Antarctic Ice Sheet

 

Antarctica is the coldest and driest continent on Earth, as well as the one with the highest average elevation. Because Antarctica is so dry, there is little water vapor, so its air doesn't conduct heat well. Further, it is surrounded by the Southern Ocean, which is far more effective at absorbing heat than any other ocean. It also has extensive year-around sea ice, which has a high albedo (reflectivity) and adds to the albedo of the ice's sheet own bright, white surface. Antarctica is so cold that it is the only place on Earth where atmospheric temperature inversion occurs every winter. Elsewhere, the atmosphere on Earth is at its warmest near the surface and it becomes cooler as elevation increases. During the Antarctic winter, the surface of central Antarctica instead becomes cooler than middle layers of the atmosphere. This means that greenhouse gases trap heat in the middle atmosphere and reduce its flow towards the surface and towards space, instead of simply preventing the flow of heat from the lower atmosphere to the upper layers. This effect lasts until the end of the Antarctic winter. Thus, even the early climate models predicted that temperature trends over Antarctica would emerge slower and be more subtle than they are elsewhere.

Moreover, there were fewer than twenty permanent weather stations across the continent, with only two in the continent's interior, while automatic weather stations were deployed relatively late, and their observational record was brief for much of the 20th century. Likewise, satellite temperature measurements did not begin until 1981 and are typically limited to cloud-free conditions. Thus datasets representing the entire continent only began to appear by the very end of the 20th century. The only exception was the Antarctic Peninsula, where warming was both well-documented and strongly pronounced: It was eventually found to have warmed by 3 °C (5.4 °F) since the mid-20th century. Based on this limited data, several papers published in the early 2000s suggested that there had been an overall cooling over continental Antarctica (that is outside of the Peninsula).

 

A 2002 analysis led by Peter Doran received widespread media coverage after it also indicated stronger cooling than warming between 1966 and 2000, and found that McMurdo Dry Valleys in East Antarctica had experienced cooling of 0.7 °C per decade - a local trend confirmed by subsequent research at McMurdo. Multiple journalists suggested that these findings were "contradictory" to global warming, even though the paper itself noted the limited data, and still found warming over 42% of the continent. What became known as the "Antarctic Cooling Controversy" received further attention in 2004, when Michael Crichton wrote a novel State of Fear which alleged a conspiracy amongst climate scientists to make up global warming, and claimed that Doran's study definitively proved there was no warming in Antarctica outside of the Peninsula. Relatively few scientists responded to the book at the time, but it was subsequently brought up in a 2006 US Senate hearing in support of climate change denial, and Peter Doran felt compelled to publish a statement in The New York Times decrying the misinterpretation of his work. The British Antarctic Survey and NASA also issued statements affirming the strength of climate science after the hearing.

By 2009, research was finally able to combine historical weather station data with satellite measurements to create consistent temperature records going back to 1957, which demonstrated warming of >0.05 °C/decade since 1957 across the continent, with cooling in East Antractica offset by the average temperature increase of at least 0.176 ± 0.06 °C per decade in West Antarctica. Subsequent research confirmed clear warming over West Antarctica in the 20th century with the only uncertainty being the magnitude. Over 2012-2013, estimates based on WAIS Divide ice cores and the revised Byrd Station temperature record even suggested a much larger West Antarctica warming of 2.4 °C (4.3 °F) since 1958, or around 0.46 °C (0.83 °F) per decade, although there has been some uncertainty about it. In 2022, a study narrowed the warming of the Central area of the West Antarctic Ice Sheet between 1959 and 2000 to 0.31 °C (0.56 °F) per decade, and conclusively attributed it to increases in greenhouse gas concentrations caused by human activity.

 

Local changes in atmospheric circulation patterns like the Interdecadal Pacific Oscillation or the Southern Annular Mode, slowed or even partially reversed the warming of West Antarctica between 2000 and 2020, with the Antarctic Peninsula experiencing cooling from 2002. While a variability in those patterns is natural, ozone depletion had also led the Southern Annular Mode (SAM) to be stronger than it had been in the past 600 years of observations. Studies predicted a reversal in the SAM once the ozone layer began to recover following the Montreal Protocol starting from 2002, and these changes were consistent with their predictions. As these patterns reversed, the East Antarctica interior demonstrated clear warming over those two decades. In particular, the South Pole warmed by 0.61 ± 0.34 °C per decade between 1990 and 2020, which is three times the global average. The Antarctica-wide warming trend also continued after 2000, and in February 2020, the continent recorded its highest temperature of 18.3 °C, which was a degree higher than the previous record of 17.5 °C in March 2015.

