Under the worst-case scenario, by 2100 average temperatures are projected to rise by 6.3 degrees Fahrenheit (3.5 degrees Celsius) and sea level by at least 12 inches (30 centimeters).
The second study was authored by Tom Wigley, also with the National Center for Atmospheric Research. He looked at what would happen to temperatures and sea levels if greenhouse gas concentrations stay constant and if humans continue to add greenhouse gases into the atmosphere.
Under the fixed-concentration scenario, the surface air temperature rise could exceed 1.8 degrees Fahrenheit (1 degree Celsius) by 2400, and sea levels may rise at a rate of 4 inches (10 centimeters) per century.
If humans keep emitting greenhouse gases at present rates, the surface air temperatures could rise between 3.6 and 10.8 degrees Fahrenheit (2 and 6 degrees Celsius) by 2400, and sea levels may edge up at a rate of 9.8 inches (25 centimeters) per century.
"Avoiding these changes requires, eventually, a reduction in emissions to substantially below present levels," Wigley writes in Science.
Gavin Schmidt is a climate modeler with the NASA Goddard Institute for Space Studies in New York. He was not involved with either of the studies, but he is not surprised by the thermal-inertia warming demonstrated by the models.
"We have been talking about this for years," he said. Schmidt added that several other teams, including his own, are planning to publish modeling studies in the months ahead that will show similar results.
Like Meehl's team, Schmidt said climate scientists are up against a deadline to publish their results so that they will be included in the next report of the Intergovernmental Panel on Climate Change, which is due out in 2007. For inclusion, results must be published by the end of 2005.
"All of them are going to show very similar numbers. These are just the first out of the gate," he said.
Meehl said the modeling results demonstrate the contribution humans have already made to warming temperatures and rising sea levels. To reverse this trend would require humans to at least stabilize the concentration of greenhouse gases in the atmosphere, which means a significant reduction in the amount of oil and coal humans burn each year.
"I'm not an expert in the policy area, but it doesn't seem likely to happen in the next few years," he said. Rather, he added, global warming is a multigenerational problem: The choices we make now set the stage for what our grandchildren will be forced to deal witha little warming and sea level rise, or a lot.
According to Wigley, the most alarming aspect of his study is the finding of a 4-inch-per-century (10-centimeter-per-century) rise in sea level under the fixed-concentration scenario.
"Although such a slow rate may allow many coastal communities to adapt, profound long-term impacts on low-lying island communities and on vulnerable ecosystems such as coral reefs seem inevitable," he writes.
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