Facing a rising sea, and wondering how far to step back

Hanna

Tropical Storm Hanna pounds the pier at Oak Island, N.C. in 2008. Scientists say an exact estimate of how much the seas will rise as a result of global warming will not be had anytime soon but that policy makers must make plans for some increase. Photo courtesy William Redman/flickr.

4 November 2010

Updated 10 November 2010

Uncertainty over sea-level predictions makes planning for change difficult, but that's no excuse for inaction, says Princeton's Michael Oppenheimer.

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By Douglas Fischer

Daily Climate editor

ROTTERDAM, Netherlands - Yes, the seas are rising. But by just how much? 

A concrete answer sure could help coastal cities plan for the future. Problem is, nobody really knows, and a credible number isn't going to appear anytime soon. But that doesn't mean public officials should wait for one, warned Michael Oppenheimer, director of Princeton University's Program in Science, Technology and Environmental Policy.

Seas have been rising about 3 millimeters a year, with the Intergovernmental Panel on Climate Change estimating between six inches and two feet by century's end. 

But the IPCC view on sea-level rise - based on the best science as of 2007 - is limited: It accounts only for thermal expansion of the world's seas as rising global temperatures warm the seawater, the melting of mountain glaciers and the continuation of current melting on Greenland and Antarctica. It does not account for potential acceleration of the latter; dump the contents of just one of the world's major ice sheets into the ocean, and the equation changes dramatically.

Greenland and the West Antarctic sheet - considered the two most vulnerable - contain enough water to raise sea levels worldwide by 40 feet. 

But predicting if or how ice sheets in Greenland and Antarctica will melt is tricky business, Oppenheimer said.

And the sea will not rise evenly everywhere, he said. The ice sheets are so large they warp the Earth's gravitational pull; melting one alters the planet's equilibrium, causing the seas to rise more in certain regions while they may actually fall in others. All that fresh water could also warp ocean circulation patterns, further altering distribution. In one scenario, the Northeast United States could see almost two feet more sea-level rise than elsewhere in the globe.

The bottom line is that this makes for some extremely complex math, Oppenheimer said. And uncertainty about sea-level rise is going to be with us for a while: Policy makers aren't going to have a clear signal from the science until the ice sheets start moving.

By then, scientists caution, it will be too late to turn back.

"Not only is there uncertainty in the system, but we're stuck with whatever we build into the system," Oppenheimer said. "We shouldn't wait for scientific knowledge to get us out of this problem."

DailyClimate.org editor Douglas Fischer's trip to the Netherlands was paid for by the Dutch trade ministry. 

Photo of Michael Oppenheimer courtesy Heniz Awards

Correction (10 Nov 2010): An earlier version of this post misstated both the current sea-level rise and the components of the IPCC sea-level rise projections.

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