Seas were nearly 10 metres higher than now in previous interglacial period.
With climate talks stalling in Copenhagen, a study suggests that one problem, sea level rise, may be even more urgent than previously thought.
Robert Kopp, a palaeoclimatologist at Princeton University in New Jersey, and his colleagues examined sea level rise during the most recent previous interglacial stage, about 125,000 years ago. It was a time when the climate was similar to that predicted for our future, with average polar temperatures about 3-5°C warmer than now.
Other studies have looked at this era, but most focused on sea level changes in only a few locales and local changes may not fully reflect global changes. Sea level can rise, for example, if the land is subsiding. It can also be affected by changes in the mass distribution of Earth. For example, says Kopp, ice-age glaciers have enough gravity to pull water slightly polewards. When the glaciers melt, water moves back towards the Equator. To adjust for such effects, Kopp's team compiled sea-level data from over 30 sites across the globe.
"We could go to a lot of different places and look at coral reefs or intertidal sediments or beaches that are now stranded above sea level, and build a reasonably large database of sea-level indicators," says Kopp.
The team reports1 in Nature today that the sea probably rose about 6.6–9.4 metres above present-day levels during the previous period between ice ages. When it was at roughly its present level, the average rate of rise was probably 56–92 centimetres a century. "[That is] faster than the current rate of sea level rise by a factor of about two or three," Kopp says, warning that if the poles warm as expected, a similar accelleration in sea-level rise might occur in future.
The study is "very sophisticated", says Peter Clark, a geologist at Oregon State University in Corvallis. "A lot more of the existing ice sheets at the time must have melted than was thought to be the case," he says, such as parts of Greenland and Antarctica.
The implications are disconcerting, says Clark. If the world warms up to levels comparable to those 125,000 years ago, "we can expect a large fraction of the Greenland ice sheet and some part of the Antarctic ice sheet, mostly likely West Antarctica, to melt. That's clearly in sight with where we're heading."
Jonathan Overpeck, a climate scientist at the University of Arizona in Tucson agrees. "Earth's polar ice sheets may be more vulnerable to climate change than commonly believed," he says.
Furthermore, even if global warming causes seas to start rising toward the levels seen 125,000 years ago, there is no reason to presume that it will proceed at the relatively sedate rate of 6-9 millimeters a year seen by Kopp's study. In part, that's because his study didn't have the resolution to spot changes on a year-by-year basis, so there's nothing to say that the rise during the last interglacial didn't occur in shorter, faster spurts, undetectable in Kopp's data.
Near future warming will also be driven by potentially faster-moving processes than those of the last interglacial. "The driver of [climate change during the last interglacial period] was slow changes in Earth's orbit, happening over thousands of years," says Stefan Rahmstorf, an ocean scientist at the Potsdam Institute for Climate Impact Research in Germany. "We're now set to cause several degrees of global warming within just a century. I would expect this to drive a much faster sea level rise."
Some scientists think that we may already be committed to a future with higher seas than had been expected. "There could be a global warming tipping point beyond which many metres of sea level rise is inevitable unless global greenhouse-gas emissions are cut dramatically, and soon," warns Overpeck.
"I have spent a lot of time talking with national security decision-makers in this country and abroad about the security implications of climate change," says Marc Levy, deputy director of the Center for International Earth Science Information Network at Columbia University's Earth Institute in New York. "I've consistently witnessed an inability on their part to take sea-level risks seriously. This study helps frame the risks in ways that decision-makers can better understand."
Kopp, R. E., Simons, F. J., Mitrovica, J. X., Maloof, A. C. & Oppenheimer, M. Nature 462, 863-867 (2009).