Models predict that under the most intense climate change scenario, known as RCP8.5, Antarctic temperatures will be up 4 °C (7.2 °F), on average, by 2100 and this will be accompanied by a 30% increase in precipitation and a 30% decrease in total sea ice. RCPs were developed in the late 2000s, and early 2020s research considers RCP8.5 much less likely than the more "moderate" scenarios like RCP 4.5, which lies in between the worst-case and the Paris Agreement goals.

Contrasting temperature trends across parts of Antarctica, as well as its remoteness, mean that some locations lose mass, particularly at the coasts, while others that are more inland continue to gain it, and estimating an average trend can be difficult. In 2018, a systematic review of all previous studies and data by the Ice Sheet Mass Balance Inter-comparison Exercise (IMBIE) estimated an increase in West Antarctic ice sheet annual mass loss from 53 ± 29 Gt (gigatonnes) in 1992 to 159 ± 26 Gt in the final five years of the study. On the Antarctic Peninsula, the study estimated −20 ± 15 Gt per year with an increase in loss of roughly 15 Gt per year after year 2000, with a significant role played by the loss of ice shelves. The review's overall estimate was that Antarctica lost 2720 ± 1390 gigatons of ice from 1992 to 2017, averaging 109 ± 56 Gt per year. This would amount to 7.6 millimeters of sea level rise. Then, though, a 2021 analysis of data from four different research satellite systems (Envisat, European Remote-Sensing Satellite, GRACE and GRACE-FO and ICESat) indicated annual mass loss of only about 12 Gt from 2012-2016, due to much greater ice gain in East Antarctica than estimated earlier, which had offset most of the losses from West Antarctica. The East Antarctic ice sheet can still gain mass in spite of warming because effects of climate change on the water cycle increase precipitation over its surface, which then freezes and helps to build up more ice.^: 1262

 

By 2100, net ice loss from Antarctica alone is expected to add about 11 cm (5 in) to global sea level rise.^: 1270 However, processes such as marine ice sheet instability, which describes the potential for warm water currents to enter between the seafloor and the base of the ice sheet once it is no longer heavy enough to displace such flows, and marine ice cliff instability, when ice cliffs with heights greater than 100 m (330 ft) may collapse under their own weight once they are no longer buttressed by ice shelves (which has never been observed, and only occurs in some of the modelling) may cause West Antarctica have a much larger contribution. Such processes may increase sea level rise caused by Antarctica to 41 cm (16 in) by 2100 under the low-emission scenario and 57 cm (22 in) under the high-emission scenario.^: 1270 Some scientists have even larger estimates, but all agree it would have a greater impact and become much more likely to occur under higher warming scenarios, where it may double the overall 21st century sea level rise to 2 meters or more. One study suggested that if the Paris Agreement is followed and global warming is limited to 2 °C (3.6 °F), the loss of ice in Antarctica will continue at the 2020 rate for the rest of the century, but if a trajectory leading to 3 °C (5.4 °F) is followed, Antarctica ice loss will accelerate after 2060 and start adding 0.5 cm to global sea levels per year by 2100.

 

Sea level rise will continue well after 2100, but potentially at very different rates. According to the most recent reports of the Intergovernmental Panel on Climate Change (SROCC and the IPCC Sixth Assessment Report), there will be a median rise of 16 cm (6.3 in) and maximum rise of 37 cm (15 in) under the low-emission scenario. On the other hand, the highest emission scenario results in a median rise of 1.46 m (5 ft) metres, with a minimum of 60 cm (2 ft) and a maximum of 2.89 m (9+1⁄2 ft)).

