice in a glass of water does not significantly raise the level of water in the glass.
Over land, there are also regions where ice mass is increasing. A 2012 study estimates the East Antarctic Ice Sheet gained 14 gigatons a year of mass between 1992 and 2011. As continent-wide surface temperature rises, the air over Antarctica warms, and warmer, saturated air can hold and release more moisture—in this case, increasing snowfall and overall mass in parts of Eastern Antarctica. Research has shown, however, that any gains in the East Antarctic Ice Sheet are being offset by the more significant losses from the West Antarctic Ice Sheet and the Antarctic Peninsula Ice Sheet. Over the same period (1992 to 2011) that the East Antarctic Ice Sheet gained mass, the whole of Antarctica lost an estimated 1,350 gigatons of ice.
Research shows that melting from below is causing most of the continent's ice shelves to grow thinner, some at a rate of up to seven meters per year. Land ice sheets are melting too, at an accelerating rate of over 246 billion tons per year. Unlike sea ice, land ice melt contributes to sea level rise. In summary, Antarctica is both warming in temperature and contributing significantly to sea level rise. While some localized areas may be cooling and/or gaining ice, these examples are not enough to reverse the trend.
Antarctic ice sheet melt is key to the rate of global sea level rise
Antarctica holds about 90 percent of the world's ice, which raises major concerns about the impacts on sea level rise of warming temperatures and melting in Antarctica. The Antarctic ice sheet stores the equivalent of approximately 58 meters of global mean sea level rise. Its potential sea-level contribution within the next 10,000 years far exceeds that of all other possible sources.
According the the Fifth Assessment Report of the Intergovernmental Panel on Climate Change:
[There is high confidence that] the Antarctic ice sheet has been losing ice during the last two decades. There is very high confidence that these losses are mainly from the northern Antarctic Peninsula and the Amundsen Sea sector of West Antarctica, and high confidence that they result from the acceleration of outlet glaciers. The average rate of ice loss from Antarctica likely increased from 30 gigatons per year (sea level equivalent, 0.08 mm per year) over the period 1992–2001, to 147 gigatons per year over the period 2002–2011 (0.40 mm per year).
The graph to the right shows the cumulative ice mass loss from the Antarctic ice sheet over the period 1992–2012 derived from recent studies conducted by 10 different research groups.
More recent analysis finds that from 2002 to the beginning of 2016, the mass of the Antarctic ice-sheet decreased on average by about 100 gigatons a year. This would translate into 0.27 millimeters over the ocean each year, corresponding to 9 percent of the global mean sea-level rise.
The reduced rate of 100 gigatons a year for the central estimate of annual melt (compared to the IPCC's 147 [72 to 221] gigatons per year) is likely due to improved modeling that better accounts for minor gains in Antarctic ice accrued through additional snowfall in recent years. This increase in snowfall, as discussed in the previous section, is also linked to climate change, because a warmer atmosphere holds more moisture when fully saturated, allowing for greater precipitation.
Antarctic ice loss mechanisms
The characteristic of melting in Antarctica is very different than that of Greenland (which mostly experiences surface melting). In Antarctica, ice shelves along the coast—which support adjacent glaciers on land—play a key role, and their disappearance leads to possible strong acceleration of ice sheet mass loss.
- Warm air melts ice sheets and ice shelves from above, thinning them, and meltwater runs off.
- Formation of melt water ponds. Compared to ice, meltwater ponds are dark, and thus absorb more light, converting it into heat energy, which leads to melting and run-off.
- Meltwater lubrication: Surface meltwater stored in ponds and crevasses can weaken and fracture ice shelves, triggering their rapid disintegration. Meltwater may be acting as a lubricant, increasing the glacier/ice sheet flow, and by doing so the transportation of ice towards the ocean. This is not fully established, even for Greenland, as a current mechanism, and is very theoretical for Antarctica. While two April 2017 studies find that the direct effects of surface meltwater, which generally refreezes in winter, are probably negligible for now, glaciologists are concerned this could change in the future.
- Hydrofracturing: Meltwater, warm temperatures and rainfall can fracture ice shelves making them vulnerable to collapse.
- Basal melting: Warm ocean currents melt away, from underneath, both ice shelves and sheets plunging down off the continent and over the surrounding sea.
- Changing ocean circulation brings warmer currents up under floating ice sheets/shelves, driving basal melting. These changing ocean currents may be driven by changing winds that in turn are driven by global warming.
- Collapse of floating ice shelves (due to basal melting, thinning due to warming air temperatures, and hydrofracturing) releases the formally buttressed ice sheets, triggering faster flow into the ocean.
- Marine Ice Cliff Instability (MICI) facilitates ice sheet disintegration. MICI is a weakening or structural failure of shear ice cliffs as warming temperatures increase crevasses and reduce the maximum supported cliff heights.
- Warming sea and air temperatures can melt sea ice that otherwise blocks the flow of ice sheets.
Changes in West Antarctica linked to climate change
Shifting ocean and air processes linked to global warming are changing the landscape of West Antarctica. The biggest changes are occurring where warm ocean water is able to access thick, floating ice shelves from below in the Amundsen and Bellingshausen Seas, a process known as basal melting. Ice-shelf thinning by basal melt implies an increase in the amount of heat supplied by the ocean. The mechanisms behind ocean heat flux in the region are an active area of research (see below).
Ice shelves are collapsing
Antarctica's ice shelves—pictured to the right—form through the extension of glaciers into the ocean and float in water where they provide structural support to the glaciers that rest on land. Warming temperatures are causing Antarctica's ice shelves to retreat.
