18 March 2005

Global Warming Unstoppable for 100 Years, Study Says

Even if humans stop burning oil and coal tomorrow, we've already spewed enough greenhouse gases into the atmosphere to cause temperatures to warm and sea levels to rise for at least another century.

That's the message from two studies appearing in tomorrow's issue of the journal Science.

Researchers used computer models of the global climate system to put numbers to the concept of thermal inertia - the idea that global climate changes are delayed because it water takes longer to heat up and cool off than air does. The oceans are the primary drivers of the global climate.

"Even if you stabilize the concentration of greenhouse gases, you are still committed to a certain amount of climate change no matter what you do because of the lag in the ocean," said Gerald Meehl, a climate scientist with the National Center for Atmospheric Research in Boulder, Colorado.

Greenhouse gases such as carbon dioxide collect in the atmosphere and are believed to act as a blanket, trapping heat and causing the Earth to warm. To stop this warming, many scientists say humans must reduce the amount of greenhouse gases they emit.

Human activities that make the largest contributions to greenhouse gases include exhaust fumes from automobiles and commercial jets and emissions from power stations and factories.

"The longer you wait to do something, the more climate change you are committed to in the future," Meehl said.

Warming Lag
Meehl co-authored one of the Science studies. He and his colleagues found that even if no more greenhouse gases are added to the atmosphere, globally averaged air surface temperatures will rise about 1 degree Fahrenheit (0.5 degree Celsius) and global sea levels will rise at least 4.3 inches (11 centimeters) by 2100.

The sea level rise estimate is conservative, because the models Meehl and colleagues used only account for thermal expansion?water expands as it warms, causing sea levels to rise. Melting glaciers and ice sheets will likely at least double the sea level rise.

Since humans are unlikely to stop pumping greenhouse gases into the atmosphere anytime soon, Meehl and his colleagues also ran their computer models under scenarios in which the gases continue to accumulate at low, medium, and high rates.

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.

Reversing Direction
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 with?a 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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