Grey Hautaluoma
Headquarters, Washington
202-358-0668
Leslie McCarthy
Goddard Institute for Space Studies, New York
212-678-5507
RELEASE: 07-37
NASA STUDY FINDS WARMER FUTURE COULD BRING DROUGHTS
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.
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