Over even longer timescales, West Antarctic ice sheet, which is much smaller than the East Antarctic ice sheet is and grounded deep below the sea level, is considered highly vulnerable. The melting of all the ice in West Antarctica would increase the total sea level rise to 4.3 m (14 ft 1 in). However, mountain ice caps not in contact with water are less vulnerable than the majority of the ice sheet, which is located below the sea level. Its collapse would cause ~3.3 m (10 ft 10 in) of sea level rise. This kind of collapse is now considered practically inevitable, because it appears to have already occurred during the Eemian period 125,000 years ago, when temperatures were similar to the early 21st century. The Amundsen Sea also appears to be warming at rates which would make the ice sheet's collapse effectively inevitable.

The only way to reverse ice loss from West Antarctica once triggered is by lowering the global temperature to 1 °C (1.8 °F) below the preindustrial level. This would be 2 °C (3.6 °F) below the temperature of 2020. Other researchers suggested that a climate engineering intervention to stabilize the ice sheet's glaciers may delay its loss by centuries and give more time to adapt. However this is an uncertain proposal, and would end up as one of the most expensive projects ever attempted. Otherwise, the disappearance of the West Antarctic ice sheet would take an estimated 2000 years. The absolute minimum for the loss of West Antarctica ice is 500 years, and the potential maximum is 13,000 years. Once the ice sheet is lost, there would also be an additional 1 m (3 ft 3 in) of sea level rise over the next 1000 years, caused by isostatic rebound of land beneath the ice sheet.

 

If global warming were to reach higher levels, then the EAIS would play an increasingly larger role in sea level rise occurring after 2100. According to the most recent reports of the Intergovernmental Panel on Climate Change (SROCC and the IPCC Sixth Assessment Report), the most intense climate change scenario, where the anthropogenic emissions increase continuously, RCP8.5, would result in Antarctica alone losing a median of 1.46 m (4 ft 9 in) (confidence interval between 60 cm (2.0 ft) and 2.89 m (9 ft 6 in)) by 2300, which would involve some loss from the EAIS in addition to the erosion of the WAIS. This Antarctica-only sea level rise would be in addition to ice losses from the Greenland ice sheet and mountain glaciers, as well as the thermal expansion of ocean water. If the warming were to remain at elevated levels for a long time, then the East Antarctic Ice Sheet would eventually become the dominant contributor to sea level rise, simply because it contains the largest amount of ice.

Sustained ice loss from the EAIS would begin with the significant erosion of the so-called subglacial basins, such as Totten Glacier and Wilkes Basin, which are located in vulnerable locations below the sea level. Evidence from the Pleistocene shows that Wilkes Basin had likely lost enough ice to add 0.5 m (1 ft 8 in) to sea levels between 115,000 and 129,000 years ago, during the Eemian, and about 0.9 m (2 ft 11 in) between 318,000 and 339,000 years ago, during the Marine Isotope Stage 9. Neither Wilkes nor the other subglacial basins were lost entirely, but estimates suggest that they would be committed to disappearance once the global warming reaches 3 °C (5.4 °F) - the plausible temperature range is between 2 °C (3.6 °F) and 6 °C (11 °F). Then, the subglacial basins would gradually collapse over a period of around 2,000 years, although it may be as fast as 500 years or as slow as 10,000 years. Their loss would ultimately add between 1.4 m (4 ft 7 in) and 6.4 m (21 ft 0 in) to sea levels, depending on the ice sheet model used. Isostatic rebound of the newly ice-free land would also add 8 cm (3.1 in) and 57 cm (1 ft 10 in), respectively.

The entire East Antarctic Ice Sheet holds enough ice to raise global sea levels by 53.3 m (175 ft). Its complete melting is also possible, but it would require very high warming and a lot of time: In 2022, an extensive assessment of tipping points in the climate system published in Science Magazine concluded that the ice sheet would take a minimum of 10,000 years to fully melt. It would most likely be committed to complete disappearance only once the global warming reaches about 7.5 °C (13.5 °F), with the minimum and the maximum range between 5 °C (9.0 °F) and 10 °C (18 °F). Another estimate suggested that at least 6 °C (11 °F) would be needed to melt two thirds of its volume.

If the entire ice sheet were to disappear, then the change in ice-albedo feedback would increase the global temperature by 0.6 °C (1.1 °F), while the regional temperatures would increase by around 2 °C (3.6 °F). The loss of the subglacial basins alone would only add about 0.05 °C (0.090 °F) to global temperatures due to their relatively limited area, and a correspondingly low impact on global albedo.