Climate change in Antarctica

Climate change caused by greenhouse gas emissions from human activities occurs everywhere on Earth, and while Antarctica is less vulnerable to it than any other continent, climate change in Antarctica has already been observed. There has been an average temperature increase of >0.05 °C/decade since 1957 across the continent, although it had been uneven. While West Antarctica warmed by over 0.1 °C/decade from the 1950s to the 2000s and the exposed Antarctic Peninsula has warmed by 3 C-change since the mid-20th century, the colder and more stable East Antarctica had been experiencing cooling until the 2000s. Around Antarctica, the Southern Ocean has absorbed more heat than any other ocean, with particularly strong warming at depths below 2000 m and around the West Antarctic, which has warmed by 1 C-change since 1955.

The warming of Antarctica's territorial waters has caused the weakening or outright collapse of ice shelves, which float just offshore of glaciers and stabilize them. Many coastal glaciers have been losing mass and retreating, which causes net annual ice loss across Antarctica, even as the East Antarctic ice sheet continues to gain ice inland. By 2100, net ice loss from Antarctica alone is expected to add about 11 cm to global sea level rise. However, marine ice sheet instability may cause West Antarctica to contribute tens of centimeters more if it is triggered before 2100. With higher warming instability would be much more likely, and could double overall 21st century sea level rise.

The fresh meltwater from the ice, 1100-1500 billion tons (GT) per year, dilutes the saline Antarctic bottom water, thus weakening the lower cell of the Southern Ocean overturning circulation. Some research tentatively suggests a full collapse of the circulation may occur between 1.7 C-change and 3 C-change of global warming, although the full effects are expected to unfold over multiple centuries. They include less precipitation in the Southern Hemisphere but more in the Northern Hemisphere, and an eventual decline of fisheries in the Southern Ocean with a potential collapse of certain marine ecosystems. Furthermore, while many Antarctic species remain undiscovered, there are already documented increases in flora and large fauna such as penguins are already seen struggling to retain suitable habitat. On ice-free land, permafrost thaws, releasing not only greenhouse gases, but also formerly frozen pollution.

The West Antarctic ice sheet is likely to melt completely,  unless temperatures are reduced by 2 C-change below the levels of the year 2020. The loss of this ice sheet would take between 2,000 and 13,000 years, although several centuries of high emissions could shorten this timeframe to 500 years. A sea-level rise of 3.3 m would occur if the ice sheet collapses leaving ice caps on the mountains and 4.3 m if those ice caps also melt. Isostatic rebound may contribute an additional 1 m to global sea levels over another 1,000 years. In contrast, the East Antarctic ice sheet is far more stable and may only cause a sea-level rise of 0.5 m - 0.9 m from the current level of warming, a small fraction of the 53.3 m contained in the full ice sheet. With a global warming of around 3 C-change, vulnerable areas like the Wilkes Basin and Aurora Basin may collapse over a period of around 2,000 years,  potentially adding up to 6.4 m to sea levels. The complete melting and disappearance of the East Antarctic ice sheet would require at least 10,000 years, and it would only occur if global warming reaches 5 C-change to 10 C-change.

Temperature and weather changes


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-change 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-change since 1958, or around 0.46 C-change 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-change 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-change, 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.

Effects on ocean currents


Between 1971 and 2018, over 90% of thermal energy from global heating entered the oceans. Southern Ocean absorbs the most heat by far - after 2005, it accounted for between 67% and 98% of all heat entering the oceans. The temperature in the upper layer of the ocean in West Antarctica has warmed 1 C-change since 1955, and the Antarctic Circumpolar Current (ACC) is also warming faster than the average. It is also a highly important carbon sink. These properties are connected to Southern Ocean overturning circulation, one half of the global thermohaline circulation. It is so important that estimates on when global warming will reach 2 C-change (inevitable in all scenarios where greenhouse gas emissions have not been strongly lowered) depend on the strength of the circulation more than any factor other than the overall emissions.

The overturning circulation itself consists of two parts - the smaller upper cell, which is most strongly affected by winds and precipitation, and the larger lower cell, which is defined by the temperature and salinity of Antarctic bottom water. Since the 1970s, the upper cell has strengthened by 50-60%, while the lower cell has weakened by 10-20%. Some of this was due to the natural cycle of Interdecadal Pacific Oscillation, but there is also a clear impact of climate change, as it alters winds and precipitation through shifts in the Southern Annular Mode pattern, while the salty Antarctic bottom water is diluted by fresh meltwater from the erosion of the West Antarctic ice sheet,  which flows at a rate of 1100-1500 billion tons (GT) per year. During the 2010s, a temporary reduction in ice shelf melting in West Antarctica had allowed for the partial recovery of Antarctic bottom water and the lower cell of the circulation. Yet, greater melting and more decline of the circulation is expected in the future.

