Image courtesy of W. T. Pfeffer, INSTAAR/University of Colorado

Big trouble. Glaciers, including Columbia in Alaska, may contribute more to sea level rise than previously thought.

Seas to Rise Faster This Century

Dick writes about Earth and planetary science for Science magazine.

Hot on the heels of findings that tropical cyclones have been intensifying over the past few decades (ScienceNOW, 3 September), researchers report tomorrow in Science that global warming will cause sea levels to rise much faster by the end of the century than officially projected. The rising temperatures will cause the oceans to swell with melted glacial ice, the study finds, likely flooding substantial portions of Florida and Bangladesh, as well as many other low-lying, densely populated areas of the world.

Warming glaciers raise sea level in two main ways. They add more water as they melt, and they also add water when ice breaks off from glacial flows. The incidence of this latter phenomenon has soared in recent years for some glaciers draining the southern Greenland Ice Sheet, much to the mystification of glaciologists. Unable to model such accelerated ice losses, members of the Intergovernmental Panel on Climate Change declined to include them in their widely cited projection of up to 60 centimeters of sea level rise by 2100 (ScienceNOW, 6 April 2007).

Glaciologist W. Tad Pfeffer of the University of Colorado, Boulder, and his colleagues tackled glacier flow anyway. They calculated how fast glaciers would have to flow in order to raise sea level by a given number of meters and then considered whether those flow rates were plausible or even physically possible. In Greenland, they calculated ice loss through specific rock-bounded "gates," which are carved in the edges of the island. In West Antarctica, the gates are not well defined, so the team used approximations of how flow might respond to rising temperatures. The resulting "improved estimate" ranges from 80 centimeters to 200 centimeters of sea level rise by the end of the century. That being said, estimates of several meters of sea level rise made by some other researchers are "physically untenable" because not enough ice could be pushed through the glacial gates, according to the paper's authors.

The new, higher sea level rise "is a useful number," says glaciologist Richard Alley of Pennsylvania State University in State College. "It's a reality check." Geoscientist Michael Oppenheimer of Princeton University agrees. "Where these guys end up is plausible." Even 1 meter of sea level rise "is a big deal," he notes, as it would threaten people in many parts of the world.

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