Has Antarctic sea ice lost its connection to the atmosphere? | Polarjournal
In the Southern Ocean, almost two million square kilometres of sea ice are missing shortly before the annual maximum extent. (Photo: Julia Hager)

Sea ice cover in the Southern Ocean has remained well below the long-term average practically since the beginning of this year, leading scientists to conclude that Antarctic sea ice has entered a new state.

For the third time in the past seven years, summer sea ice cover in Antarctica set a new negative record in February this year. And currently, in the Antarctic winter just before the annual maximum, more than 1.8 million square kilometres of sea ice are missing (17 September 2023, source: Sea Ice Portal) compared to the long-term average on this day (1991 – 2020) and around 1.1 million square kilometres compared to the last record minimum in 2017.

Antarctic sea ice extent has been declining since September 2016 with a never-before-documented negative anomaly of 2.33 million square kilometres in June 2023, after increasing from September 2007 to August 2016 despite rising global temperatures and reaching record maxima since sea ice can be observed by satellites in 1979.

The annual cycle of sea ice extent in Antarctica. The light blue line marks this year’s sea ice cover. Light green marks the long-term average in the period 1991 to 2020. The years 2017 and 2022 are shown in yellow and blue dotted lines, respectively. Graphic: meereisportal.de

Dr Ariaan Purich from Monash University, Australia, and Dr Edward Doddridge from the Australian Antarctic Program Partnership at the University of Tasmania analysed the abnormal behaviour of sea ice. In the Nature journal Communications Earth & Environment, they describe that there are signs of a regime change in the Southern Ocean that will have far-reaching implications for Antarctic ecosystems and global climate.

“Our study suggests that since 2016, ocean warming due to global heating has pushed Antarctic sea ice into a new state of diminished coverage that it will struggle to recover from,” Dr Purich said in a Monash University press release. “The characteristics of this new state suggest that the underlying processes governing Antarctic sea ice have fundamentally changed. It appears we’re seeing the decline of sea ice long predicted by climate models.”

In their study, the researchers state that the recent sea ice extremes are not associated with large-scale natural climate modes such as the Antarctic Oscillation, El Niño and others. Instead, the last three summers with record minima in sea ice extent had one thing in common: higher temperatures in the upper 100 to 200 metres of the Southern Ocean. To them, this is an indication that the sea ice has lost its connection to the atmosphere.

Five-day anomalies of Antarctic sea ice extent in millions of square kilometres for each year, with data from the National Snow and Ice Data Center. November 1978 to August 2007 – grey, September 2007 to August 2016 – blue, September 2016 to June 2023 – orange. The bold orange line represents the period January to June 2023. Graph: Purich & Doddridge 2023

Antarctic sea ice plays an extremely important role in the global climate system. If its extent is smaller, less solar radiation is reflected back into space while at the same time the dark ocean absorbs more heat and creates less sea ice – a self-reinforcing vicious circle. With less sea ice, deep water formation also slows down, which has an impact on global ocean circulation.

As a physical barrier, sea ice also protects ice shelves from the waves of the Southern Ocean and from even faster melting, and provides a habitat for emperor penguins, the icons of Antarctica, and other unique animals that depend on the sea ice to raise their young.

Sea ice loss also has a dramatic impact on organisms at the bottom of the food chain that require sea ice for their survival: Antarctic krill, for example, a key component and the foundation of the Antarctic food web, feeds on unicellular algae that grow on the underside of the sea ice. Its decline would have fatal consequences for the entire Antarctic ecosystem.

“Our results demonstrate that a lot of what we thought we knew about sea ice has changed,” Dr Doddridge said. “Previously, the ocean’s memory would be wiped every winter – there was no discernible relationship between the summer minimum and the following winter maximum. But, since 2016, this annual reset appears to have broken down. This raises the possibility that sea ice has entered a new state in which previously important relationships no longer dominate sea ice evolution.”

Like many other scientists before them, Dr Purich and Dr Doddridge reiterate the urgent need to reduce fossil fuel emissions.

Julia Hager, PolarJournal

Link to the study: Purich, A., Doddridge, E.W. Record low Antarctic sea ice coverage indicates a new sea ice state. Commun Earth Environ 4, 314 (2023). https://doi.org/10.1038/s43247-023-00961-9

Sea ice data from www.meereisportal.de (Funding: REKLIM-2013-04)

Spreen, G.; Kaleschke, L. and Heygster, G. (2008), Sea ice remote sensing using AMSR-E 89 GHz channels J. Geophys. Res.,vol. 113, C02S03, doi:10.1029/2005JC003384.

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