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Global 'Sunscreen' Has Likely Thinned, Report NASA
Scientists
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03.15.07
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A new NASA study has found
that an important counter-balance to the warming of our planet
by greenhouse gases – sunlight blocked by dust, pollution and
other aerosol particles – appears to have lost ground.
Image above: The average amount
of dust, pollution and other aerosol particles in the
atmosphere has dropped since the 1990s. Global averages were
relatively low in the period 2002 to 2005, shown here (highest
aerosol levels in light blue, lowest in purple). Credit: NASA
Global Aerosol Climatology Project
The thinning
of Earth’s “sunscreen” of aerosols since the early 1990s could
have given an extra push to the rise in global surface
temperatures. The finding, published in the March 16 issue of
Science, may lead to an improved understanding of recent
climate change. In a related study published last week,
scientists found that the opposing forces of global warming
and the cooling from aerosol-induced "global dimming" can
occur at the same time.
"When more sunlight can get
through the atmosphere and warm Earth's surface, you're going
to have an effect on climate and temperature," said lead
author Michael Mishchenko of NASA's Goddard Institute for
Space Studies (GISS), New York. "Knowing what aerosols are
doing globally gives us an important missing piece of the big
picture of the forces at work on climate."
The study
uses the longest uninterrupted satellite record of aerosols in
the lower atmosphere, a unique set of global estimates funded
by NASA. Scientists at GISS created the Global Aerosol
Climatology Project by extracting a clear aerosol signal from
satellite measurements originally designed to observe clouds
and weather systems that date back to 1978. The resulting data
show large, short-lived spikes in global aerosols caused by
major volcanic eruptions in 1982 and 1991, but a gradual
decline since about 1990. By 2005, global aerosols had dropped
as much as 20 percent from the relatively stable level between
1986 and 1991.
Image above: Sun-blocking
aerosols around the world steadily declined (red line) since
the 1991 eruption of Mount Pinatubo, according to satellite
estimates. The decline appears to have brought an end to the
"global dimming" earlier in the century. Credit: Michael
Mishchenko, NASA
The NASA study also sheds light
on the puzzling observations by other scientists that the
amount of sunlight reaching Earth's surface, which had been
steadily declining in recent decades, suddenly started to
rebound around 1990. This switch from a "global dimming" trend
to a "brightening" trend happened just as global aerosol
levels started to decline, Mishchenko said.
While the
Science paper does not prove that aerosols are behind the
recent dimming and brightening trends -- changes in cloud
cover have not been ruled out -- another new research result
supports that conclusion In a paper published March 8 in the
American Geophysical Union's Geophysical Research Letters, a
research team led by Anastasia Romanou of Columbia
University's Department of Applied Physics and Mathematics,
New York, also showed that the apparently opposing forces of
global warming and global dimming can occur at the same
time.
The GISS research team conducted the most
comprehensive experiment to date using computer simulations of
Earth's 20th-century climate to investigate the dimming trend.
The combined results from nine state-of-the-art climate
models, including three from GISS, showed that due to
increasing greenhouse gases and aerosols, the planet warmed at
the same time that direct solar radiation reaching the surface
decreased. The dimming in the simulations closely matched
actual measurements of sunlight declines recorded from the
1960s to 1990.
Image above: Computer
simulations of Earth's climate over the entire 20th century
show the effect of airborne particles called aerosols:
sunlight reaching the surface decreased over most of the globe
(blue) and in some regions remained unchanged (white) or
slightly increased (yellow). Credit: Anastasia Romanou,
Columbia University
Further simulations using
one of the Goddard climate models revealed that aerosols
blocking sunlight or trapping some of the sun's heat high in
the atmosphere were the major driver in 20th-century global
dimming. "Much of the dimming trend over the Northern
Hemisphere stems from these direct aerosol effects," Romanou
said. "Aerosols have other effects that contribute to dimming,
such as making clouds more reflective and longer-lasting.
These effects were found to be almost as important as the
direct effects."
The combined effect of global dimming
and warming may account for why one of the major impacts of a
warmer climate -- the spinning up of the water cycle of
evaporation, more cloud formation and more rainfall -- has not
yet been observed. "Less sunlight reaching the surface
counteracts the effect of warmer air temperatures, so
evaporation does not change very much," said Gavin Schmidt of
GISS, a co-author of the paper. "Increased aerosols probably
slowed the expected change in the hydrological
cycle."
Whether the recent decline in global aerosols
will continue is an open question. A major complicating factor
is that aerosols are not uniformly distributed across the
world and come from many different sources, some natural and
some produced by humans. While global estimates of total
aerosols are improving and being extended with new
observations by NASA's latest generation of Earth-observing
satellites, finding out whether the recent rise and fall of
aerosols is due to human activity or natural changes will have
to await the planned launch of NASA's Glory Mission in
2008.
“One of Glory's two instruments, the Aerosol
Polarimetry Sensor, will have the unique ability to measure
globally the properties of natural and human-made aerosols to
unprecedented levels of accuracy," said Mishchenko, who is
project scientist on the mission.
Related
Links:
+
GISS research
+
Global Aerosol Climate Project
+
NASA's Glory mission
Stephen Cole
Goddard Space Flight
Center
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