Climate Change Could Cause Rapid Rise in Sea Levels

Before the last ice age, global sea level was as much as 20 feet higher than it is today. Global warming is likely to push air temperatures back to the levels found 129,000 years ago in the next century so sea levels could rise much faster than expected.

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MELISSA BLOCK, host:

This is ALL THINGS CONSIDERED, from NPR News. I'm Melissa Block.

ROBERT SIEGEL, host:

And I'm Robert Siegel. Research that jangled nerves around the world about global warming way back in the 1970s may have been dead on. A new study says the huge glaciers in Antarctica and Greenland could start melting substantially in our children's lifetimes. The impact would not be felt overnight. But eventually the world's sea level could rise by 20 feet or more. NPR's Richard Harris reports.

RICHARD HARRIS reporting:

By filling the air with carbon dioxide, humans are running an experiment with planet Earth. The best way to guess how it's going to turn out is to look back, to see what the Earth was like last time it really heated up, that time due to natural causes. Ellen Mosley-Thompson, at Ohio State University, says that was 129,000 years ago, between the last two Ice Ages.

Professor ELLEN MOSLEY-THOMPSON (Geography, The Ohio State University): We know that sea level was five or six meters higher at that time. And the question is, where did that come from?

HARRIS: The answer, of course, is that water came from melted ice. And that raises some questions. Where exactly did that ice come from, and how likely is it to happen again? In 1978, Mosley-Thompson's friend and colleague, John Mercer, put that issue squarely on the table with a prescient scientific paper.

Professor MOSLEY-THOMPSON: It was called West Antarctic Ice Sheet and CO2 Greenhouse Effect: A Threat of Disaster?

HARRIS: Mercer died years ago, but scientists have spent the last three decades trying hard to address the issues he raised. Two papers in the journal Science now take a sophisticated new look at the earth back then, and also peek into the future. Jonathan Overpeck, at the University of Arizona, says it appears that melting 129,000 years ago was actually biggest in the Northern Hemisphere.

Professor JONATHAN T. OVERPECK (Geosciences, University of Arizona): Most of southern Greenland melted, and then around the margins of Greenland melted, and all of the, almost all of the other ice fields, such as in Canada and Iceland, melted at that time as well.

HARRIS: But Overpeck doesn't think that was enough to explain all the sea level rise. Instead, his simulation suggests that some of the water came from melting ice in Antarctica as well.

Professor OVERPECK: It appears that the ocean warmed, and that the big ice shelves around much of West Antarctica were warmed from below, and that also the sea level rise driven by melting ice in the Northern Hemisphere was able to help float the big ice shelves in and around Antarctica.

HARRIS: Not only did that floating ice melt, but ice behind it on the continent was then freed to flow into the ocean. At least, that's what Overpeck's computer simulations suggest. Now, this was exactly the scenario that caused John Mercer to write his alarming scientific paper back in 1978. Overpeck and his colleagues now turn around the problem to make a projection based on that history.

Professor OVERPECK: If we get the same level of Arctic warming we had 129,000 years ago, we're likely to get at least four to six meters of sea level rise.

HARRIS: So 13 to 20 feet, and current estimates are that the earth will get that warm sometime between 2050 and 2100. Co-author Richard Alley, at Penn State University, says the big question is how fast the ice will respond to that warming. Right now, sea level increases about an inch a decade. But Alley says that could easily accelerate to…

Professor RICHARD B. ALLEY (Geosciences, Penn State University): That's the hard part. We don't really have the answer that everyone would like, which is how much, how fast? It's one of these things that when we're done chatting I need to hang up and get back to work, because this really motivates me. This is a reason to get out of bed in the morning.

HARRIS: Presumably this ice melt will happen over the course of several centuries. But glaciers have a way of moving suddenly and unexpectedly. In fact, scientists have witnessed some unexpected behaviors in both Antarctica and in Greenland over the past few years. And, again, that was exactly what John Mercer had fretted about in his landmark paper of 1978.

Richard Harris, NPR News.

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