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NASA Study
Finds Warmer Future Could Bring Droughts
02.12.07-NASA
scientists may have discovered how a warmer climate in the future could increase
droughts in certain parts of the world, including the southwest United States.
The researchers compared
historical records of the climate impact of changes in the sun's output with
model projections of how a warmer climate driven by greenhouse gases would
change rainfall patterns. They found that a warmer future climate likely will
produce droughts in the same areas as those observed in ancient times, but
potentially with greater severity.
"These findings strongly
suggest that greenhouse gases and long-term changes in solar activity both can
have major influences on climate via similar processes," said Drew Shindell,
NASA Goddard Institute for Space Studies, New York. Shindell is lead author of a
paper that appeared in the Dec. 27, 2006, issue of the American Geophysical
Union's Geophysical Research Letters.
"There is some evidence that
rainfall patterns already may be changing," Shindell added. "Much of
the Mediterranean area, North Africa and the Middle East rapidly are becoming
drier. If the trend continues as expected, the consequences may be severe in
only a couple of decades. These changes could pose significant water resource
challenges to large segments of the population."
Using the NASA Goddard Institute
for Space Studies climate model, researchers found that changes in solar output
in the ancient past increased surface warming and altered atmospheric moisture
and circulations. These changes likely led to the severe droughts seen in
paleoclimate records.
The same model showed that
greenhouse-gas warming has similar effects on the atmosphere, suggesting drier
conditions may become more common in the subtropics. Rainfall could decrease
further in already water-stressed regions such as the southwest United States,
Mexico, parts of North Africa, the Middle East, and Australia. Meanwhile,
precipitation may increase across the western Pacific, along much of the equator
and in parts of southeast Asia.
The computer model considers
changes in the oceans, weather, and chemistry of the atmosphere, like ozone
concentrations, and accurately reproduced the broad rainfall shifts toward
regionally drier or wetter conditions during the past several hundred years.
Sunspot and ice core data also link the historical rainfall shifts to variations
in the amount of energy released by the sun. Since the size of solar changes is
uncertain, the study focused on the location and pattern of precipitation
shifts, not their precise amount.
Increases in solar output break
up oxygen molecules, raising ozone concentrations in the upper atmosphere. This
adds to upper atmospheric heating that leads to shifts in circulations down to
the surface. In turn, surface temperatures warm, and the Earth's basic rainfall
patterns are enhanced. For instance, in wet regions such as the tropics,
precipitation usually increases, while dry areas become more prone to drought
since rainfall decreases and warmer temperatures help remove the small amount of
moisture in the soil.
"Precipitation is hard to
predict because it is so highly variable, but these results increase our
confidence that continued warming will be associated with large-scale changes in
rainfall," said Shindell.
Researchers also considered
numerous tree-ring, fire, and lake sediment records from across the Americas,
including Mexico, Peru, and the Yucatan Peninsula. These data are reliable
indicators of historical climate and confirm a pronounced increase in drought
frequency in the southern United States, Mexico, and other subtropical locations
during periods of increased solar output in the past 1,200 years. This long-term
record of solar output is based on chemical isotopes whose production is related
to the sun's brightness. Conversely, in parts of the tropics, ocean sediment
data, key indicators of precipitation changes, reflect increased rainfall.
According to the researchers, the
same processes identified by this new research very likely also affected past
civilizations, such as the Pueblo people of New Mexico and Arizona who abandoned
cities in the 1300s.
Credit: NASA Goddard Institute for Space
Studies
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