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April 2005

CGD research:
Climate change is inevitable


Jerry Meehl. (Photo by Carlye Calvin.)

Even if concentrations of greenhouse gases were stabilized in the atmosphere after 2000, we would still face warmer temperatures and a rise in sea level over the next century. That's the conclusion of a new study by a team of CGD climate modelers published last month in Science.

The modeling study quantified the relative rates of sea level rise and global temperature increase that society is already committed to in the 21st century. With no additional increases in concentrations, globally averaged surface air temperatures would rise about a half-degree Celsius (1 degree Fahrenheit) and global sea levels would rise another 11 centimeters (4 inches) from thermal expansion alone by 2100

"Many people don't realize we are committed right now to a significant amount of global warming and sea level rise because of the greenhouse gases we have already put into the atmosphere," says lead author Jerry Meehl. "The longer we wait, the more climate change we are committed to in the future."

Jerry's co-authors were Warren Washington, Bill Collins, Julie Arblaster, Aixue Hu, Lawrence Buja, Warren Strand, and Haiyan Teng. The article, "How much more global warming and sea level rise," was published in the March 18 issue of Science (307, 5716: 1769-71).

Though temperature rise shows signs of leveling off 100 years after stabilization in the study, ocean waters continue to warm and expand, causing global sea level to rise unabated.

The inevitability of the climate changes described in the study is the result of thermal inertia, mainly from the oceans, and the long lifetime of carbon dioxide and other greenhouse gases in the atmosphere. Thermal inertia refers to the process by which the ocean heats and cools more slowly than air because of its greater heat capacity.

The new study is the first to quantify the commitment to future climate change using coupled global three-dimensional climate models. The CGD team ran the same scenario a number of times and averaged the results from two global climate models--the NCAR/Department of Energy Parallel Climate Model and the NCAR-based Community Climate System Model, version 3. Then they compared the results from each model.

The scientists also compared possible climate scenarios in the two models during the 21st century in which greenhouse gases continue to build in the atmosphere at low, moderate, or high rates. The worst-case scenario projects an average temperature rise of 3.5°C (6.3°F) and sea level rise from thermal expansion of 30 centimeters (12 inches) by 2100.

All scenarios analyzed in the study will be assessed by international teams of scientists for the next report by the Intergovernmental Panel on Climate Change, due out in 2007.

• by Anatta

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