Over the past 30 years, temperatures in the Arctic have been creeping up, rising half a degree Celsius with attendant increases in glacial melting and decreases in sea ice. Experts predict that at current levels of greenhouse gases--carbon dioxide alone is at 375 parts per million--the earth may warm by as much as five degrees Celsius, matching conditions roughly 130,000 years ago. Now a refined climate model is predicting, among other things, sea level rises of as much as 20 feet, according to research results published today in the journal Science.
Modeler Bette Otto-Bliesner of the National Center for Atmospheric Research in Boulder and paleoclimatologist Jonathan Overpeck of the University of Arizona matched results from the Community Climate System Model and climate records preserved in ice cores, exposed coral reefs, fossilized pollen and the chemical makeup of shells to determine the accuracy of the computer simulation. Roughly 130,000 the Arctic enjoyed higher levels of solar radiation, leading to increased warming in the summer and the retreat of glaciers worldwide. The model correctly predicted the extent of the resulting Arctic ice melt, enough to raise sea levels by roughly nine feet.
"Getting the past climate change correct in these models gives us more confidence in their ability to predict future climate change," Otto-Bliesner says. "These ice sheets have melted before and sea levels rose. The warmth needed isn't that much above present conditions."
But sea levels rose as much as 20 feet 130,000 years ago and Overpeck speculates that may have been the result of additional melting in Antarctica. After all, the ice there is not all landlocked; some rests in the ocean and a little warming in sea temperatures could melt it or pry it loose. And this time around, the warming is global, rather than concentrated in the Arctic. "In the Antarctic, all you have to do is break up the ice sheet and float it away and that would raise sea level," he says. "It's just like throwing a bunch of ice cubes into a full glass of water and watching the water spill over the top."
Such a sea level rise would permanently inundate low-lying lands like New Orleans, southern Florida, Bangladesh and the Netherlands. Already sea level rise has increased to an inch per decade, thanks to melting ice and warm water expansion, according to Overpeck. And evidence that the Arctic is exponentially warming continues to accumulate. Indeed, in another paper in the same issue of Science, Goran Ekstrom of Harvard University reported a marked increase in so-called glacial earthquakes (seismic events recorded throughout the world when Greenland's glaciers slip past rock) since 2002. In fact, last year alone saw twice as many quakes as in previous years, with most of that increase coming during the summer months.
"We need to start serious measures to reduce greenhouse gases within the next decade," Overpeck says. "If we don't do something soon, we're committed to [13 to 20 feet] of sea level rise in the future."