What’s happening to the world’s ice?
The loss of Arctic sea ice and the plight of the polar bears often makes headlines (and cute pictures) but what’s happening elsewhere in the cryosphere (the planet’s ice sheets and ice caps) is at least as important.
Monitoring and research from NASA and the University of California adds increasing evidence to the contention that what’s happening in the Antarctic is something we should be worried about.
Why the Antarctic Ice Sheet Matters
Though lacking permanent habitation (except for its scientific bases) the Antarctic is crucial to humans because of its role in climate. Its area, at around 5.4 million square miles, is, as the National Snow and Ice data centre (NSIDC) observes, “roughly the area of the contiguous United States and Mexico combined” and the volume of the ice sheet includes around 7.2 million cubic miles of ice.
That’s a lot. And one of the reasons it’s important is that it’s land ice rather than sea ice. Sea ice is frozen sea water and has no impact upon sea level. But land ice, when it melts, becomes additional water in the oceans – and if all the water locked up in the Antarctic ice sheet were to melt, it would release enough water to increase global sea level by up to 200 feet.
It isn’t just sea level. The existence of such a large area of extreme cold on the surface of the planet means that the Antarctic ice sheet is already a key player in global climate. Climate is influenced by heat transport by winds and by ocean currents; the polar ice caps are key drivers of the latter, affecting the transport of the sun’s heat from low to high latitudes.
What’s Happening at the Bottom of the World?
Although the fluctuations of Arctic sea ice are a key media focus, climate scientists have been studying the characteristics of the Antarctic ice sheet (and Decoded Science reports regularly upon their findings). Ice sheets are dynamic and climate changes naturally; but studies are showing unusual changes within the ice sheet – changes which may have key implications for sea level change.
Ice is moved from the ice sheet by massive glaciers which discharge into the oceans; and the rate at which these glaciers move is determined by a number of factors, including the underlying topography. A study from NASA and the University of California focussed upon a selection of glaciers in a part of the continent known as the West Antarctic Ice Sheet (WAIS).
Using a variety of techniques (including satellite radar altimetry), the team, led by Professor Eric Rignot of University of California Irvine, mapped changes in the point at which the glaciers lift from the sea bed (the grounding line) based on data from 1992-2001. Professor Rignot told Decoded Science the key findings of the study were: “The fast retreat rate of the grounding lines and the high resolution reconstructed bed that shows no bed barrier.” The lack of an upstream barrier being crucial in that it offers no means to stop the retreat.
The West Antarctic Glaciers
As glaciers retreat they become thinner and lift up, allowing sea water to get beneath them and increase the rate of melting – an example of a positive feedback mechanism. The changes which the team mapped for the half dozen glaciers they studied are, to say the least, alarming. Over the study period one, the Smith/Kohler glacier retreated by 35km, the Pine Island Glacier retreated by 31km and the Thwaites Glacier by 14km.
“This is not an unprecedented rate of retreat,” said Professor Rignot, “but for the West Antarctic ice sheet, this is what you might refer as a tipping point.” In other words, the retreat of the ice may have become so rapid and so significant that it cannot be reversed and the melting of this part of Antarctica, at least, may now be inevitable.
And with that come implications for sea level. “The evolution of this sector will contribute to push sea level toward the upper part of IPCC projection for 2100 (90cm) and even beyond (more like 120cm)” warns Professor Rignot.
The Longer Term Perspective
The WAIS makes up just part of the Antarctic Ice Sheet as a whole – and a range of studies have shown that melting rates are lower elsewhere on the continent (although overall the mass of ice is decreasing). But NSIDC data suggest that the wasting off the WAIS glaciers contributes to a loss overall of 136 ± 60 gigatons in 2006, most of it in the WAIS.
The volume of ice locked up in Antarctica is so large that even at rapid rates of melting it would take many hundreds, possibly thousands, of years before it all melted. But even a minor increase in sea level, such as the 90-120cm projected by the IPCC, will have significant impacts on low-lying coastal areas – including some of the world’s major cities.