Glacier impact on sea-level rise depends on its shape

Glacier impact on sea-level rise depends on its shape

31 May 2013, by Harriet Jarlett

As glaciers shrink because of melting and the formation of icebergs, the ice they lose contributes to sea-level rise. Over the past decade the glaciers across Greenland have, been retreating at an accelerating rate and scientists were increasingly concerned about runaway losses causing marked sea-level rise in the future.

But without a better understanding of the processes that form icebergs and whether the recent rise in calving – chunks of ice breaking off where the glacier meets the ocean – is due to climate change, scientists couldn’t accurately predict how much melting glaciers would add to sea-level rise in the future, until now.

An international team of scientists created computer models of the four most important glaciers in Greenland, chosen as together they drain around 22 per cent of Greenland’s ice sheet into the ocean. The team then used these models to predict how Greenland’s glaciers could contribute to sea-level rise by 2200.

The research, published in Nature, shows that how fast a glacier retreats depends on the shape and the landscape it flows over. While all four glaciers are retreating, because of their different shapes they accelerate at different rates and in different ways. By understanding the processes which affect different glaciers, scientists are able to better predict how they will react to warming oceans and atmosphere and can then use models to show how much each individual glacier will contribute more or less to sea level under different warming scenarios.

For these four glaciers, the study showed that their shape’s suggest their retreat is unlikely to continue accelerating at the same pace. Instead, under a mid-range climate scenario, where carbon emissions increase through the 21st century so global temperatures rise by 2.8oC, the four glaciers could contribute up to three centimetres to sea-level rise by 2200– less than previous estimates based on current rates of retreat.

‘We were mainly interested in understanding the behaviour of these glaciers; is it the warmer ocean currents controlling their rapid retreat, or a warmer atmosphere resulting in more surface melt runoff, or is it a combination of these forcings resulting in icebergs being calved off faster? We wanted to see which of these caused the recent acceleration,’ says Dr Faezah Nick, of the Université Libre de Bruxelles and the lead author of the paper. ‘We tried to model the glaciers so we could see how things would change over the next century and how glaciers would react to different forcing.’

The computer model showed that the shape of the glacier and its fjord – the seawater-filled valley it leaves behind – can alter how it responds to a changing climate.

Two of the four glaciers studied, Helheim and Kangerdlugssauaq, were found to be most sensitive to changes happening where they meet the ocean, which results in large amounts of calving. The area in front of these glaciers is filled with floating chunks of broken ice, and sea ice called ice mélange. Ice mélange may protect the glacier front from calving, but as ocean temperatures rise it melts and more calving occurs.

This means the glacier can move faster down into the ocean, which in turn results in surface crevasses opening and even more calving.

Because of its shape, the Petermann glacier’s retreat was unaffected by ice calving. Instead its retreat was mostly affected by the amount of ice melting underneath it. The Petermann glacier has a very long and thin ice shelf so changes happening at its front, where it meets the ocean, affect it much less. Instead, because the glacier has so much ice in contact with seawater along its base, rising ocean temperatures can cause that ice to melt and accelerate its retreat.

‘In Summer 2012 and 2010 two large icebergs broke off of the Petermann glacier, but neither had a big influence on the ice flow. There was no speed up because that kind of glacier has a very long and thin shelf and the glacier flow is insensitive to changes at the front,’ says Nick.

The team are now confident that, despite the rapid changes observed for these glaciers over the last decade, the accelerated retreat is unlikely to continue. But they warn that other glaciers, which have not shown rapid changes yet, may start retreating.

‘It’s now clear that each glacier behaves different depending on the geometry of land they sit on and whether they have an ice shelf or not. So if we really want to understand how much sea level rise will be in the next century we need to study each of the major outlet glaciers individually, therefore it is necessary to measure the topography beneath them,’ Nick concludes.

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