As bottom water weakens while the flow of warmer, fresher waters strengthens near the surface, the surface waters become more buoyant and less likely to sink and mix with the lower layers. Consequently, ocean stratification increases. One study suggests that the circulation would lose half its strength by 2050 under the worst climate change scenario, with greater losses occurring afterwards. Paleoclimate evidence shows that the entire circulation has weakened a lot or completely collapsed in the past: some preliminary research suggests that such a collapse may become likely once global warming reaches levels between 1.7 C-change and 3 C-change, but this estimate is much less certain than for the majority of tipping points in the climate system. Such a collapse would also be prolonged: one estimate indicates it would occur some time before 2300. As with the better-studied AMOC, a major slowdown or collapse of the Southern ocean circulation would have substantial regional and global impacts. Some likely impacts include a decline in precipitation in the Southern Hemisphere countries like Australia (with a corresponding increase in the Northern Hemisphere), and an eventual decline of fisheries in the Southern Ocean, which could lead to a potential collapse of certain marine ecosystems. These impacts are expected to unfold over multiple centuries, but there has been limited research to date and few specifics are currently known.

Observed changes in ice mass
Contrasting temperature trends across parts of Antarctica mean that some locations lose mass, particularly at the coasts, while others that are more inland continue to gain mass. Estimating an average trend can be difficult due to these contrasting trends and the remoteness of the region. 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 a loss of 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.

Black carbon pollution
Black carbon from incomplete fuel combustion is carried long distances by wind. If it reaches Antarctica, it accumulates on snow and ice, which reduces ice reflectivity and so causes it to absorb more energy. This accelerates melting and can create an ice-albedo feedback loop where meltwater itself absorbs more sunlight. Due to its remoteness, Antarctica has the cleanest snow in the world, and some research suggests that the impact of black carbon across West and East Antarctica is "minimal", with an albedo reduction of only ~0.5% in one 47-year core.

On the other hand, human activity is higher on the Antarctic Peninsula, and so the highest black carbon concentrations are found there. Near common tourist sites or research stations, this black carbon increases summer seasonal melting by around 5 to 23 extra kilograms of snow per m2.

21st century ice loss and sea level rise
By 2100, net ice loss from Antarctica alone is expected to add about 11 cm to global sea level rise. Other processes may cause West Antarctica to contribute more to sea level rise. One such process is 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 the sheet is no longer heavy enough to displace such flows. Another potential process is marine ice cliff instability, when ice cliffs with heights greater than 100 m may collapse under their own weight once they are no longer buttressed by ice shelves. This process has never been observed and it only occurs in some models. Such processes may increase sea level rise caused by Antarctica to 41 cm by 2100 under the low-emission scenario and 57 cm under the high-emission scenario.

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 m or more. One study suggested that if the Paris Agreement is followed and global warming is limited to 2 C-change, 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-change is followed, Antarctica ice loss will accelerate after 2060 and start adding 0.5 cm to global sea levels per year by 2100.

Long-term sea level rise
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 and maximum rise of 37 cm under the low-emission scenario. On the other hand, the highest emission scenario results in a median rise of 1.46 m metres, with a minimum of 60 cm and a maximum of 2.89 m).

Over even longer timescales, the 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. 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. The collapse of the West Antarctic ice sheet would cause ~3.3 m 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-change below the preindustrial level. This would be 2 C-change 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. 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, the isostatic rebound of the land previously covered by the ice sheet would result in an additional 1 m of sea level rise over the following 1000 years. The East Antarctic Ice Sheet as a whole is far more stable than the West Antarctic sheet. It would take global warming in a range between 5 C-change and 10 C-change, and a minimum of 10,000 years for the entire ice sheet to be lost. Some of its parts, such as Totten Glacier and Wilkes Basin, are located in vulnerable locations below sea level, known as subglacial basins. Estimates suggest that irreversible loss of those basins would begin once global warming reaches 3 C-change, although this loss may become irreversible at warming as low as 2 C-change or as high as 6 C-change. After global warming reaches the critical threshold for the collapse of these subglacial basins, their loss will likely unfold over a period of around 2,000 years (although the loss may be as fast as 500 years or as slow as 10,000 years).

The loss of all this ice would ultimately add between 1.4 m and 6.4 m to sea levels, depending on the ice sheet model used. Isostatic rebound of the newly ice-free land would also add 8 cm and 57 cm, respectively. Evidence from the Pleistocene shows that partial loss can also occur at lower warming levels: Wilkes Basin is estimated to have lost enough ice to add 0.5 m to sea levels between 115,000 and 129,000 years ago, during the Eemian, and about 0.9 m between 318,000 and 339,000 years ago, during the Marine Isotope Stage 9.

Permafrost thaw
Antarctica has much less permafrost than the Arctic, but what permafrost is there is also subject to thaw. Similar to how soils have a variety of chemical contaminants and nutrients in them, the permafrost in Antarctica traps various compounds. These include persistent organic pollutants (POPs) like polycyclic aromatic hydrocarbons, many of which are known carcinogens or can cause liver damage, and polychlorinated biphenyls such as HCB or DDT, which are associated with decreased reproductive success and immunohematological disorders. There are also heavy metals like mercury, lead and cadmium, which can cause endocrine disruption, DNA damage, immunotoxicity and reproductive toxicity. When contaminated permafrost thaws, these compounds are released. This can change the chemistry of surface waters. Bioaccumulation and biomagnification spread these compounds throughout the food web. While permafrost thaw also results in greenhouse gas emissions, the limited volume of Antarctic permafrost relative to Arctic permafrost means that Antarctic permafrost is not considered a significant factor affecting climate change.

