Catastrophic sea level rise could occur with only two degrees Celsius warming

Jeremy Hance
December 17, 2009

Allowing the climate to rise by just two degrees Celsius—the target most industrialized nations are currently discussing in Copenhagen—may still lead to a catastrophic sea level rise of six to nine meters, according to a new study in Nature. While this rise in sea levels would take hundreds of years to fully occur, inaction this century could lock the world into this fate.

To discover how sea levels will react to 'moderate' warming, scientists looked to the past. Researchers turned back the clock approximately 125,000 years to the last interglacial stage when polar temperatures were likely warmer than today by 3 to 5 degrees Celsius (or 5 to 9 degrees Fahrenhiet). Since temperatures in the poles rise faster than the global average, scientists say this is approximately what they expect of polar temperatures if today's global temperatures are allowed to reach 2 to 3 degrees above preindustrial levels.

"The last interglacial stage provides a historical analog for futures with a fairly moderate amount of warming; the high sea levels during the stage suggest that significant chunks of major ice sheets could disappear over a period of centuries in such futures," explains lead author Robert Kopp, who conducted the work as a postdoctoral researcher in Princeton's Department of Geosciences and Woodrow Wilson School of Public and International Affairs. "Yet if the global economy continues to depend heavily on fossil fuels, we're on track to have significantly more warming by the end of century than occurred during the last interglacial."

Rate of sea level rise computed directly from the climate model simulation (red), and estimated using Eq. 1 driven by the global mean temperature from the climate simulation (blue).
In the study researchers determined that there is 95 percent probability that during the last interglacial stage global sea level peaked at more than 6.6 meters (22 feet) above today's present level. On the other end of the spectrum, they found that there was only a one-in-three chance that global sea level was higher than 9.4 meters (31 feet) 125,000 years ago, providing them with the range of approximately 6-9 meters.

Given that the last interglacial period was relatively recent, researchers believe it may be a good indicator of how polar sheets, and thereby sea levels, will respond to current warming. However, one of the major caveats of the study, according to the researchers, is that they are uncertain how long polar ice sheets would need to be exposed to peak temperatures in order to cause sea levels to rise to this extent.

"Despite the uncertainties inherent in such a study, these findings should send a strong message to the governments negotiating in Copenhagen that the time to avoid disastrous outcomes may run out sooner than expected." Michael Oppenheimer said, a professor of geosciences and international affairs in Princeton's Woodrow Wilson School.

In the US a sea level rise of 6-9 meters would permanently submerge New Orleans, most of southern Florida, and part of the East Coast. Low-lying islands—now pushing in Copenhagen for a target of 1.5 degrees Celsius—would sink meters underwater, while the ocean would come to cover most of Bangladesh and the Netherlands.

Citation: R. E. Kopp, Frederik J. Simons, J. X. Mitrovica, A. C. Maloof & M. Oppenheimer. Probabilistic assessment of sea level during the last interglacial stage. Nature, 462, 863–867, 2009, doi:10.1038/nature08686.

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Jeremy Hance
mongabay.com (December 17, 2009).

Catastrophic sea level rise could occur with only two degrees Celsius warming.