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Warmer is not always wetter

Compared to global warming caused by solar radiation, global warming caused by greenhouse gases results in less rainfall, simulations suggest

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Not all warming is the same. For the same increase in temperature, global warming caused by greenhouse gases results in less rainfall than does warming caused by the sun’s radiation, climate simulations suggest.

Because wet places should get more rain as the climate heats up, the new results may explain the mystery of why a warm period 1,000 years ago was wetter than the warm late 20th century. Jian Liu of the Chinese Academy of Sciences in Nanjing and colleagues describe these results in the Jan. 31 Nature.

“If what they show holds up, it’s good news in that it helps reconcile an apparent contradiction,” says oceanographer Gabriel Vecchi of the National Oceanic and Atmospheric Administration’s Geophysical Fluid Dynamics Laboratory in Princeton, N.J. But it would also limit how scientists can use some past episodes of climate change as analogs for the future, he says.

During the Medieval Warm Period from 1000 to 1250, temperatures rose because the sun started to release more radiation and there were few volcanic eruptions spewing sun-blocking aerosols into the atmosphere. Climate simulations indicate global precipitation was higher during this warm spell than at the end of the 20th century, when temperatures increased because of rising levels of carbon dioxide and other greenhouse gases.

Liu and colleagues investigated this rain disparity by simulating climate over the last millennium, comparing the effects of solar radiation during pre-industrial times with those of greenhouse gases during the industrial period. For every one degree Celsius increase in temperature, greenhouse gases boosted global rainfall by 1.2 to 1.3 percent. Meanwhile, solar warming caused a 2.1 percent increase in rainfall.

These rainfall differences may be a consequence, the researchers say, of how solar radiation and greenhouse gases alter the distribution of heat in the atmosphere and influence sea surface temperatures in the tropical Pacific Ocean — factors that have a big effect on rainfall. 

Warming of any kind increases global rainfall by raising surface temperatures, the researchers reason, and by increasing the difference in temperature between Earth’s surface and the upper boundary of the lower atmosphere. But greenhouse gas warming counteracts the effect somewhat because the gases also trap heat in the atmosphere. The result is a smaller heat difference than would result from solar warming, the researchers say.

In turn, these heat differences appear to affect temperatures in the Pacific. Normally, the western Pacific is several degrees warmer than the eastern Pacific. With solar warming, trade winds may strengthen and amplify this temperature gradient. This leads to La Ni±a-like conditions in which rainfall increases in already wet regions, thereby increasing the total amount of rainfall on Earth. Fossil corals suggest a temperature change like this may have occurred during the really wet Medieval Warm Period, says study coauthor Mark Cane, a climate scientist at Columbia’s Lamont-Doherty Earth Observatory in Palisades, N.Y.

During greenhouse-gas warming, the simulations indicate, the opposite trend in the Pacific occurs: The temperature differential between the eastern and western Pacific shrinks. This shifts normal rainfall patterns east, but doesn’t lead to the same kind of rainfall intensification seen during solar warming.

Cane says the team needs to do much more work to really understand how warming influences all of the mechanisms that interact to modify sea surface temperatures in the Pacific and thus the Earth’s rainfall. But the new findings already could influence attempts to mitigate climate change, he says. If countries try to cool the planet by somehow shielding it from incoming solar radiation, that won’t reverse all of global warming’s changes. “We don’t have a way of exactly undoing the effects of greenhouse gases,” he says.

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