An international research team, including the Alfred Wegener Institute, has now discovered that West Antarctica's most vulnerable ice streams - including its giant Thwaites Glacier - repeatedly collapsed and retreated far inland during natural warm periods of the last 800,000 years. This occurred even when global temperatures were similar to those of today. The researchers have published their findings in the journal Nature Communications.
How can we assess the way, our planet will respond to current and future climate changes? Looking to the past is often a reliable way of determining whether conditions similar to those of today have ever existed before, and how different regions of the Earth responded at that time.
The question of how the West Antarctic Ice Sheet (WAIS) responded to previous warm periods – known as interglacials - has long divided researchers. Now, an international team led by the British Antarctic Survey (BAS) under direct involvement of the Alfred Wegener Institute (AWI) has found one of the most convincing pieces of evidence to date. The focus is a distinctive mineral called kaolinite – what most people know as china clay. It typically forms in warm, wet, tropical climates or swampy environments. More than 34 million years ago conditions like that existed on the Antarctic continent.
Today, this ancient kaolinite is locked away in the bedrock beneath the WAIS. When massive glaciers such as the Thwaites Glacier or the neighbouring Pine Island Glacier flow towards the sea, they grind down the underlying terrain and wash out the mineral. Icebergs and water currents then transport it down to the seabed, which is why sediment samples show a corresponding kaolinite signal. However, when the researchers analysed the sediment cores they pulled from a depth of around 4,000 metres below the sea surface on Antarctica’s continental margin, they made a remarkable discovery: they could no longer detect any kaolinite in the samples from any of the interglacial periods over the past 800,000 years.
“When the kaolinite signal at our drill-core sites suddenly collapses, it tells us the marine-based ice in the Amundsen Sea sector of the West Antarctic - including Thwaites and Pine Island glaciers - must have melted and retreated far back into the continent’s interior,” says Dr Claus-Dieter Hillenbrand, the BAS marine geologist who led the study. “It's the only way to explain why the delivery of kaolinite-rich debris to the ocean floor stops.“
Water temperatures in West Antarctica have risen rapidly
In their search for the cause of past ice retreat, the team examined another clay mineral that is not naturally found in Antarctica: magnesium-rich smectite. This is typically found in the deep Pacific Ocean floor far to the north. The presence of this form of smectite in the sediment samples suggests that it must somehow have been transported southwards and deposited here at the same time as the kaolinite disappeared from coastal sediments.
The obvious explanation is warm deep-water currents intruding on to the West Antarctic continental shelf. These would have transported the smectite south towards the continent, simultaneously melting the grounded ice, which in turn halted the erosion of the kaolinite. To further support this hypothesis,the team then turned to the fossilized remains of diatoms in the mud layers. These microscopic single-celled organisms form shell-like structures take different shapes depending on water temperature. The analysis shows that the global average temperature during earlier warm periods was only about 1.0 to 1.5 degrees Celsius above pre-industrial levels – in other words, on a scale similar to what we see today, or even slightly milder. At the same time, however, the waters off West Antarctica warmed locally by around one to three degrees Celsius compared with today’s conditions.
The critical difference is timescale. Under past natural conditions Thwaites Glacier might have taken several thousand years to disappear. In the modern world, human greenhouse gas emissions are warming the oceans at an unprecedented rate, threatening to compress that process into centuries. “What we found, to my surprise, is that even under natural conditions we had at least a partial collapse of the West Antarctic Ice Sheet. What we are doing now is making it much worse,” says AWI marine geologist and co-author of the study Dr Johann Klages. If Thwaites Glacier were to completely collapse and melt into the ocean, it would raise global sea levels by 65 centimeters, with Pine Island having the potential to add another 51 centimeters. But if wider areas of the WAIS go with it, levels could rise by as much as three to five meters.
“I'm personally convinced that future generations will see substantial sea-level rise,” says Dr Karsten Gohl, an AWI marine geophysicist, co-chief scientist of the corresponding drilling expedition and study co-author. “How fast we'll see that rise is really difficult to say. This study shows that even in past natural warm periods we had a lot of ice loss, but what we're living in now is an interglacial on steroids.”
