Antarctic ice shelf thinning is accelerating, reveals new study

A new study reveals ice shelves in the western part of the continent are melting much faster than a decade ago. Satellite data from three separate missions shows melting of these vast, floating ice shelves has increased by 70% in the last decade.

If current warming trends continue, the researchers say the ice could thin so much that these icy ‘gatekeepers’ risk collapsing, unlocking parts of the ice sheet to faster ice loss.

Floating sheets of ice

Ice shelves form where a glacier on land reaches the coast and flows into the ocean. They surround  75% of the Antarctic continent. If the ocean is cold enough, the ice doesn’t melt but instead forms a floating sheet of ice that extends over the ocean.

Ice shelf diagram. Credit: Professor Helen Fricker, Scripps Institution of Oceanography, UC San Diego.

The new study, published in Science, looks at changes in ice shelf thickness over an 18-year period. The scientists analysed radar measurements of ice thickness taken by three different satellites between 1994 and 2012.

The study is the first to pull together such a long satellite record, lead author Fernando Paolo, from the Scripps Institution of Oceanography at the University of California, tells Carbon Brief. This allows the researchers to look for trends in ice shelf changes, beyond just the year-to-year fluctuations.

Accelerating decline

You can see the changes in ice shelf thickness in the map below. Red dots show ice shelves that have got thinner over the 18-year study period, blue dots show ones that have got thicker.The bigger the circle, the bigger the percentage change.

Map shows changes in Antarctic ice shelves from 1994 to 2012. Shading of ice shelves shows rate of thickness change (in metres per decade), from thinning (red) to thickening (blue). Dots show percentage of thickness lost (red) or gained (blue). The central circle shows the area not covered by the satellites. Source: Paolo et al. ( 2015)

In West Antarctica, large red dots show the ice shelves have been thinning rapidly. But that’s not the case everywhere. On the east side of the continent, the blue dots show that some shelves have got thicker.

Between 1994 and 2003, these gains in East Antarctica compensated for the losses in the west, meaning the total volume of ice shelves across the continent remained steady.

But since 2003, losses in the west have increased while gains in the east have stopped, meaning the total volume of ice has decreased markedly.

For West Antarctic ice shelves, the rate of ice loss has increased by 70% in the last decade, the paper finds. For example, ice shelves in the Amundsen and Bellingshausen seas have thinned by 8% and 5% respectively over the last 18 years. These two regions alone account for around 20% of West Antarctic ice shelves.

Both the Venable and Crosson ice shelves have lost 18% of their thickness in just 18 years, the paper says. You can see some of these results in the video below.

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Video of the changing rate of ice shelf thickness in the Antarctic. Red colours show areas of greatest thinning. Credit: Scripps Institution of Oceanography, UC San Diego

The scale of ice loss is striking, says  Professor David Vaughan, Director of Science at British Antarctic Survey, who was not involved in the paper:

“Most of the pattern of change, the strong loss around West Antarctica and the marginal gain around East Antarctica are what we expected, but the rate of ice loss – especially when considered in terms of the percentage of ice lost in the last two decades – is dramatic.”

Warm ocean water

The main cause of the thinning ice shelves in West Antarctica is warm water coming into contact with the underside of the ice, melting it from below, explains Paolo. The warm water is pulled towards the ice shelves by changes in wind and ocean circulations, he says.

These wind circulation changes have also resulted in more sea ice forming around Antarctica, Paolo says:

“Westerly winds around Antarctica are getting stronger, blowing more sea ice away from the coast. This allows more sea ice to form as cold air from the continent blows across the extra open water.”

Ice shelves can also thin because of warmer air temperatures and changing amounts of snowfall, Paolo says.

‘Gatekeepers of ice flow’

So why does it matter if ice shelves are thinning?  As ice shelves melt, they may become unstable and collapse, as seen with the Larsen B ice shelf in 2002.

While melting ice shelves don’t directly contribute to sea level rise, they effectively hold back, or ‘buttress’, the glacier behind. Without the ice shelf, there would be nothing stopping the glacier from flowing into the ocean, contributing to sea level rise. Dr Ted Scambos, lead scientist at the National Snow and Ice Data Center in the US, explains to Carbon Brief:

“Ice shelves are the gatekeepers of the ice flow off of Antarctica. The study implies that ice shelves are going to thin more and more, and this will have a direct impact on how much ice is lost from the main continental ice sheet.”

Two studies looking at rapid ice loss in the Amundsen Sea region attracted a lot of media attention last year. The studies noted how the the loss of ice shelves at the edge of the West Antarctic Ice Sheet had amplified the speed of glaciers flowing into the ocean.

Other studies suggest that glaciers behind ice shelves may accelerate as much as five times following a rapid ice shelf collapse.

The new study shows how quickly ice shelves can change from one decade to another, Paolo says. And if warming trends continue, this will put more ice shelves at greater risk of collapse.

Main image: Antarctic ice shelf in the mist.