Glimpse beneath iconic glacier reveals how it’s adding to sea-level rise


Researchers have dropped a submersible vehicle down a hole in Antarctic ice to get their closest-ever look at the underside of Thwaites Glacier — a massive and increasingly unstable body of ice that has become an icon of climate change — and the first-ever glimpse at the spot where the ice meets the land.

The observations, published in two papers in Nature on 15 February, could help to pin down one of the biggest uncertainties in current projections of rising global sea levels. The studies imply that models of how the West Antarctic Ice Sheet and glacier flow respond to climate change are missing some important details. Incorporating these insights should clarify how and why the ice will change in the future.

For now, the work brings “neither good news nor bad news” in terms of sea-level rise, says co-author Peter Davis, a physical oceanographer at the British Antarctic Survey in Cambridge, UK. “The glacier is still moving as quickly as it ever has been.”

Reducing uncertainty

The Intergovernmental Panel on Climate Change predicts that sea levels will probably rise by between 38 and 77 centimetres by 2100, but the collapse or melting of ice sheets in Greenland and the Antarctic could theoretically contribute an additional metre. “All of this is to beat down those uncertainties,” says Britney Schmidt, an Earth scientist at Cornell University in Ithaca, New York, who is a co-author of both papers.

Thwaites Glacier is a fast-moving block of ice, the size of Florida, in the West Antarctic. Satellite studies have shown that its ‘grounding line’ — where ice attached to bedrock transitions to ice floating in the sea — has shifted 14 kilometres inland since the late 1990s, and some parts of it are retreating as fast as 1.2 kilometres per year.

‘Grounding-line retreat’ is what makes Thwaites responsible for about 4% of today’s global sea-level rise (see ‘Retreating glacier’). As the grounding line moves inland, it levers up more, ever-thicker ice to float on the sea. This, in turn, raises the sea level and makes the glacier move faster. The process can lead to accelerating collapse, as the erosion of the coastal ice allows kilometres of ice behind it to flow ever more rapidly out to sea.

Researchers think that grounding-line retreat is driven by warm ocean water melting the underside of the ice. Climate change has shifted wind patterns in the region, allowing a patch of warm water to flow towards the West Antarctic.

Drilling for data

To investigate this process, Davis, Schmidt and their colleagues decided to drill down into the glacier and have a look at the grounding line.

They drilled a hole roughly 30 centimetres wide through nearly 600 metres of ice, using hot water, and lowered down instruments and a remotely operated vehicle called Icefin. This allowed them to observe the underside of the ice, the grounding line, more closely than ever before. Over five days in January 2020, they took images and videos of the underside of the glacier and collected data about water temperature, salinity and more. Some instruments left on site have now been taking data for more than a year.

The researchers found that melt rates on the underside of the ice were just 2–5.4 metres per year, much lower than the 14–32 metres predicted by models1. “That was very surprising,” says Davis.

The water was about 1.5 °C above the freezing point. However, they found that a thin layer of cold, fresh melt water was coating the underside of the ice — and, because the water was very still, this prevented heat from being transferred to the ice. “There’s more than enough heat, actually, to drive really rapid melting, but you need to get that heat through the protective layer,” says Davis.

The melt rate was highest in areas under the ice where there were cracks and steep, staircase-like features2. These divert the cold, protective melt water, allowing the heat to reach the ice, and melt it to widen crevasses.

Sensitive ice

Both papers might help to clarify what’s missing from simple models of Antarctic ice that don’t seem to capture large changes thought to have happened during warmer periods of Earth’s history, says Eric Steig, a glaciologist at the University of Washington in Seattle. The results show ways in which the ice might be more sensitive: glacial retreat can be rapid despite low rates of melting from underneath; and the most pronounced melting is helping to carve out crevasses from below, which might encourage large bits of ice to break off. “Maybe you don’t need as much melt to affect the structural integrity,” Steig says.

“These types of hard-fought observations are absolutely critical to refining the treatment of these processes in the models we use to predict the ice sheet’s future,” says Robert DeConto, a geophysicist and ice modeller at the University of Massachusetts Amherst. “We need more of them.”

When this information does get built into models, it should tighten predictions of what will happen to Antarctic ice and global sea levels. It’s unknown whether this will paint a more frightening picture or a more reassuring one. “I really couldn’t speculate,” says Schmidt.

Nature, 15 February 2023