The Washington Post considers one of the challenges in climate change modeling: how much can we expect sea level to rise? If the Greenland and West Antarctic ice sheets melted completely, sea level could rise about 39 feet. However, the current consensus predicts something well short of that. Most estimates range from several inches to several feet over the next century, with a few outliers.
The divergent predictions derive from the problems inherent in modeling glaciers. Most melting occurs on the edges of glaciers, where an ice flow meets the ocean. How quickly melting occurs depends on the interaction of sea, ice, and topography.
David Vaughan, a glaciologist with the British Antarctic Survey in Cambridge, said the terrain beneath the ice streams helps determine how they move, but the contours of the land are largely unknown because it is buried so far under the ice. The streams may run aground on elevated bedrock, slow down as they move past rocky fjord walls or speed up as they move over mud.The good news is that there seems to be more effort and funding to collect data on the edges of ice sheets and develop the computer models to interpret it. In the meantime, we will have continued uncertainty about what to expect.
Researchers are also trying to measure the layer of water that lies under the ice sheets, as that also helps regulate ice stream flows.
"They're essentially afloat on their own sub-glacial water, even if there's not much water there," said Garry Clarke, a glaciology professor at the University of British Columbia. "We don't know very much about how water flows underneath ice sheets."
Another uncertainty is how much the oceans surrounding the ice sheets are warming, something that is difficult to measure because the areas are remote. Vaughan and his colleagues suspect that warmer waters around Antarctica have contributed to melting the Western Antarctic ice sheet, but there is little good data because few ships venture there.
Even with better data, scientists find it difficult to enter the information into computer models. Most models do not attempt to calculate what could happen to ice sheets at their edges.
Adding to the challenge, Oppenheimer said, is that models "are only good at explaining things that happen at a large scale. Ice sheets are very complex beasts, and the water moves at a very small scale."
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Update: Here is an explanation of why climate change science is sound (even if sea level predictions vary).