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| Beijing engolfed by smog |
a professor of atmospheric sciences at Texas A&M University in College Station. Zhang is a co-author, along with several scientists from the U.S. and China, of a study released in the Proceedings of the National Academy of Sciences recently.
Zhang continued by saying "The scientists say pollution from Asia is likely leading to stronger cyclones in the midlatitudes of the Pacific, more precipitation, and a faster movement of heat from the tropics toward the North Pole. As a result of these changes, "it's almost certain that weather in the U.S. is changing,".
Zhang and fellow scientists used computer modeling to study
the effects on the weather of aerosols, which are fine particles
suspended in the air. The main natural aerosols over the Pacific are sea
salt tossed up by waves and dust blown off the land.
But
those natural particles are now increasingly outnumbered by human-made
ones. According to Zhang, the most significant aerosols the team
considered are sulfates, which are emitted primarily by coal-fired power plants. Other aerosol pollutants are released by vehicle emissions and industrial activities.
In
the atmosphere, such aerosols scatter and absorb sunlight, and thus
have both cooling and warming effects on climate. But they also affect
the formation of clouds and precipitation—and the magnitude of that
indirect effect on clouds is one of the biggest uncertainties hampering
scientists' ability to forecast climate change.
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| water pollution |
Clouds
form when water vapor condenses around aerosol particles to form liquid
droplets. Because pollution increases the number of particles, it leads
to more water droplets—but smaller ones. Those smaller droplets in turn
rise to greater heights in the atmosphere—and even form ice—before they
precipitate back out.
In an
earlier paper, Zhang and his colleagues used satellite data to show that
the amount of "deep convective clouds," including thunderstorms, had
increased over the North Pacific between 1984 and 2005. The most likely reason, they concluded, was an increase in aerosol pollution from Asia. "The intensified Pacific storm track likely has profound implications for climate," they wrote.
EFFECT GLOBALLY
In
the recent study the scientists took a first stab at considering those
global implications. Standard global climate models simulate the
atmosphere at grid points that are too widely spaced to resolve the
fine-scale processes involved in cloud formation—which is one reason
clouds remain such a knotty problem for climate scientists. But the
researchers found a way to embed a "cloud resolving model" into a
conventional climate model.
They
then used that "multi-scale" model to compare the preindustrial
atmosphere of 1850, when levels of aerosol pollution over the Pacific
were low, with the present atmosphere.
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| air pollution |
The
simulations confirmed that human-made aerosols are now spreading across
the Pacific and having large effects on the storms that sweep east
during winter. The storms are more vigorous than they would be without
pollution, with more ice and a broader "anvil" shape to the cloud tops.
And those more vigorous storms are having a significant effect on the
global atmosphere: They're increasing the flow of heat from the
equatorial region toward the Arctic, says Zhang.
What about North America? The Pacific storm track has a big effect on American weather, and large-scale natural changes like El Niño and La Niña are known to disrupt its usual pattern, leading to floods and droughts.
"What
we have shown is that aerosols from Asia can get transported over the
Pacific and change weather in North America," Zhang says—but nailing
down the nature of the change will require more research.
"We've
been getting some weird weather, such as a very cold winter [in the
eastern U.S.], so the next question is, does that have something to do
with Asian pollution?"
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