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    What Can Past Warming Tell Us About Future Rainfall?

    By By Christa Marshall, E&E reporter
    January 31, 2013, 5:52:14 AM EST

    When it comes to how climate change influences rainfall, temperature may be only part of the puzzle.

    In a new study, scientists report that warming spurred by greenhouse gases causes less overall precipitation than similar warming caused by solar heating, such as what occurred hundreds of years ago.


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    The research provides an explanation of why the Medieval Warm Period (A.D. 1000-1250) was wetter than now, even though temperatures are higher today than they were then.

    "Warming due to increased greenhouse gases produces a climate signature different from that of warming due to solar radiation changes," states the study released yesterday in Nature.

    The new research should help scientists better predict rainfall patterns in the future, said Mark Cane, a professor at Columbia University's Lamont-Doherty Earth Observatory.

    It also should boost confidence in existing climate models by explaining discrepancies between the past -- when warming was caused by a natural variation in solar output -- and now, he said. The fact that the Medieval Warm Period didn't entirely jell with analyses of the future "seemed to be an indictment of the models," he said.

    The topic of how much precipitation will change for a given temperature rise has been a subject of "intense debate," the study notes.

    The scientists examined global precipitation changes over the last millennium and anticipated changes through the end of the century.

    Connection between rain and heat

    They reported that for roughly every 1 degree Celsius rise in global temperature, the global rainfall rate since the Industrial Revolution has been roughly 40 percent lower than past warming phases of the Earth.

    The reason that greenhouse gases affect rainfall differently than solar heating stems from a natural heating-cooling balance in the atmosphere, Cane said.

    Solar energy heats the Earth's surface, which in turn radiates that heat upward into the atmosphere and out into space. "The atmosphere loses heat this way," he said.

    He compared the process to a frying pan, in the sense that the heat originating from the burner flame eventually is radiated upward and away from the pan.

    The cooling effect on the planet of this heat radiation loss into space is countered by the warming effect of rainfall, he explained. "Without rain, the planet would cool and cool."

    Rain has this warming effect because it releases the heat that helped convert water into vapor in the first place, he said.

    For the planet to be in equilibrium, the heat from rain has to roughly equal the net cooling of the atmosphere, he said. In essence, that means the higher the cooling effect from heat loss into space, the higher the level of countering rainfall to maintain balance.

    The issue with greenhouse gases, he said, is that they trap heat more in the upper atmosphere, allowing less radiation to head into space than what is found with solar heating.

    "Today, the top [of the atmosphere] is more like the Earth's surface, because the greenhouse gases have kept the heat from getting out," Cane said.

    A further connection to trade winds

    This dynamic of less contrast between the surface and the top of the atmosphere in turn weakens trade winds, resulting in stronger warming in the eastern than the western Pacific -- a situation similar to El Niño conditions, the scientists said. That results in a weaker east-west gradient with greenhouse gas warming.

    "For the same average global surface temperature increase, the weaker [sea surface temperature] gradient produces less rainfall, especially over tropical land," Bin Wang, a study co-author and a professor at the University of Hawaii, Manoa, said in a statement.

    In the Medieval Warm Period, for example, rainfall over tropical land rose 5.5 percent for a 1 C rise in global mean temperature caused by solar and volcano forces, compared with a 2.4 percent increase in rainfall in contemporary times, according to the study.

    With solar radiation, on the other hand, the eastern Pacific remains cool, creating a sharper difference with the western part of the ocean, according to the study. That, in turn, helps boost global rainfall more than what occurs with greenhouse-gas-fueled warming.

    "The climate system [with greenhouse gas heating] has to find a way to have it rain less ... and it seems to use the tropical Pacific to accomplish this," Cane said.

    Cane said the research raises questions about the idea of cooling the Earth through some geoengineering techniques, such as injecting particles into the upper atmosphere. That might lower temperatures, but it wouldn't eliminate the heat-trapping ability of still-present greenhouse gases in the atmosphere.

    Their ongoing presence would mean that rainfall patterns might not come back to normal, even if geoengineering generally lowered the temperature, he said.

    The study should not lower concerns about global warming or shift general assumptions about rainfall patterns with climate change, Cane said. Just because the present is somewhat different than the past doesn't mean the present is less problematic, he said.

    The study notes a 2007 assessment from the Intergovernmental Panel on Climate Change, which reported that climate change now is still likely to increase precipitation intensity and increase precipitation in the high latitudes and the tropics.

    Reprinted from ClimateWire with permission from Environment & Energy Publishing, LLC. 202-628-6500. E&E Publishing is the leading source for comprehensive, daily coverage of environmental and energy issues. Click here to start a free trial to E&E's information services.

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