Soot Linked to Flooding, Drought, Global Warming

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http://ens-news.com/ens/sep2002/2002-09-27-06.asp

Soot Linked to Flooding, Drought, Global Warming

By Cat Lazaroff

WASHINGTON, DC, September 27, 2002 (ENS) - Large amounts of black carbon or

soot particles and other pollutants are causing changes in precipitation and

temperatures over China, a new study suggests. The study's authors say soot

pollution may be at least partially responsible for the tendency toward

increased floods and droughts in Asian regions over the last several

decades.

In a paper appearing in today's issue of the journal " Science, " the

researchers explained that black carbon can affect regional climate by

absorbing sunlight, heating the air, and altering large scale atmospheric

circulation and the hydrologic cycle. The study's U.S. authors include

Surabi Menon of the National Aeronautics and Space Administration (NASA) and

Columbia University, and her colleague Hansen of NASA's Goddard

Institute for Space Studies in New York.

For this man in Zhangxi, Anhui, there is no chance of cleaning up his house

until the flood waters recede - but more rain is predicted. (Photo courtesy

International Federation of Red Cross and Red Crescent Societies)

Using the NASA Goddard climate computer model and aerosol data from 46

ground stations in China, Menon and Hansen conducted four sets of computer

simulations to monitor the effects of black carbon on the hydrologic cycle

over China and India. The aerosol data from the Chinese ground stations were

provided by Yunfeng Luo, a co-author on the study from the Institute of

Atmospheric Physics at the Chinese Academy of Sciences.

In the four numerical simulations, Menon and Hansen isolated specific

factors such as sea surface temperature, and concentrations of greenhouse

gases and aerosols, and analyzed whether changes in those factors would be

responsible for hydrologic cycle changes.

Out of the four scenarios, the effect of increased amounts of soot over

southern China created a " clear tendency " toward the flooding that has been

occurring in southern China, and the increasing drought over northern China

that has persisted over the last several years.

" If our interpretation is correct, then reducing the amount of black carbon

or soot may help diminish the intensity of floods in the south and droughts

in the northern areas of China, in addition to having human health

benefits, " Hansen said. Research is now being conducted to verify a similar

pattern over India.

The desert regions of southern Mongolia and northern China are visible in

this true color MODIS image acquired March 31, 2002. A haze of air pollution

is visible over the middle and lower right portion of the image. (Photo

courtesy Jacques Descloitres, MODIS Land Rapid Response Team, NASA/GSFC)

Black carbon or soot is generated from industrial pollution, traffic,

outdoor fires and household burning of coal, wood and other biomass fuels.

Soot is produced when these fuels are not burned completely.

China and India both produce large amounts of soot pollution because much of

their cooking and heating is done with wood, agricultural slash, cow dung

and coal, at a low temperature that does not allow for complete combustion.

These dark soot particles absorb sunlight, heating the air and reducing the

amount of sunlight reaching the ground. The heated air makes the atmosphere

more unstable, creating rising air, or convection, which forms clouds and

brings rainfall to heavily polluted regions. When soot blocks the Sun's

energy from reaching the ground, it can also reduce crop yields.

The increase of rising air in southern China is balanced by an increase of

sinking air, or subsidence, in northern China. When air sinks, clouds and

rain cannot form, creating dry conditions.

Much of the cooking and heating in China and India is done with coal, like

the fuel piled behind this hotel in Chengdu, Sichuan province. (Photo

courtesy U. of Minnesota)

In recent years, northern China has suffered from an increased severity of

dust storms, while southern China has had increased rainfall that is thought

to be the largest change in precipitation trends since the year 950 AD.

Menon and Hansen believe that human made sunlight absorbing soot particles

may be responsible for these changes.

As soot heats the lower atmosphere over China some of this warm air can get

transported to the other regions of the world, causing surface warming in

distant locations. But the role that soot plays in global climate change may

be much more complex than this, argue scientists from the Georgia Institute

of Technology.

In a perspectives article published today with the NASA report on black

carbon soot, atmospheric researchers at Georgia Tech describe some of the

report's policy implications, arguing that the role of atmospheric soot

particles in global warming is not been well documented by current climate

models. The authors add that the newly understood impacts of soot support

arguments to shift more responsibility for curbing climate changing

pollution to developing nations such as China and India.

In their perspectives article, Tech professor Chameides and

assistant professor Bergin point out the differences between black

carbon soot and greenhouse gases such as carbon dioxide and methane. For

instance, soot particles are removed from the atmosphere on time scales of

weeks to months, while carbon dioxide lingers for hundreds of years.

Thick layer of smog and smoke over China. July 11, 2002. (Satellite photo

courtesy U.S. National Aeronautics and Space Administration (NASA))

The authors argue that this finding could point toward a better near term

control strategy for global warming than attempts to reduce carbon dioxide

emissions. " In the past, researchers have felt that soot didn't really

have a significant warming effect, " said Bergin. " But as we've learned more

about the amount of black carbon emitted by countries like China and India,

it appears now that soot could have important climate effects, and that

these effects may be almost as much as those of carbon dioxide. "

Most soot emissions come from developing nations such as India and China. If

these emissions do in fact play a large role in global warming, that could

shift the pressure for environmental control to those nations, the authors

write. Industrialized nations like the United States are responsible for the

bulk of carbon dioxide emissions.

Breathing smoggy air containing fine pollution particles can be as dangerous

as breathing second hand cigarette smoke. (Photo courtesy U.S. Environmental

Protection Agency)

" From a policy standpoint, the payoff for controlling soot could be on the

scale of years rather than centuries, " Bergin added.

Efforts to control soot may also bring immediate improvements in human

health, the Georgia Tech writers said, since the small particles thought to

be most active in affecting climate are the same particles that cause

respiratory distress when trapped deep in the lungs. The U.S. Environmental

Protection Agency has concluded that emissions from diesel engines,

including soot, increase cancer risks in humans.

Little is known about the worldwide impact of soot emissions or even how

best to measure them, so new research will be needed before the role of

black carbon emissions can be reliably assessed, the authors add.

" There are a lot of possible atmospheric effects from soot, " Bergin said.

" We really don't yet understand all the feedback cycles involved. "

Incompletely burned coal releases soot particles into the atmosphere. (Photo

by Parsons, courtesy National Renewable Energy Laboratory)

A key uncertainty is the amount of soot going into the atmosphere. Localized

studies in China and India, where crops wastes are burned for heating and

cooking, show very high levels. In developed nations, elevated soot levels

are found in urban areas - which have often been excluded from climate

studies to avoid confusing global climate change with local urban heat

island effects.

" The nature of the particles and how they absorb light could be different, "

Bergin explained. " So one gram of soot from one part of the world could be

different from a gram of soot from another part of the world. We are really

at the beginning of trying to understand the influences of soot on climate.

Right now, there is a great deal of uncertainty in any estimate of the

climatological impact of soot. "

Copyright Environment News Service (ENS) 2002.