walked out on the deck of my boat and smelled Africa," says marine ecologist Virginia Garrison, who was traveling through the Caribbean at the time. "You know how a smell can totally remind you of something? The smell is warm and dry, and it pricks your nose." Garrison is speaking figuratively, but in fact she was smelling Africa -- tiny particles of it that had blown off the distant continent about a week earlier during a massive dust storm. Flowing on the wind in a broad, golden river a couple of miles above the Atlantic, the dust had headed west. And now, much diluted, it was settling. Vast clouds of dust regularly blow out of the Sahara in aerial streams thousands of miles long. During winter months, a single storm might eventually sprinkle a quarter of a million tons of dust over the Amazon. In summer, when the dust in northern Africa is boiling up almost constantly, the rivers tend to undulate due west, toward the Caribbean and -- a few times each summer -- toward Florida. But, while Africa has probably tossed desert dust across the Atlantic for millions of years, today's dust is different.
"People ask, 'If this dust has always come over, what's changed?'" says Garrison, a member of a new U.S. Geological Survey (USGS) team formed to study the hazards of incoming dust. What has changed is northern Africa itself. Parched by drought, heavily farmed by a hungry population, and chemically altered by the use of plastics, pesticides, and other industrial substances, the region is offering different dusts to the wind.
ast year, USGS microbiologist Dale Griffin conducted the first microbial census of African dust trapped over the Caribbean. His results rattled a pillar of microbiology. Traditionally, scientists did not expect many microbes to survive long in the air, where the organisms are assaulted by oxygen and ultraviolet radiation. Only a few microbiologists had speculated that microorganisms could travel long distances. "When I came into this project, I thought if I got three or four organisms I'd be pretty lucky," Griffin says. "Oh, my God, it was surprising!"
No one had predicted the bouquets of pink, yellow, orange, and gray that bloomed in Griffin's petri dishes: "A hundred and twenty species of fungi and bacteria," he says. Griffin thinks the dust may actually protect the microbes from the elements, sheltering them in microscopic crevices like tiny caves. The particles at the top of a dust cloud may also shade those traveling beneath them.
These biological hitchhikers may be the result of a crucial change in the airborne rivers of African dust. Before the 1970s, mineral chips from the Sahara probably dominated the dust that crossed the Atlantic. But in the 1970s and 1980s drought gripped the Sahel, the region south of the Sahara. More than a million people died. Drought and livestock erased vegetation, and Lake Chad shrank, exposing fine sediments. The survivors of the Sahel have had to farm harder, forcing food out of the fragile land and cutting trees to clear farmland and fuel cooking fires, and desertification has taken hold. Dust is one by-product of all this change. Dust from Lake Chad and the Sahel, unrestrained by roots or water, has come billowing west.
Compared with the Sahara, the Sahelian soils are rich in natural fungi, bacteria, and other microorganisms. Griffin believes his 120 species are only the fringes of what's actually there. "If there's one growing on the plate," he explains, "there are probably ninety-nine more that won't grow on a common medium." Of the microbes he did find, one in four is a plant pathogen. A plague of Aspergillus sydowii fungus that maimed and killed sea-fan corals throughout Floridian and Caribbean reefs in the 1980s may have flown in on dust from Africa.
Griffin has yet to sieve the dust for viruses -- which could outnumber bacteria by a hundred to one. Most viruses are harmless, but a few agricultural bad guys are on his most-wanted list. "We're looking for the really nasty ones," he says. "We'll probably go after foot-and-mouth this summer. And we're looking for citrus canker." This scabbing bacterial disease turned up in Miami in 1995. Hoping to prevent it from reaching central Florida's fruit groves, state crews began culling both commercial and backyard trees within the known blowing distance of infected ones. But with more than $200 million spent and about 2 million trees destroyed, the scourge persists, and Griffin suspects the disease is an import. Citrus canker, he points out, is endemic to northern Africa.
ut the sheer volume of dust would cause trouble even if it were pathogen-free. The Sahel catastrophe opened up such vast new sources of dust to the wind that, when the flow peaked in the mid-1980s, the amount trapped by a sampling station on Barbados had quadrupled. Today, dust storms in the Caribbean can still be heavy enough to foul laundry and cars. A dust storm once delivered a swarm of locusts to the region. And now, African dust has been linked to red tide outbreaks.
For decades, scientists have theorized that iron-rich dust might spark plankton blooms. In 1999, Jason Lenes, a graduate student at the University of South Florida, analyzed the fallout of an African dust cloud that settled into the shallow waters off Florida's west coast. Iron in the water shot up 300 percent. An explosion of a plantlike ocean bacterium, Trichodesmium, ensued. Then, organic nitrogen, a by-product of the Trichodesmium bloom, rose 300 percent. And the nitrogen served as a power lunch for a notorious red alga, Karenia brevis.
A single red tide can kill hundreds of manatees and millions of fish and cause respiratory ills in swimmers. This bloom was no exception. "It was a particularly nasty year," Lenes recalls. "There were millions of dollars' worth of damage." He is now trying to determine what percentage of Florida's red tides are fired up by dust. Florida's big Trichodesmium blooms almost always follow the arrival of African dust, he says, but how often a red tide follows Trichodesmium isn't known.
Lenes hopes his work will help Florida cut its losses. "If you could predict when a red tide is coming," he says, "you could close beaches and fisheries and prevent the waste of harvesting shellfish that are toxic. You could save millions of dollars."
irginia Garrison recently visited the nation of Mali, which straddles the Sahara and the Sahel, to set up an air-monitoring station. "It's the worst air I've ever breathed, including Jakarta's," she says. "I developed a really bad sinus problem, and it hurt my lungs to breathe." In her travels, she saw impoverished farmers burning garbage in their fields. The burning is a traditional technique for fertilizing poor soils -- but today's garbage is increasingly rich in plastics and other pollutants.
Africa is modernizing, and so is its dust. "Now you have pesticide use," Garrison says. "Pharmaceutical use. Plastics. When you burn plastics you get phthalates, which are endocrine disrupters. And all these chemicals may adsorb onto the tiny dust particles." Her dust monitor in Mali, and another in the Virgin Islands, are designed partly to investigate the dust's chemical content.
African dust is already known to carry radioactive beryllium, which forms naturally in the atmosphere and probably builds up as the dust travels. "We couldn't believe how high the beryllium-7 was" in the Virgin Island samples, says USGS geologist Gene Shinn. "One sample was three times the upper limit for the workplace." Then there's radioactive lead, a product of the natural decay of radon in rocks. The dust also ferries toxic mercury in concentrations a thousand times higher than are typical in U.S. soils.
"If dust is contaminated with this stuff, then it's a direct delivery to the lungs," Garrison warns. "And [it]'ll stay right there in your lungs." The average African dust particle is a hundredth of a hair's width in diameter, perfect for lodging deep in human lung tissue. But how much of this dust people are actually breathing in the Caribbean and in Florida -- where African dustfalls are much lighter than those in the Caribbean -- and how it may affect them aren't known. Some doctors and scientists suspect a correlation between dust and asthma, but the research is just beginning.
Even in its early stages, however, dust research has demonstrated how the most remote environmental tragedy can ripple across a great distance. If scientists continue to link expensive U.S. problems to African dust, then the United States might eventually recognize the benefits of helping to heal the Sahel. "Florida's dust problem is one more proof that Americans can't turn their backs on the rest of the world and hope to live untouched in our castle," says James Gustave Speth, former head of the UN Development Program and an NRDC trustee. "We have ignored the suffering in the Sahel, and it is to our own peril."
