If there's one thing that will get climate modelers steamed it's the question of how much water vapor will worsen the global warming caused by carbon dioxide. In the 26 April issue of Science, researchers report that the 1991 eruption of Mount Pinatubo in the Philippines has helped them get a handle on this question.
Conventional models of global warming assume that rising temperatures will pack more water vapor into the air, providing an extra layer of insulation that will heat up the atmosphere even more. But critics complain that no one has seen this happen--and if it doesn't, temperature hikes from carbon dioxide will be much smaller than predicted by current climate models. When Mount Pinatubo erupted, researchers were able to track global changes in water vapor and temperature. Although the natural experiment was a bit backwards--sulfate particles from the eruption cooled the globe--it still offered a way to check the climate models.
Geophysicist Brian Soden of the Geophysical Fluid Dynamics Laboratory in Princeton, New Jersey, and colleagues compared satellite observations of atmospheric water vapor after the eruption with the changes predicted by a standard climate model. Because colder air holds less water, the atmosphere dried after the eruption--essentially a reverse greenhouse effect. Soden's team found that the model did a good job of predicting satellite measurements of the average global cooling. But when the team removed the water vapor feedback loop from the model, it predicted milder cooling. And the cooling slacked off by the same amount that researchers expect water vapor would enhance warming, says Soden.
The new research validates the way climate models treat the connection between surface temperature, the amount of water vapor, and its greenhouse effect, says climate researcher V. Ramanathan at the Scripps Institution of Oceanography.