Climate research: regime change in Antarctica

For several decades, summer ice on the Arctic Ocean has been decreasing. The waters along the North American and Russian coasts are often navigable for a few weeks from August onwards. However, around the sixth continent, Antarctica, the climate change clock seemed to tick differently for a long time. There, too, the kilometer-thick ice sheets pushing into the water are threateningly losing mass. But the surrounding sea ice, which forms on the oceans in winter and in the
The summer seems to be receding somewhat, but rather it seems to be expanding. The trend has been upward for almost four decades.

Until 2014. There was February, the peak of summer ice loss
In the southern hemisphere, sea ice extent is greater than ever since systematic observation via satellites began in the 1970s. Just two years later, the ice had retreated further than ever before in the summer months of December, January and February. And this development continued. The lowest coverage to date was in February 2023 and the minimum in February 2024 was only slightly larger. The ice surface in the Antarctic summer was only about half as large as ten years earlier.

Indicators of system change

One recently published in the Journal of Climate of the (US) American Meteorological Society Study concludes that this is not a coincidence. Rather, the authors, a team from the University of Tasmania in Australia, demonstrate a real system change in the Antarctic sea ice. There has been a significantly increased fluctuation in summer ice cover for almost two decades. The standard deviation – that is, the spread around the mean – doubled between the periods 1976 to 2006 and 2007 to 2022. In addition, the autocorrelation increased significantly. The latter means that the ice cover in winter can now be used to estimate how strong the decline will be in summer. This was not the case before 2007.

From the analysis of dynamic systems in various fields such as ecology, economics and climate science, it is known that high standard deviation and autocorrelation are two indicators of a regime change, of a system transitioning into a different mode. A third indicator of such a transition also seems to exist: the high spatial coherence, that is, small regional differences in the changes.

Such a regime change in a system means a rapid change, but not necessarily a so-called tipping point, which is accompanied by irreversible changes. Thomas Frölicher, who researches and teaches at the Oeschger Center for Climate Research at the University of Bern, does not assume a tipping point and points out that the authors are not committed to this question. Torsten Albrecht from the Potsdam Institute for Climate Impact Research also shares Frölicher’s view. The situation is analogous to sea ice in the Arctic, the extent of which has been shrinking more and more in the summer for several decades. However, there is widespread agreement in climate science regarding the Arctic that the sea ice there could quickly grow again in a cooler climate. If there were finally an end to the burning of fossil fuels such as coal, gasoline and natural gas, which continue to build up carbon dioxide in the atmosphere, and if this greenhouse gas were removed from the atmosphere again. But this is not in sight, but rather a further escalation of the climate crisis.

And in this context, the news from Antarctica is anything but reassuring. So far, the sea ice there has had a certain protective function for the several hundred meter thick ice shelf that the glaciers push from the land into the sea, as Albrecht notes. As long as there is a mixture of icebergs and sea ice in front of the ice shelf, this dampens the effect of wind and waves and thus the break-off of further icebergs.

Sea ice slows further melting

Additionally, the white ice on the sea prevents the sun from warming the water below. Otherwise it would penetrate under the ice shelf and further accelerate the thaw there. This is already a problem. It wasn’t until the end of February that one appeared in the journal »Nature« Published work by two scientists from the University of Edinburgh showed that the ice shelf has actually only been rapidly thinning since the 1990s, somewhat delayed, so to speak, with global warming. The thick ice sheet shrinks primarily as relatively warm seawater from deeper ocean layers penetrates under the ice shelf and melts there. It has been known for several years that these rising currents of warmer water have increased.

At the same time, deep water is also formed around Antarctica, for which the sea ice is responsible. When the ocean surface freezes, most of the salt remains in the cold, near-freezing water. As in the northern North Atlantic, relative cold and high salinity lead to water sinking into the depths, thereby driving the overturning circulation that encompasses all of the world’s oceans and, at the same time, the CO dissolved in the water₂ as well as thermal energy with it into the depths. Both together have a cooling effect on the planet, as Frölicher and Albrecht independently emphasize. However, there is still no reliable knowledge as to whether the reduced sea ice cover already influences deep water formation.

It is also still unclear what actually led to the regime change in sea ice extent. The Australian authors were only able to rule out that it was due to changes in air pressure or shifts in the temperature differences on the sea surface in various regions of the southern hemisphere. A possible cause could be the increased rising water, but too little is known about it. Antarctica is still a very inaccessible place where it is not so easy to carry out long series of measurements.

Algae are the basis of the ecosystem

However, it is clear that the decrease in sea ice cover will have consequences for the species-rich and very productive ecosystems of Antarctic waters. For example, on the seabed, where more light does not necessarily mean more biological activity, says Hauke ​​Flores from the Alfred Wegener Institute, Helmholtz Center for Polar and Marine Research in Bremerhaven. The communities there are specialized in the types of algae that trickle down from the thawing ice in summer. As in many other ecosystems, climatic changes are disrupting the coordinated rhythms of growth, reproduction and death, explains Flores, whose work focuses on sea ice ecosystems and who did his doctorate on the subject of under-ice communities in the Antarctic.

The krill will also be affected by the changes. The small crustaceans, which occur in huge schools, are not only an important source of food for penguins and some whale species, but also play a role in fishing. Studies from the last two decades have shown that where sea ice declines regionally, the amount of krill also declines. This feeds primarily on algae that live on the underside of the ice and which it scrapes off with its front legs. The krill offspring in particular are dependent on this ice algae, for which they have hardly any food alternatives in winter. Since, unlike adult animals, it cannot build up fat reserves, it has to constantly graze on algae from the ice so that it does not starve. However, according to Flores, it is not yet possible to say whether the recent retreat of the ice has already had an impact on the krill.

Consequences for penguins and seals

The animals living on the ice also get into trouble if that happens
Sea ice retreats faster and further in summer. In the past
Last year, a mass death of young animals in some large penguin colonies made headlines. Due to dwindling ice, the penguin offspring had to go into the water before they had even developed a waterproof down coat. Accordingly, most of the young birds froze to death. Such events are becoming more likely as the ice continent changes, Flores said. The crabeater seal, which needs the sea ice to give birth to its offspring, has a particular problem. If the ice is gone too soon, the young seals have little chance of survival. Important details of the causes and effects of the significantly reduced sea ice may still be unclear. However, the rapid change of mode makes it clear that changes in the climate system often occur abruptly and that sometimes the extent of the effects only becomes fully clear once they have already occurred. And these can, as in the case of the overturning circulation, be irreversible, even if the trigger, the change in the sea ice, was not. Or in other words: Climate change still has some nasty surprises in store for us.