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Sea ice-free Arctic makes permafrost vulnerable to thawing

New research published in Nature provides evidence from Siberian caves on the essential role that summer sea ice in the Arctic Ocean plays in stabilising permafrost and its large store of carbon.

Permafrost is ground that remains frozen throughout the year and covers nearly a quarter of the Northern Hemisphere’s land. The frozen state of permafrost enables it to store large amounts of carbon; about twice as much as in the atmosphere. The rate and extent of future thawing of permafrost, and consequent release of its carbon, is hard to predict from modern observations alone.However, a new study has discovered a crucial past relationship between summer sea ice in the Arctic and permafrost. This has significant implications for the future as observations show Arctic sea ice has decreased in recent years.

A team of international researchers, including Dr Sebastian Breitenbach from Northumbria’s Department of Geography and Environmental Sciences, have found that times when permafrost melted in the past did not simply match up with times when the Earth was at its warmest. The new research relies on challenging fieldwork to discover and explore Siberian caves. Caves are a powerful recorder of periods when permafrost was absent in the past.

Why are caves important?

Stalagmites, stalactites and flowstones can only form when there is liquid water, and therefore do not form when overlying land is permanently frozen. The presence of stalagmites in caves under permafrosted land therefore demonstrates that there were periods when permafrost was absent in the past.

The researchers used new approaches to date the formation of stalagmites using the decay of natural uranium to lead.  This allows them to assess the timing of periods when permafrost was absent over the last one and a half million years. They found that stalagmites in Siberian caves grew intermittently from 1.5 million to 400,000 years before the present day and have not grown since, due to the overlying land being frozen.

The timing of stalagmite formation during the absence of permafrost does not relate simply to global temperatures in the past but was notably more common when the Arctic Ocean was free of summer sea-ice. The researchers say that several processes may lead to the relationship between Arctic sea-ice and permafrost. Read more.

Thinning ice shelves around Antarctica are causing more ice to move from the land into the sea

A Northumbria University researcher is part of a team that has produced the first physics-based quantifiable evidence that thinning ice shelves in Antarctica are causing more ice to flow from the land into the ocean. Their findings have been published in Geophysical Research Letters

Satellite measurements taken between 1994 and 2017 have detected significant changes in the thickness of the floating ice shelves that surround the Antarctic Ice Sheet. These shelves buttress against the land-based ice, holding them in place like a safety band. While it has been suggested that the thinning ice shelves were responsible for a direct loss of ice from the land-based ice sheet into the ocean, there was no actual evidence linking data and physics that could demonstrate this, until now. 

Researchers in the UK and US have now undertaken the first continent-wide assessment of the impact the thinning ice shelves are having on the flow of ice in Antarctica. They were particularly interested in seeing how much ice flowed across the ‘grounding line’. This is the point where the land-based ice sheet meets the sea-based ice shelves.

They used a state-of-the-art ice-flow model developed at Northumbria and newly available measurements of changes in the geometry of ice shelves to calculate the changes in grounded ice flow. When the modelled results were compared with those obtained by satellites over the last 25 years, the researchers found what they described as ‘striking and robust’ similarities in the pattern of ice flowing from the ice sheet into the ocean. 

The largest impact was found in West Antarctica, which already makes a significant contribution to sea level change. The largest changes are taking place around the Pine Island and Thwaites glaciers. On Pine Island Glacier, evidence of these changes could be seen almost 100 miles (150km) inland, upstream of the grounding line.

Caption:A detailed view of changes in ice flow around the Pine Island and Thwaites glaciers on the Amundsen Sea, due to thinning sea-based ice shelves. Pine Island Glacier is at the top right of the image, with changes in ice seen almost 100 miles inland.

A detailed view of changes in ice flow around the Pine Island and Thwaites glaciers on the Amundsen Sea, due to thinning sea-based ice shelves. Pine Island Glacier is at the top right of the image, with changes in ice seen almost 100 miles inland. 

Hilmar Gudmundsson, Professor of Glaciology and Extreme Environments, led the study. He said there has been a long-standing question as to what was causing the changes we have observed in land-based ice over the last 25 years, and that while the thinning of the floating ice-shelves had been suggested as a reason, the idea had never been put to the test before now.  “I found it striking how well our modelled changes agree with the pattern of observed mass loss,” he said. Read more.

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