When ice shelves collapse, they contribute very little to global sea level rise because they already rest on and displace water. Of greater concern is how the collapse of ice shelves can affect the glaciers behind them, because the melting of those glaciers can cause much higher levels of ocean rise.
In the Antarctic Peninsula's Weddell Sea, two ice shelves have already disintegrated—Larsen A in 1995 and Larsen B in 2002. A September 2014 study finds that the collapse of Larsen B was the result of warmer air temperatures.
Larsen C—Antarctica's fourth-largest ice shelf—has a large crack due to a combination of surface melting and warm water melting at its base (known as "basal melting"). The crack is expected to reach across the entire ice shelf in a matter of months. When this happens, the resulting iceberg will likely be one of the largest ever recorded, and what remains of Larsen C will likely constitute the ice sheet's smallest size on record.
On the opposing side of the Antarctic Peninsula, in the Bellinghausen Sea, there has also been a partial collapse of a large ice shelf in recent years: the Wilkins shelf in 2008—a collapse that also continued in the Antarctic winter.
A December 2014 study finds shelf basal melt in the Bellinghausen and adjacent Amudsen Seas is being driven by ocean water that has become steadily warmer and saltier over a timeframe of four decades.The ice shelves in the Amundsen Sea are especially important, because they are connected to two large glaciers—the Pine Island and Twaites Glaciers—that have dramatically increased in flow velocity.
Glaciers are picking up speed
In recent decades, hundreds of glaciers draining the Antarctic Peninsula have undergone systematic and progressive change. These changes are widely attributed to rapid increases in regional surface air temperature. A July 2016 study finds that ocean temperatures also have a strong impact on glacial melt in the region.
In West Antarctica, a May 2014 study finds that the Pine Island and the Twaites Glacier, together with four other large West-Antarctic glaciers has picked up speed. The study concludes that the retreat and thereby final collapse of the entire West Antarctic ice sheet had become ‘unstoppable’. The findings were supported by another study published that year. The Twaites Glacier partially holds all the other glaciers in place and is essential to the entire West Antarctic Ice Sheet.
Winds may explain warming in the Southern Ocean
Increases in the winds that circle the South Pole (circumpolar winds) are one of the primary mechanisms thought to be causing strong warming in the Southern Ocean. Since the 1950s, increasing circumpolar winds have warmed the Southern Ocean by causing a poleward shift in the ocean current encircling the Antarctic continent (the Antarctic Circumpolar Current, or ACC) and by increasing the rate that circular currents of water in the region (known as eddies) transfer heat.
Changes in Antarctica's circumpolar winds are also contributing to another mechanism implicated in the warming of the Southern Ocean: fluctuating incursions of Circumpolar Deep Water (CDW). Circumpolar Deep Water is relatively warm, saline, and dense, increasing the melt rate in places where it approaches the Antarctic coast.
These changes in Antarctica's circumpolar winds have been attributed to a combination of changing tropical Pacific sea surface temperatures, and stratospheric ozone loss and increased greenhouse gases.
The flux of heat from the ocean into the Amundsen and Bellingshausen Seas region of West Antarctica helps to explain the rapid acceleration of glacier flows as well as the rapid mass loss from the West Antarctic Ice Sheet (WAIS). The WAIS has long been marine-based and susceptible to unstable retreat, whereas the East Antarctic Ice Sheet (EAIS) is more stable because it rests on bedrock and is isolated from warm ocean waters.
Ice sheet dynamics are not linear: large scale ice sheet collapse and rapid sea level rise
Large-scale melting of both the Antarctic and Greenland Ice Sheets include large-scale losses of ice, potentially leading to tens of feet of sea level rise. Although most of these losses are projected as being unlikely to occur before 2100, we may pass the point where these losses will be set in motion in the coming decades, with at least a slight chance that we have already done so.
A sea level rise speed of multiple meters (many feet) per century is possible if positive melting feedbacks are activated. Many positive feedbacks have already been suggested and are all subject of ongoing investigation, through modeling, paleo research and in situ (on site) observations.
In Antarctica, the instability of ice shelves, glaciers, and ice sheets threatens abrupt and large losses from both the West Antarctic Ice Sheet (WAIS) and portions of the East Antarctic Ice Sheet. Any significant ice loss likely would be irreversible for thousands of years. Simulations of warming and ice loss during earlier warm periods of the past 5 million years indicate these areas can contribute 23 feet of sea level rise.
A March 2016 study by Hansen et al shows why the Greenland and Antarctic ice sheets might be far less stable than previously assumed, raising the possibility of 2 to 5 meters (6.6 to 16.4 feet) sea level rise, within this century.
A second study published in March 2016 by DeConto and Pollard identifies a mechanism for accelerated melting of the Antarctic ice sheet, which could raise global sea levels to 2 meters by 2100.
Over the long-term, paleoclimate records indicate that greenhouse gas emissions in the 20th and 21st centuries could cause 95 to 180 feet (29 to 55 meters) of sea level rise, depending on future emissions levels.
Many current projections of global sea level rise do not account for the complicated behavior of Antarctica's giant ice slabs as they interact with the atmosphere, the ocean and the land. Lack of knowledge about the ice sheets and their behavior is the primary reason that projections of global sea level rise include such a wide range of plausible future conditions.
The above image shows that sea level changes in response to climatic changes are intrinsically slow – following the large thermal mass of oceans and ice sheets. But if you look at the millennium-scale you see that sea level changes to several degrees of warming are also very large – and what happened between the cold and mild state of Earth (Pleistocene-Holocene boundary) could repeat itself between the mild and hot state of Earth (Holocene-Anthropocene boundary – following human carbon emissions). The big uncertainty is the speed of change.