The future of Earth's seas changes with the research tides

Where's King Canute when you need him? The ancient Viking, legends tell, once commanded the tides from a throne that was trotted out onto the beach. A handy man to have in an age of global warming, when one of the most alarming concerns is that rising seas will one day drown the globe's coastal cities.

Whether those waters will be a flood or a trickle in this century, however, remains an open question among climate researchers. Dueling reports in science journals point to the different directions in which researchers are facing the rising seas.

Last year's Intergovernmental Panel on Climate Change report concluded with "very high confidence" that an average 2-foot sea level rise would erode coasts by 2100. By that time, higher ocean surface temperatures, as much as 5 degrees Fahrenheit warmer, would raise sea levels in two ways: by heating and expanding ocean waters and by melting glacial ice now resting on land. The same panel found that industrial releases of greenhouse gases, which trap heat in the atmosphere, along with deforestation and other activities would likely raise global average temperatures 3-to-7 degrees Fahrenheit by 2100.

But the IPCC report left out estimates of sea level rise from ice sheets breaking up and dropping into the ocean, such as the 2002 breakup (or "calving") of Antarctica's Larsen B ice shelf, which dumped 1,150 square miles of ice into the ocean in about three days. That exclusion made the IPCC's conclusions about rising sea levels inherently low-ball figures in the eyes of many scientists. A 2006 Science magazine analysis led by Jonathan Overpeck of The University of Arizona in Tucson, for example, found that a 6 to 10-foot sea-level rise worldwide by the end of the century might result if Greenland and Antarctic ice sheets melt.

So how much extra sea-level rise will result from ice sheets' calving? Not a lot, suggests a study of glacial melting rates out last week in Science. Led by Tad Pfeffer of the University of Colorado in Boulder, the study suggests the low-ball estimate is the better bet, with a 2.5-foot sea level rise "most likely" by 2100. That's an extra half-foot from ice sheets. If ice sheets susceptible to calving in Antarctica and Greenland break up, a worst-case scenario, global sea-level rise would reach about 6.5 feet by 2100. Anything above that, they conclude, is "physically untenable."

Another study out this week in Nature Geoscience looks at the question from another angle, focusing on the long retreat of the vast ice sheet that 20,000 years ago covered much of North America during the last Ice Age. Led by Anders Carlson of the University of Wisconsin, a team combines geological records of the ice sheet with climate models of the most likely atmospheric heating to come in this century.

The geologic records suggest the ice sheets retreated rapidly, in two waves that were millennia apart, in response to sudden changes in atmospheric temperatures during the summer. Plugging similar ice response to global warming in Greenland finds that increases up to 2 extra feet in sea-level rise might result from calving, contributing to a global sea-level rise of about 4 feet, concludes Anders.

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