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February 12, 2007
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.
Writer: Mike Bettwy, NASA Goddard Space Flight Center
##
Contact:
Leslie McCarthy
NASA Goddard Institute for Space Studies
212-678-5507
Leslie.M.McCarthy@nasa.gov
This text is
derived from:
http://www.nasa.gov/home/hqnews/2007/feb/HQ_0737_Solar_Climate.html
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