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despite global warming, the sea ice is expanding. here’s why

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Work using paleoclimate methods is helping to better understand why sea ice has gained ground east of Antarctica in recent decades. The results were published in the journal Geoscience of nature this February 22.

Although it presents itself as a continent isolated from the rest of the world, Antarctica is not spared by the ongoing global warming. Nevertheless, the regional variation of the latter is complex. Indeed, if the west of the continent has undergone significant warming since the end of the 1950s, the central and eastern parts show little or no change.

North pole and south pole: very differentiated trends

In addition, sea surface temperatures signify a slight cooling in the continental periphery. Only the seas of Amundsen and Bellingshausen are experiencing a warming trend. A phenomenon which should not, however, make us forget the rapid rise in temperature a little deeper. The latter being responsible for erosion of the ice barriers from below. For more information on this worrying process, you can read our dedicated article.

Due to this complexity, the Antarctic sea ice shows a rather counter-intuitive evolution in a context of global warming. While the annual extent of arctic sea ice decreased from 3.5% to 4% per decade between 1979 and 2012, that bordering the southern continent increased from 1.2% to 1.8% over the same period. Trends that have not fundamentally changed since, despite a break at the South Pole after theThe Boy from 2015 to 2016.

Extension of Antarctic sea ice in September (top) and February (bottom), between 1979 and 2020. The graphs correspond to the month of the annual maximum and minimum, respectively. Credits: NASA.

The gain in sea ice observed at the South-East Pole is mainly due to the changes that have occurred off the coast of East Antarctica. Until now, the precise causes of this expansion were not well known. And for good reason, the operational monitoring provided by satellites has only existed for about forty years. Too short a period to separate the part attributable to natural variations from that linked to human-made climate change. Finally, numerical simulations are known to present biases in the fine representation of the southern climate. They do not therefore constitute in themselves a way out.

The importance of natural variability in Antarctica

In a new study, researchers therefore had the idea of ​​reconstructing the history of the ice floe by taking advantage of natural archives. Thanks to the scrupulous analysis of sedimentary cores taken in Terre Adélie and in the south of the Indian Ocean, scientists were able to characterize the changes that have occurred in the sector over the past 2000 years. The results highlight a significant variability on a multi-decadal scale.

By constraining a climate model by the paleoclimatic data obtained, the authors have shown that this mobility comes from the combined effect of two modes of natural variability. Climatologists speak ofENSO and of SAT for El Niño / Southern Oscillation and South ring mode. The point to remember is that these fluctuations in wind and sea current regimes bring more or less heat to the south pole. In addition, they influence the power of the katabatic winds of the ice sheet which push the ice to a greater or lesser extent out to sea.

Antarctic
Positive (left) and negative (right) phase of SAT. The colors represent pressure anomalies: low pressure in blue, high pressure in red. In the positive phase, the westerly winds are stronger and closer to the pole compared to the negative phase. Credits: ESRL / NOAA.

Depending on the way in which the circulation redistributes the heat, the pack ice thus extends more or less easily towards low latitudes. Remember that these are phenomena that are essentially internal to the climate machine. Also, the results provided by this study underline the need to consider natural variability in the study of recent changes. Compared to the Arctic Ocean, Antarctica has longer typical timescales, hence the need to work with more extensive observational series.

Our results therefore indicate that natural variability is important in the Southern Ocean and suggest that it has played a crucial role in recent sea ice trends and their decadal variability in this region. »Concludes the study in its summary.

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