Marine ecosystems
Nearly all of the Antarctic species are marine. Nearly 8,354 species had been discovered and taxonomically accepted in Antarctica by 2015, and only 57 were not marine. There could be as many as 17,000 species in total, as while 90% of the Antarctic region is greater than 1000 m deep, only 30% of the benthic sample locations were taken at that depth. On the Antarctic continental shelves, Benthic zone biomass may increase due to the warming, although it is likely to benefit seaweed the most: around 12% of the native benthic species may end up outcompeted and lost. These estimates are preliminary, as most Antarctic species lacked detailed assessments of their vulnerability.

Unlike the Arctic, there has been very limited change in marine primary production across the Southern Ocean in the available observations. Estimates suggest that an increase in Southern Ocean primary production could occur after 2100, but it would block many nutrients from travelling to other oceans and so decrease production elsewhere. Some microbial communities appear to have been negatively affected by ocean acidification, and there is a risk that future acidification would threaten the eggs of pteropods, a type of zooplankton.

Antarctic krill are a keystone species in the Antarctic food web, as they feed on phytoplankton and are the main food for fish and penguins. They are likely to abandon the fastest-warming areas, such as the Weddell Sea, while icefish may find shelf waters around Antarctic islands unsuitable. The shifts or outright declines in krill and copepod numbers are known to prevent baleen whales to recover from the declines caused by historical whaling. Without a reversal in temperature increases, baleen whales are likely to be forced to adapt their migratory patterns or face local extinction. At the same time, many other marine species are expected to invade the Antarctic Ocean as it continues to warm, forcing native species to compete with them. Some research suggests that at 3 C-change of warming, Antarctic species richness would decline by nearly 17% and the suitable climate area by 50%.

Penguins
Penguins are the highest species in the Antarctic food web, and they are already being substantially affected by climate change. Adélie penguins, chinstrap penguins, emperor penguin and king penguins have already been declining, while gentoo penguins have increased in number. Gentoo penguins are ice-intolerant and use mosses as nesting material, so they have been able to spread into previously inaccessible territories and substantially increase in number. The vulnerable penguin species can respond through acclimatization, adaptation, or by range shift. Range shift through dispersal leads to colonization elsewhere, yet results in local extinction.

Terrestrial ecosystems
On the Antarctic continent, plants are mainly found in coastal areas, and are dominated by lichens, followed by mosses and ice algae. In the Antarctic Peninsula alone, green snow algae have a combined biomass of ~1300 tonnes. As glaciers retreat, they expose areas which often become colonized by the pioneer lichen species. The reduction in precipitation in East Antarctica had turned many green mosses from green to red or brown, as they respond to this drought. Schistidium antarctici had declined, while the desiccation-tolerant Bryum pseudotriquetrum and Ceratodon purpureus have increased. The Antarctic ozone hole has also led to an increase in UV-B radiation, which also causes observed damage to plant cells and photosynthesis.

The only vascular plants on continental Antarctica, Deschampsia antarctica and Colobanthus quitensis, are found on the Antarctic Peninsula. Increased temperatures have boosted their photosynthesis and allowed them to increase their population size and spread further. At the same time, other plant species are increasingly likely to invade the Antarctica as the temperatures continue to warm and since human activity on the continent is likely to increase.

Impacts on human development
Tourism in Antarctica has been significantly increasing for the past 2 decades with 74,400 tourists in the summer of 2019/2020. The development of Antarctica for the purposes of industry, tourism, or an increase in research facilities may put direct pressure on the continent and threaten its status as largely untouched land. On the other hand, regulated tourism in Antarctica already brings about awareness and fosters the investment and public support needed to preserve Antarctica's distinctive environment, although an unmitigated loss of ice on land and sea could greatly reduce its attractiveness.

Policy can be used to increase climate change resilience through the protection of ecosystems. The Polar Code is an international code abided by ships that operate in Antarctica. This code includes regulations and safety measures that aid this fragile ecosystem. These regulations include operational training and assessments, the control of oil discharge, appropriate sewage disposal, and preventing pollution by toxic liquids. Antarctic Specially Protected Areas (ASPA) and Antarctic Specially Managed Areas (ASMA) are areas of Antarctica that are designated by the Antarctic Treaty for special protection of the flora and fauna. Both ASPAs and ASMAs restrict entry but to different extents, with ASPAs being the highest level of protection. Designation of ASPAs has decreased 84% since the 1980s despite a rapid increase in tourism which may pose additional stress on the natural environment and ecosystems. In order to alleviate the stress on Antarctic ecosystems posed by climate change and furthered by the rapid increase in tourism, much of the scientific community advocates for an increase in protected areas like ASPAs to improve Antarctica's resilience to rising temperatures.