The oceans have so far been our most important ally in the fight against runaway climate change. They form a huge earth-spanning sponge: a quarter of the climate-damaging emissions and even ninety percent of the energy that the planet accumulates through these emissions are swallowed by the oceans. The water absorbs the gas, converts it, and the water carries the excess heat with the large-scale currents into the deep sea. But this by no means eliminates the problem. In the depths off the southern continent of Antarctica, but also on the edges of the ice, researchers have been observing for years and decades how far these changes reach and what consequences they can sometimes have.
One of the most worrying signs of change in the Southern Ocean has long been the melting of glaciers. The Thwaites Glacier in West Antarctica is a paradigmatic example. It is almost as large as Great Britain and up to four kilometers thick. It has been called the “Doomsday” glacier since a change in the speed of the ice flow into the Amundsen Sea was recorded two decades ago. This makes it clear: melting has more than doubled since the 1990s, and almost a tenth of global sea level rise is due to inflow from the Thwaites Glacier alone. If it melted completely, that alone would raise global sea levels by more than half a meter. If the surrounding ice sheets of West Antarctica were to melt completely, the sea would rise to three meters. If – a distant hypothesis – all of Antarctica’s ice flows into the ocean, that would be enough water to raise coastlines around the globe by seventy to eighty meters.
Warmer water is eroding the glaciers
It’s not that far yet. But the influence of ocean water has already been demonstrated in numerous measurement trips from the behavior of the Thwaites Glacier, which, like the land masses of the rest of West Antarctica, lies largely below sea level. Where the ice sheets rest on the edge of the continent, increasingly warmer water penetrates, which logically also affects the ice shelves in front of the coasts, which float on the water as a continuation of the glacier tongues.
In Nature Geoscience, American polar researchers from the University of Colorado in Boulder recently described an almost incredible phenomenon off the West Antarctic Peninsula – one of the fastest warming regions in the world. The glacier known as Hektoria on the peninsula became an incredible eight kilometers shorter between the end of November and the end of December 2022 – and thus shrank by almost half in two months. “This kind of withdrawal is crazy,” the study’s lead author, Ted Scambos, said in an interview after the publication. In fact, glacier melt rates in West Antarctica can vary greatly. But for ice sheets like Hektoria, which move almost entirely on the rock towards the coast, shortening of the glacier tongue in the range of a few hundred meters per year is more common in times of climate change.
There remains great uncertainty regarding the long-term stability of the West Antarctic ice sheet. An international team, in which researchers from the University of Kiel were also involved, provided remarkable paleoclimatic data just a few weeks ago. The starting point was sediment drilling off Antarctica, which took place as part of the International Ocean Drilling Program. In Nature Communications, the team reported widespread instability in the West Antarctic ice sheet during a natural ice age around 400,000 years ago. In fact, the period known as Marine Isotope Stage 11 is considered one of the longest and most stable warm periods of the past million years. At that time, global temperatures were estimated to be two degrees higher than pre-industrial values, and the ice masses in West Antarctica were significantly smaller. The results of chemical studies of the calcareous shell sediments showed that the West Antarctic ice sheet was sensitive to warming in the Southern Ocean in the past. Above all, the deep water, which flows around the continent to depths of over 5,000 meters in some cases, had an influence.
Particularly interesting were those layers in the sediment in which the oxygen content fell to a minimum. These were apparently the phases that occurred with a warming of the rising circumpolar deep water. This led to melting at the bottom of the ice shelves and at the edge of the Antarctic ice sheet and destabilized the glaciers beyond the coastline. This was followed by meltwater that flowed into the ocean and slowed the formation of deep bottom water. But because this Antarctic deep water also plays a crucial role in the global overturning circulation, it may also have had consequences for the global conveyor belt of ocean currents. The Kiel geoscientist Lena Jebasinski sees parallels to today: “This is an important warning signal, as we are observing the same trends in temperature development in the region today and the stability of the Antarctic ice sheet could be at risk in the short or long term.”
The decisive deep sea
No less exciting with regard to the climate past was a discovery reported by researchers from the Kiel GEOMAR Helmholtz Center for Ocean Research in “Nature Geoscience”. It was about the end of the last ice age around eleven to twelve thousand years ago. The chemical composition of the sediments was examined, which provided information about the situation in the deep sea at that time. Conclusion of the researchers led by Marcus Gutjahr: At that time, the ground water around Antarctica was apparently expanding significantly, whereupon the carbon stored in the deep sea for centuries was released, thus contributing to warming and the end of the ice age.
The process is partly reminiscent of a future scenario that a GEOMAR group also recently presented in the magazine “AGU Advances”. The starting point was model calculations with a climate model that was spatially roughly resolved and physically simplified, but which was therefore suitable for long-term simulations over several centuries. Particular attention was paid to the Southern Ocean and the time when the global temperature would cool again – that is, when climate protection measures take effect and even carbon dioxide is actively removed from the atmosphere. As it turns out, this cooling may have a surprising effect. Then, within a few decades, the huge heat reservoirs in the deep sea, which are currently being built up due to man-made warming, could literally be emptied. The researchers speak of a warm “burp” that could warm the actually cooling atmosphere over a longer period of time just as much as emissions do today. First in the south, later worldwide. However, the carbon dioxide that is currently being absorbed into the ocean water would probably remain in the deep sea – biologically bound. At least that, one could say, if the model calculations are correct: the sea will continue to be favorable to us.
