Two new studies point to significant ice melt-driven sea level rise this century

Posted on December 16, 2009

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laseralticesheet-smIn 2007, the Fourth Assessment Report (AR4) of the Intergovernmental Panel on Climate Change (IPCC) refused to stake a firm position on how fast and how high sea levels would rise. The IPCC claimed that, while there was widespread agreement on sea level rise due to thermal expansion of seawater, scientists did not yet know enough about how the ice sheets on Greenland and Antarctica would respond to climate disruption. The science has advanced considerably since 2007 and the majority of the new results (for example, this paper, this paper, and this consensus statement from earlier this year) have confirmed that the IPCC estimates were too low.

Two recent studies measuring different changes on the Greenland and Antarctic ice shelves have added more evidence that sea levels are going to rise higher and faster than the IPCC estimates. One used highly accurate measurements of the changes in ice sheet thickness to estimate how much ice was exiting the ice caps on Greenland and Antarctica via glaciers dumping ice into the ocean. The other used the GRACE gravity measurement satellites to estimate the total amount of mass being lost from Antarctica. Both found significant losses in ice, but GRACE found something more significant – a loss of ice mass from the East Antarctic Ice Sheet, a mass of ice that was previously believed to be stable or even adding ice mass.

Laser altimetry bounces a laser off the surface of the earth to measure how far the surface is from the satellite, and as the satellite passes over the surface, changes in surface’s height can be tracked over months and years. The first study, published in the journal Nature, use laser altimetry to determine whether the ice sheets in Greenland and Antarctica were getting thicker or thinner, and the results revealed significant thinning of the ice sheets along their edges.

The Greenland data revealed that the majority of the ice cap’s edges showing significant thinning, with those areas feeding fast-flowing glaciers thinning the most. Only a few areas showed thickening, and most of those were in the interior and/or above 2000 m altitude. As the ice thins, the total mass of ice going into the ocean increases and sea level rises. However, the authors couldn’t estimate how much ice was being lost due to thinning because most of Greenland is thinning at a slow enough rate that it didn’t exceed the detection limit and so wasn’t considered significant. Even so, the fast moving glaciers thinned dramatically, and in three areas the thinning has penetrated deep into the ice sheet – at the outlet glaciers Jakobshavn Isbrae, Helheim, and Kangerdlugssuaq, where thinning is detectable 120, 95, and 100 km inland. Jakobshavn Isbrae glacier is one of the largest outlet glaciers, draining 6.5% of the Greenland ice cap. Helheim and Kangerdlugssuaq are known for having accellerated unexpectedly and retreated from the coast rapidly in the 1990s and 2000s.

The Nature study’s conclusions on Antarctica provide another independent confirmation of what scientists have known for a long time – the West Antarctic Ice Sheet (WAIS) is thinning dramatically. In addition, there are areas of the East Antarctic Ice Sheet (EAIS) that are thinning as well, although the bulk of the EAIS appeared to be relatively stable between 2003 and 2007. And the authors found that some parts of the WAIS were actually thickening, even as the outlet glaciers were thinning.

These results led the authors to conclude that air temperatures are less a factor in the loss of ice mass than changes in ocean temperature and currents. The areas that thinned the fastest were those where the outlet glaciers ground out into the ocean in areas where warm water can melt the bottoms of the glacier, speeding it up by reducing friction on the sea bottom. The authors say

This is an apparently widespread phenomenon that does not require climate warming sufficient to initiate ice-shelf surface melt. (emphasis mine)

Furthermore, the authors conclude that the thinning of the ice sheets at their edges is “more sensitive, pervasive, enduring, and important than previously realized.”

graceEAIS-smWhile the laser altimetry study had difficulties determining the mass lost to thinning and melt, the GRACE satellite measures mass directly, albeit over a much larger spatial area than a reflected laser beam. And according to the study, published in the journal Nature Geoscience, not only has there been significant mass loss from the WAIS, the previously stable EAIS is apparently losing mass as well. If the EAIS were to melt significantly, it would add a massive amount of water to the oceans and dramatically increase sea level rise, especially since melting of the EAIS has not been included in the climate models.

Something to notice is that the two studies agree with each other qualitatively – the areas that are the reddest in the image above are the areas with the most thinning. They match the bluest areas in the figure at right, which have the greatest mass loss.

According to the GRACE study, the WAIS lost 132 ± 26 Gt of ice per year while the EAIS lost 57 ± 52 Gt per year over the period from 2002 to 2009. According to the paper, the large error in both regions is largely a result of limitations in the model of post-glacial rebound in Antarctica, but the larger EAIS error is a result of barometric pressure problems that occur over the EAIS but not over the WAIS. Regardless, the error is still smaller than the estimated loss of ice mass, and the combined total (190 ± 77 Gt per year) shows that the Antarctic ice sheets are losing mass.

Furthermore, the EAIS data from two areas where there’s been the most change show a potential breakpoint at around 2006, as seen below

graceEAIS06-09-sm

If a breakpoint really did occur, then the ice loss from 2006-2009 would be greater than the average over the period of 2002-2009. Instead of 190 ± 77 Gt per year, the actual loss could be as high as 220 ± 89 Gt per year.

Taken in combination, these two papers support the conclusion that sea level rise is very likely to be greater than the IPCC estimated in 2007, and may in fact exceed recent estimates. Combined with another paper that suggests ice sheets and sea level are more tightly coupled to climate changes and carbon dioxide than previously believed, these papers should probably be read as a serious warning about the future of the world’s coastal areas.

Thanks to Dr. Pritchard and Dr. Chen, primary authors on the two papers discussed above, for providing review copies of their work.

Image Credits
Nature
Nature Geoscience

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