Are human-caused and natural global warming different? Study says yes.
A study suggests that human-caused and natural global warming episodes affect rainfall rates differently. The finding could help scientists better forecast what's ahead.
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As the water vapor condensed, it released as heat the energy that had turned it into a vapor. This heating fueled further convection, boosting the build-up of thunderstorms with intense rains. The collective action of powerful updrafts feeding the thunderstorms strengthened the surface trade winds that blow from east to west. Those prevailing winds encouraged stronger upwelling of deep, cold ocean water along the South American coast, and that sharpened the contrast in sea-surface temperatures between the eastern and western tropical Pacific.Skip to next paragraph
In Pictures Weather extremes 2013
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Greenhouse gases' new wrinkle
Dr. Liu's team showed that greenhouse gases have thrown a wrinkle into the process.
Instead of heating the surface of the western tropical Pacific – as happened the Medieval Warming Period – greenhouse gases heat the lowest layer of the atmosphere, or troposphere. This reduces the temperature contrast between the ocean surface and the air above it, easing the pace of convection and throttling back on the level of thunderstorm activity.
The trade winds relax, the upwelling slows, and the temperature contrast between the eastern and western tropical Pacific surface waters ease. The relaxed trade winds also allow the warm pool to migrate east, shifting its position along the equator in ways that allow its convective activity to alter atmospheric circulation patterns far from this center of action.
The contrast in sea-surface temperatures in the eastern and western tropical Pacific – and the location of the warmest water – can significantly shift regional rainfall patterns beyond the tropics.
Previous research into what should happen in the tropical Pacific as greenhouse gases rise had yielded opposite conclusions.
One model found that the temperature difference between the cooler eastern tropical Pacific and the warmer west intensified in a warmer climate, according to a study by Mark Cane, a climate researcher at Columbia University's Lamont-Doherty Earth Observatory in Palisades, N.Y., and a member of the team reporting the new results in the current issue of the journal Nature.
But a different, more detailed model came to the opposite conclusion that the temperature contrast would ease, according to a study by Jerry Meehl at the National Center for Atmospheric Research.
"You'd think, 'OK, we've had increasing greenhouse gases for quite awhile, and we should be able to have observations to show us how the tropical Pacific is responding,' " Dr. Meehl says. "But the observations are not that good going back into the earlier parts of the last century."
Now, the new study suggests that "we are all right," he says.
The results also may provide fresh reasons to be wary of "geoengineering" – attempts to stem global warming by changing the earth, for example building an umbrella of tiny aerosol particles high into the atmosphere to reflect sunlight back into space.
"One conclusion from this work would be: Don't fool yourself into thinking that will simply undo the damage that extra greenhouse gases do," says Dr. Cane.
If the only goal is to reduce global average temperatures, "you could probably do it," he says. But, he adds, attempts control the amount of radiation reaching Earth's surface "will not restore everything back to where it was" – in this case, precipitation patterns.