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RED WIND BLOWING


Born on a parched African plain, it loops and swirls across the Atlantic each spring, leaving a film of microscopic dust over everything it passes: islands, rain forests, huts, mansions. Once again the red wind is blowing, and this year it is especially fierce.


Eye in the Sky


Scientists know the phenomenon as aeolian, or atmospheric, dust. It mystifies and annoys those who live along its routes—which extend over much of the globe—and has been blamed for a plethora of ill effects including coral bleaching and even human disease.

“It’s been an unusually active year for dust coming out of Africa,” says Joseph M. Prospero, a University of Miami atmospheric and marine chemist who has studied the phenomenon for more than three decades. “We’ve had spectacular dust outbreaks starting in late February. I haven’t seen anything like this in many years.”

Dust pollution occurring near the source of an African outbreak has been known to contaminate food and drinking water, cause highway accidents, close airports, disrupt radio and satellite communications, and even suffocate cattle.

Borne aloft by trade winds, the heavier particles quickly drop away. Those that survive the journey across the ocean are a hundred times smaller than the diameter of the finest human hair. The dust normally arrives in the Caribbean by mid-June, as reliable as an unwelcome house guest, and doesn’t leave until fall. Islanders see the results most vividly at sunset: the opalescent glow that ordinarily lights the sky in these regions becomes a muddy shade of burnt orange.

“BLOOD SNOWS”

At various times dust from Africa can be found as far west as South America and as far north as central Europe, where it is blamed for “blood snows” in the Alps and Pyrenees.

“It originates wherever there is a desert,” says Lester Machta, of the National Oceanic and Atmospheric Administration (NOAA). “When strong winds occur, they just pick up the dust and carry it along. Most of it falls out over the water, but enough comes across to be detectable.”

Africa isn’t the only source. Springtime windstorms in western and central China pick up large quantities of yellow dust and deposit them across Japan, especially in the south, where they’re called “kosa.” Particles that bypass Japan scatter across the Pacific Ocean to destinations as far north as the Bering Strait between Alaska and Siberia, and as far south as the Antarctic Circle.

Saharan dust periodically prompts the National Weather Service Office in San Juan, Puerto Rico, to issue an air pollution alert. Thomas Cahill of the University of California at Davis recently has found traces of Saharan dust in the Southeast and Eastern United States.

“When the dust comes up here to Miami, which is not as often as farther south, it is pretty dramatic,” says the University of Miami’s Prospero. “The visibility may be reduced to five or six kilometers, and if it rains, we get mud spots on our cars. It is not a subtle change.”

Prospero, who monitors dust through a network of air-sampling stations in both the Atlantic and Pacific, says that by the time the particles reach the United States, for the most part they have lost their harmful properties.

GULF WAR SYNDROME CULPRIT?

Dust outbreaks are likely to be most harmful close to the source. Airborne dust is one of the suspected culprits in the range of maladies labeled “Gulf War Syndrome” resulting from the 1991 U.S.-led military campaign in Kuwait. Roland Draxler of NOAA’s Air Resources Laboratory estimates that roughly 30 percent of the surface of Kuwait was disturbed by the war, resulting in higher than usual levels of dust.

However, some scientists theorize that aeolian dust may have beneficial effects. Since the particles are chemically alkaline, they may dilute sulfuric acid—a major component of acid rain. They might even be helping to offset the current trend toward global warming. (See sidebar.)

Dust can make noticeable differences in weather. In a process similar to the formation of acid rain clouds from industrial air pollutants, airborne moisture may form around the tiny particles, which become cloud droplets. The chemistry of the resulting rainfall depends on the composition of the particles that made the cloud.

An obvious question is whether human activities such as deforestation and other disturbances of the land have increased the levels of aeolian dust on a global basis. Although some scientific models in specific areas have shown as much as a 50 percent increase in dust resulting from human impacts, Prospero isn’t prepared to make any global generalizations.

“The surface of the earth is very mobile,” he says. “The continents move around, materials are moving around. Dust is another manifestation of the dynamic nature of the earth.”

Eye in the Sky is a weekly series that brings you the story behind the headlines using satellite imagery, remote sensing, aerial photography, and maps. This feature is developed by National Geographic News with the sponsorship of the National Imagery and Mapping Agency (NIMA) and Earth-Info. Check out maps and imagery at http://www.earth-info.org.



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More Information
•  Dust from the Sahara normally crosses the Atlantic several times each summer, causing hazy skies in the northeastern Caribbean and occasionally reaching into Florida and even further north.
•  Rising warm air can lift particles from Saharan dust storms as much as 15,000 feet (4,572 meters) above the desert. Trade winds carry the dust westward.
•  People have noticed mineral dust (as opposed to household dust, which comes mostly from fabrics) swirling around the atmosphere for thousands of years.
•  During the epic voyage of the research ship Beagle, Charles Darwin in 1833 noted a fall of reddish-brown dust in the Cape Verde Islands in the Atlantic Ocean off Senegal.


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THE UPSIDE OF DUST

Dust kicked up from storms in Africa’s Sahel and China’s Gobi desert has been blamed for a variety of problems around the world, including the spread of disease. However, aeolian dust may have beneficial effects as well—such as possibly offsetting the effects of global warming.

Scientists at the Goddard Space Flight Center in Greenbelt, MD say that airborne dust from large deserts can be a vital source of nutrients for both ocean and terrestrial ecosystems—for example in ocean regions that are deficient in iron. Research has shown that the rain forest canopy in Central and South America derives much of its nutrient supply from dust originating in northern Africa.

When Asian dust has settled into waters around Hawaii, scientists have observed an increase in plankton—a primary food source for many forms of aquatic life.

Some suspect that airborne dust could be a built-in response to global temperature changes. During colder periods in Earth’s remote past, desert areas appear to have grown. Scientists believe that during those periods dust from deserts wound up in the oceans. There it may have stimulated tiny plant life, which in turn removed additional carabon dioxide from the atmosphere.

As a result, according to this theory, the global “thermostat” of carbon dioxide in the atmosphere remained at a low level, prolonging cooling conditions.

When the earth warms up—as it is doing now—another hypothesis is that increasing amounts of aeolian dust could help keep the temperature down by decreasing the amount of sunlight, and thus heat, reaching the earth’s surface. It’s even been suggested that with enough iron added to the oceans, a new “Ice Age” could be created.