There it is again, the catastrophe scenario that was so impressively conjured up in gloomy, apocalyptic scenes in Roland Emmerich's Hollywood shocker "The Day After Tomorrow": The global ocean circulation – the global conveyor belt – in the world's oceans, driven by different temperatures and salinity, collapses. The transport of warm surface water to Europe and the American East Coast, which has been so stable for thousands of years, dries up, temperatures literally sink into the basement – ice age-like cold snaps threaten life.
Joachim Müller-Jung
Editor in the arts section, responsible for the section "Nature and Science".
- Follow I follow
Since an Australian-American research group reported inNature on their studies on the developments around Antarctica, this specter has been haunting the climate scene again. The starting point was the observations repeatedly made since the nineties that the Antarctic bottom water and the currents running around the southern continent are changing noticeably. In addition, there are the unprecedented data collections on accelerated ice melt at the South Pole, especially on the Antarctic Peninsula and West Antarctica, where huge ice shelves protrude into the sea and are apparently melting faster and faster.
Many polar researchers fear that the enormous meltwater masses could massively disturb the deep current around Antarctica. Because the area below 4000 meters depth, where the bottom water consists of particularly dense, because saline and cold water, forms a kind of pump for the global conveyor belt. As in the north on the edge of the Greenland Sea in the North Atlantic, where the Atlantic Meridional Overturning Circulation (AMOC) sinks millions of cubic meters of dense surface water per second into the depths and drives the global conveyor belt, the deep-flowing water body off Antarctica is also a decisive driver for the overturning of the world's oceans. The cold water feeds four out of five oceans. If this pump were to dry up, heat exchange and weather around the globe would change virtually overnight.
Disturbing findings
However, for this global conveyor belt, the "thermohaline circulation", to actually collapse, a lot has to happen. In the quite cautious reports and warnings of the Intergovernmental Panel on Climate Change (IPCC), this became clear again and again. The evidence for a collapse is scarce, but the evidence for potentially decisive changes accelerating climate change is mounting.
The meltwater analysis by Qian Li of the Massachusetts Institute of Technology and Matthew England of the University of New South Wales are certainly among these disturbing findings. What they have found is that deep currents off Antarctica could slow – and warm further – by up to forty percent by mid-century. The whole thing is driven by the melting of the huge ice sheets. The fresh water flowing away from the melting glaciers dilutes the salt water. In addition, the surface water is heated and less and less ice forms. But especially the cold surface water, which cools down especially under the so-called polynyas and increases the salt concentration, because the salt does not crystallize, is urgently needed.
Normally, this water sinks along the coasts down into the depths and feeds the bottom water, so to speak. In the meantime, however, according to measurements in recent years, warm water masses have spread out at depth. The cold deep water is visibly displaced. "Particularly worrying," says Matthew England, is the warming in the Amundsen Sea and the Bellingshausen Sea off the West Antarctic coast, because there the water on the ice shelf coast warms up particularly quickly, and the meltwater inflows are particularly large.
This could quickly lead to feedback: The warmer water causes the ice masses to melt even faster. In fact, this was the strongest impression that this year's Polarstern expedition to the Bellinghausen Sea left on the Bremerhaven team of the Alfred Wegener Institute: "Where has all the ice gone?" the polar researchers wondered. Never before had they observed so little sea ice in the region. Since the beginning of systematic measurements, there has never been little sea ice around the Antarctic as in this southern summer.
The eternal ice as a tragic myth
It is not yet possible to say how meaningful these observations and the new model calculations from the ice front really are for the future of the climate. But the fact that this monstrous melting ice at the poles – also around the North Pole and on Greenland – can get rolling somewhat quickly is also shown by the findings that a British team off the coast of Norway has just reported in Nature.
Ships surveyed the seabed off the coast to document the retreat of the ice sheets after the end of the ice age more than 15,000 years ago. Undulating furrows on the seabed, which formed on the tidal touchdown lines of the ice edges, testify to the disappearance of the glaciers. What the researchers found seemed unthinkable for a long time: that the huge ice sheets temporarily retreated by up to 600 meters in a single day as they warmed. Off the Antarctic Peninsula, the historical records have so far been a good 50 meters per day. This is roughly the speed at which the fast-flowing Pope Glacier is currently shortening in West Antarctica. The so-called eternal ice thus proves more and more clearly to be a tragic myth, a mere wishful thinking.