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 James 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, George Tech professor William Chameides and assistant professor Michael 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 David 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."