Atmospheric pollutants from fossil fuel burning may cause nearly
as much global cooling as global warming, a University research
team reported in the Jan. 15 issue of the journal Nature.
Their findings add a new layer of complexity to the contentious
subject of global climate change, dismissed by skeptics as a
chimera invented by environmental alarmists. However, the cooling
effects are insufficient to counteract the warming influence of
greenhouse gases like carbon dioxide in the long run, said the
study’s principal author, atmospheric, cceanic and space
sciences Prof. Joyce Penner.
In fact, the cooling may even temporarily be masking the full
impact of fossil fuel burning on the global climate, said SNRE Dean
Rosina Bierbaum, who served four years in the administration of
President Bill Clinton.
The burning of fossil fuels like oil and coal emits suspended
particles such as soot and sulfur dioxide, collectively known as
aerosols, into the atmosphere. Aerosols have long been known to
cool the climate, partially offsetting global warming caused by
greenhouse gases like carbon dioxide, which are also produced by
fossil fuel burning.
Aerosols affect climate by changing the way clouds develop.
“When clouds form, the water can’t easily condense
without an existing particle there,” Penner said. “That
water is going to condense on these aerosol particles, so if you
have more particles, you’re going to have more drops in the
The result is that solar radiation reflects off the increased
number of cloud droplets and returns to outer space instead of
reaching and warming the earth’s surface.
To quantify the effect of aerosols on solar penetration, the
research team — comprised of Penner, University graduate
student Yang Chen and Prof. Xiquan Dong of the University of North
Carolina — compared a polluted site in Oklahoma, where
aerosol levels are high, to a pristine site in Barrow, Ak. with low
aerosol concentrations. Their measurements showed that at the
high-aerosol site, higher cloud reflectivity resulted in less solar
radiation at the surface.
“We connected the measure of the aerosols and the measure
of radiative properties,” Chen said. “It’s the
first time to use this connection to understand the aerosol’s
indirect effect.” Previous research had only shown the effect
of aerosol concentrations on cloud droplet size rather than on
cloud reflectivity, or had relied on modeled, rather than measured,
data to estimate cooling, he said.
The team’s results showed that aerosols can cause global
cooling of about 2.5 watts per meter squared — levels as high
as some estimates of global warming.
“What we get from (this study) is that the models that
have done the estimates of global cooling are actually in the right
range,” Penner said. “Even though they’re quite
high and in the range of global warming, we think they must be
Penner cautioned that due to their short lifetime, aerosols
cannot be relied upon to mediate the effects of global warming in
the long run. Aerosols break down within five years of entering the
atmosphere, she said, while carbon dioxide molecules survive for
100 to 200 years. Carbon dioxide therefore builds up over time,
while aerosols dissipate.
Moreover, aerosols pose health and environmental risks, causing
respiratory problems and acid rain. Because of this, the world is
more likely to strive to remove aerosols from the atmosphere in
coming decades, Bierbaum said.
“It’s very important to understand how much aerosols
are masking the greenhouse effect,” she said. “As the
world decreases aerosols for health reasons, warming will be more
The surprisingly large values of cooling due to aerosols are a
reminder of the uncertainties involved in climate prediction, Chen
“The global warming caused by greenhouse gases like carbon
dioxide has been studied a lot, but some other effects like
aerosols — there’s still a lot of uncertainties related
to this topic,” he said. “We still need to do more to
study the relationship between aerosols, clouds, and