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Chinese dust storms could stir up big trouble for U.S.

By JOAN LOWY Scripps Howard News Service May 23, 2001

- Poor farming practices, population pressures and drought are intensifying dust storms in China that some scientists and environmentalists believe may ultimately pose a significant pollution problem for the United States.

The situation gained dramatic attention in April when a giant Chinese dust storm tracked by weather satellites invaded North America, raining dust and other pollution as it blew eastward.

The dust cloud, measuring thousands of square miles, formed April 5 on the desert border of northwestern China and Mongolia. Over the next two weeks it moved across the Pacific Ocean and North America, blanketing large portions of the western United States and Canada as well as areas of New England with a white haze. It dissipated over the Atlantic Ocean halfway to Europe.

Dust storms have long been a major environmental and health concern not only for China but also Japan and Korea, where they are called "yellow dust." But scientists had never before tracked such a large and intense storm into North America.

"We are seeing that even the largest ocean in the world isn't a sufficient barrier to prevent pollution from crossing the sea," said Lester Brown, president of the Earth Policy Institute.

The storms gather and push air pollutants in front of them, including methane, carbon monoxide and carbon dioxide, said Russ Schnell, director of observatory operations at the National Oceanic and Atmospheric Administration, whose satellites tracked the storms.

The April storm "was very intense and it brought a lot of things with it ... this big glob of pollution," Schnell said. "Like a motorboat in a lake, it pushes what's in front of it."

Dust itself is also a pollutant, especially particles that are small enough to be inhaled and cause respiratory problems.

Right now, the dust clouds are not a serious pollution threat to the United States. But that could easily change as the economies of Asia grow and consume more energy, which increases the pollution that gets caught up in the storms, Schnell said.

The storms are an "early signal" of even greater problems to come as a result of widespread environmental degradation in northern China, including poor farming practices that have increased deforestation and desertification and severely strained water resources, Brown said.

More than 1 million square miles of China is desert, and nearly 1,000 square miles is lost to sand each year. At least 400 Chinese cities are short of water.

The Chinese government has announced a $22 billion tree-planting program in the nation's northern provinces, including a 2,000-mile-long tree berm aimed at holding back the encroaching Gobi Desert. Chinese officials have also announced a new five-year economic plan that calls for industry to recycle more water.

However, "it's going to take a much larger effort" to turn the problem around, Brown said. If not corrected, China could lose large areas of cropland to desert, which would force the dislocation of tens of millions of Chinese.

Not all the dust storms plaguing North America come from Asia. Dust storms from Africa can rise up to 20,000 feet and are carried by trade winds across the Atlantic to the United States.

Every few years, fine red-brown dust from Africa will fill the skies over Florida and some other East Coast states at levels just shy of violating the standard for fine particulate matter under the Clean Air Act, said Joseph Prospero, director of the Institute for Cooperative Marine and Atmospheric Studies at the University of Miami.

The Environmental Protection Agency's standard for fine particulate matter is no more than 65 micrograms per cubic meter. Africa dust clouds over Florida frequently reach 50 micrograms, Prospero said.

The "mother of all dust sources in the world" is an area of northern Chad in what is known as the Bodele depression, Prospero said. With satellite imagery, "you can just see this thing putting out dust day after day - very specific plumes," he said.

-- Martin Thompson (, May 24, 2001


I put picture captions in [brackets]; the link is at the bottom.

--------------- Dust Begets Dust

Everyone knows that dry weather leads to dusty soils, but new research suggests that dust might in turn lead to dry weather.

May 22, 2001 -- Windblown desert dust can choke rain clouds, cutting badly needed rainfall. This new discovery made with the help of NASA satellites suggests that droughts over arid regions, such as central Africa, are made even worse by land management practices that expose new dust and accelerate the growth of deserts.

In other words: Dust begets dust!

These findings, reported in the Proceedings of the National Academy of Sciences, present a new view of the decades-long drought in the African Sahel, a semi-arid region of Africa adjacent to the Sahara Desert. The grassy savannas of the Sahel once provided natural pasture land for livestock, but increasing demands on the land due to population growth have lead to desertification, which has been accompanied by increasing levels of airborne dust during the rainy season.

[Above: Dust and other particles in the air cause water droplets in clouds to be smaller, leading to decreased rain. In this illustration, the cloud on the right is over a forest fire, which releases tiny airborne particles (called "aerosols") in its smoke. Click on the image for an animation illustrating the principle.]

More airborne dust is not necessarily a result of decreased rainfall but rather its cause, according to scientists from Israel's Hebrew University and the Weizmann Institute. "This impact of desert dust on rainfall was not known before," says lead author Daniel Rosenfeld, Hebrew University, Jerusalem. Furthermore, it was the opposite of what scientists expected.

Before Rosenfeld et al.'s research, many scientists thought that large airborne dust particles would speed the formation of rain by forming giant cloud condensation nuclei and larger cloud droplets.

Not so.

"Our laboratory analysis of the desert dust showed that [dust] particles contained very little water-absorbing matter," says co-author Yinon Rudich of the Weizmann Institute. "As a result, even large dust particles form relatively small cloud droplets."

The research shows dust actually amplifies the process of creating deserts. Activities that expose and disrupt topsoil, such as grazing and agricultural cultivation, can increase the amount of dust blown into the air. More dust reaching rain clouds produces less rainfall, which exacerbates the drought conditions and contributes to the desertification of the landscape. In desert regions, dust storms can often kick up clouds of dust that blanket thousands of square miles of land, dramatically illustrated by the record-setting dust cloud that blew from Asia to North America last month.

[Left: Water droplets that form around dust grains and other aerosols, like those on the right, tend to be smaller than droplets that don't. The size of the water droplets in a cloud determines whether gravity will dominate over the tendency of the air molecules to keep the droplets suspended.]

Dust and other types of aerosol particles blowing into clouds act as nuclei where water vapor can condense to form cloud droplets. If a lot of dust enters a cloud, the available water is spread over many small droplets. These small droplets grow more slowly through collisions with one another to the size of a raindrop, and the cloud yields less rainfall over the course of its lifetime.

What the researchers saw in two separate cases, using different satellite observations, was that cloud droplets were smaller as dust concentrations increased.

NASA's Tropical Rainfall Measuring Mission (TRMM) spacecraft captured images of clouds over the Atlantic Ocean off the coast of northern Africa during a major March 2000 dust storm. Droplet sizes steadily increased the farther the clouds were from dust-filled air. Rain was falling only from the dust-free clouds even though all the clouds contained equal amounts of water.

The researchers also observed similar behavior in clouds over the eastern Mediterranean Sea in March 1998, using data from aircraft and a U.S. weather satellite.

[Right: By using TRMM to observe dust storms such as the one depicted in this SeaWiFS image, scientists noticed that the dust appeared to inhibit precipitation. Credit: the SeaWiFS Project and ORBIMAGE]

Rosenfeld has used TRMM observations in two other recent studies to show that aerosols from biomass-burning smoke and urban air pollution also reduce rainfall. Combined with the negative impact of desert dust, Rosenfeld believes the aerosol rainfall-suppression effect can have a major impact on regional and global climate.

"The recent observations of the impact on precipitation of all kinds of aerosols, each with a major human contribution, show a major climate change issue that has nothing to do with greenhouse gases," says Rosenfeld. "Still, this is perhaps the climate-change effect with the greatest socio-economic impact on water-scarce areas."

-- L. Hunter Cassells (, May 24, 2001.

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