Climate Change: The Scientific Mystery of Antarctic Sea Ice Growth | Climate and Environment

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Climate Change: The Scientific Mystery of Antarctic Sea Ice Growth | Climate and Environment
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The sea ice that surrounds Antarctica is a real black hole for polar science. That layer that freezes over the Southern Ocean, the ice pack, decreased in the first half of the 20th century, but then it has been increasing in the last decades of the 20th century, until today, while the ice in the rest of the world is shrinking due to the change climate. The phenomenon, verified with data from satellites and stations, reveals the extreme complexity of this continent and of a planetary climate in which the tropics and poles are linked.

Recent research, published in the journal Nature Climate Change by a scientific team from Ohio University, is the most complete carried out to date on the Antarctic sea ice. Since 1979, the authors conclude, Antarctic sea ice has been increasing, except in 2016, when there was a strong retreat that reached a minimum in 2017. It then recovered, returning 2020 to the previous average. However, at the beginning of 2022, the ice pack, the Ohio researchers point out, is again below average: “It had returned to normal, but for a short time,” they acknowledge in an official statement.

The causes behind this increase in sea ice are still a mystery. In the investigation they point, as a hypothesis, to changes in pressure since 1979, with winds that push the ice towards the Antarctic coast. They indicate that it could have to do with the Southern Hemisphere Annular Mode (SAM) phenomenon, a movement of the belt of winds from the north to the south of that hemisphere that occurs over decades. They also refer to phenomena that occur many thousands of kilometers away, in the tropical zone, such as the so-called Pacific Decadal Climate Oscillation, which occurs every 20 or 30 years.

“Tropical phenomena change the winds and that change moves to the poles. For this reason, in some areas the ice grows and in others it does not, as this work confirms. All this tells us about the complexity of Antarctica and its climate. At the same time that ice increases, we see that more warm water reaches the coast and that some glaciers increase and others recede,” says the Spanish geologist and polar scientist Carlota Escutia, who is already packing her bags to go to Antarctica in a few weeks for work related to climate change and the circumpolar current.

Precisely, his group proposes in a recent investigation that there is a relationship between that Antarctic sea ice, in whose history they have gone back thousands of years investigating ocean sediments, and phenomena such as The boy: According to their data, polar sea ice has increased in Antarctica for 4,500 years and past variations would have to do with those events much further north. Moreover, the ice floe drop detected in 2016 and 2017 coincides with one of the phenomena of The boy most intense recorded, something the Ohio scientists do not mention in their work. “This possible relationship would have to be studied,” says Escutia.

Although it is not the first time that the increase in Antarctic sea ice has been detected, it is the first reconstruction of what happened throughout the 20th century, throughout the continent and in all seasons of the year. The authors triple the data obtained so far in a continent where it is not easy to obtain them: almost all polar research is carried out in the southern summer, when the sea ice is less. “Our reconstructions show that there is a brief period [en términos científicos] of observations since 1979 that is unique in the context of the 20th century. Before the satellites, there is increasing evidence that the sea ice was decreasing until the middle of the last century, but then it has increased,” Ryan Fogt, climatologist and lead author of the research, points out via email, who later highlights “that in 2016 there is a sudden downturn like you haven’t seen before”. “Something powerful is happening recently. The trend shows how variable Antarctica’s climate is, including sea ice, and how quickly its climate can change.”

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Faced with those who may use this increase in sea ice for denial arguments, Fogt considers that “no aspect of the Earth’s climate is completely independent of anthropogenic climate change” and points out that “the signal of that change is much easier to detect at scales larger or over a longer period of time and Antarctic observations are comparatively short.” “Science is rapidly evolving to see how we can link changes in Antarctica to global climate change, and as we collect more data we know better those connections,” he says. “There is not yet enough scientific evidence to conclude that Antarctic sea ice is not affected by global climate change, in any way,” he concludes, “but this research serves to better understand the historical variations of that ice, which contrast with what that occurs in the Arctic, where there is a faster loss since 1979, consistent with the increase in greenhouse gases.

An important aspect of his conclusions are the differences he detects according to the zones: while the sea ice increases in the Ross Sea and the Amundsen Sea, it decreases in the Bellinghausen Sea. It also varies by season. “The magnitude changes seasonally and over time. For example, there are similar areas of sea ice increase between the observations and our reconstructions in East Antarctica in the autumns since 1979,” he says.

Another unresolved uncertainty is whether that ice will continue to increase in the future. “There is intense scientific research on the causes. If it can be due to changes in the tropical Pacific, it seems very likely that the patterns we have seen in the satellite age will repeat themselves. Now, if the climate continues to change so rapidly, it may not be the same in the future,” says Fogt.

What happened in the past is also an important clue and Escutia recalls that in the Antarctic geological record it was recorded that with high temperatures in the southern water, the Antarctic sea ice disappears, as it already happened during the Pleistocene, with 4 ºC more. It is also known that the Southern Ocean is the one that absorbs the most heat from the Earth’s atmosphere that we are heating up, dangerously approaching a 1.5 ºC global temperature increase.

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