This intense melting could be due to unusually high air temperatures to the west and east of the Antarctic Peninsula, which were ca. 1.5 °C above the long-term average. In addition, the Southern Annular Mode (SAM) is in a strongly positive phase, which influences the prevailing wind circulation in the Antarctic. In a positive SAM phase (like today), a low-pressure anomaly forms over the Antarctic, while a high-pressure anomaly develops over the middle latitudes. This intensifies the westerly winds and causes them to contract toward the Antarctic. As a result, upwelling of circumpolar deep water on the continental shelf intensifies in the Antarctic, promoting sea-ice retreat. More importantly, it also intensifies the melting of ice shelves, an essential aspect for future global sea-level rise.
Unravelling the geological evolution of the West Antarctic Ice Sheet, i.e., the massive glaciers that cover the Antarctic continent and fuel the ice shelves, is the proclaimed goal of the current Polarstern expedition. Doing so, it is hoped, will allow us to make more accurate statements on the ice sheet’s future development, and therefore on sea-level rise in the face of constant climate change. For example, the last interglacial, 120,000 years ago, and a prolonged warm period in the Pliocene roughly 3.5 million years ago, are considered analogous to today. In both past periods, the warming was exclusively due to gradual changes in Earth’s orbit – today, these are supplemented by carbon dioxide emissions, which are produced by the use of fossil fuels and accumulate in the atmosphere. The insights gleaned from the ice sheets’ history are intended to help estimate how rapidly and extensively they will melt when certain tipping points of today's rapid anthropogenic climate change are exceeded. In this regard, researchers use geophysical and geological methods to investigate marine sediments at the sea floor, which, as archives of past ice-sheet movements, hold valuable information.
Historical records also reflect the tremendous changes. For example, in the Antarctic summer 125 years ago, the Belgian research vessel Belgica was trapped in the massive pack ice for more than a year – in exactly the same region where the Polarstern can now operate in completely ice-free waters. The photographs and diaries of the Belgica’s crew offer a unique chronicle of the ice conditions in the Bellingshausen Sea at the dawn of the industrial age, which climate researchers often use as a benchmark for comparison with today’s climate change.
You can find more detailed analyses at the Sea Ice Portal: https://www.meereisportal.de/. For the latest news from the Polarstern expedition, check out the Polarstern app https://follow-polarstern.awi.de/ or the Polarstern blog https://blogs.helmholtz.de/polarstern/en/, not to mention the blog on the 125th anniversary of the Belgica expedition: https://125yearsbelgica.com/.