Sea levels may rise higher than predicted due to global warming

Sea levels may rise higher than predicted due to global warming

mongabay.com

December 14, 2006

Global warming could cause sea levels to rise by four-and-a-half feet (140 cm) according to new projections published in Friday’s issue of the journal Science.

Stefan Rahmstorf, a scientist at the Potsdam Institute for Climate Impact Research in Potsdam, Germany, uses air temperature measurements and past sea level changes rather than computer models to calculate that ocean levels could rise by 50-140 cm by 2100, well above the 9-88 cm projected by the United Nations Framework Convention on Climate Change. A 140 cm rise in sea levels could swamp low-lying cities like New York and Venice while causing catastrophic flooding in Bangladesh and South Pacific island nations.

“A rise by over one meter by 2100 for strong warming scenarios cannot be ruled out… [but] very low sea level rise values as reported in the [IPCC report] now appear rather implausible in the light of the observational data,” wrote Rahmstorf in the Science paper. “The possibility of a faster sea level rise needs to be considered when planning adaptation measures such as coastal defenses, or mitigation measures designed to keep future sea level rise within certain limits.”

Rahmstorf added that the lowest plausible limit to sea level rise by 2100 is 38 cm — around a foot.

Rahmstorf says that sea level changes are dependent on poorly understood factors such as the rate at which glaciers and ice sheets melt in Greenland and Antarctica and the thermal expansion of sea water.

“Understanding global sea level changes is a difficult physical problem, as a number of complex mechanisms with different time scales play a role,” he wrote. “For this reason, our capability of calculating future sea level changes in response to a given surface warming scenario with present physics-based models is very limited, and models are not able to fully reproduce the sea level rise of recent decades.”

Rahmstorf says his approach to predicting future sea level rise is not unlike estimating the height of tides along a coast based on past observations.

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).

“We expect sea level to rise as the ocean takes up heat and ice starts to melt, until asymptotically a new
equilibrium sea level is reached,” Rahmstorf wrote. “Paleoclimatic data suggest that changes in the final equilibrium level may be very large: sea level at the Last Glacial Maximum 20,000 years ago was 120 m lower, while global mean temperature was 4-7 ºC lower. Three million years ago, during the Pliocene, the average climate was about 2—3°C warmer than today and sea level was 25—35m higher.”

Rahmstorf says that this data suggest that sea levels rise on the order of 10-30 meters per degree Celsius over the course of thousands of years. The U.N. has projected temperatures will rise by 1.4-5.8 C (2.5-10.4F) by 2100 due largely to human-induced emissions of greenhouse gases that result from fossil fuel combustion and deforestation.

This article uses quotes from “A Semi-Empirical Approach to Projecting Future Sea-Level Rise,” published in Sciencexpress, the online version of Science.