It's a world traveler that affects climate and may have wiped out the dinosaurs.
Dust, scientists say, is more than something vacuumed from under our beds.
This time each year, dust from Africa hitches a ride on trade winds across the Atlantic Ocean to Florida, where it strengthens thunderstorms, pollutes the air and may be damaging sea corals. Dust storms in China routinely affect Pacific Ocean chemistry. This spring, they clouded the skies over Colorado.
But despite decades of research, scientists say, there is still much to learn about dust and its effect on our lives.
"Dust, with regard to climate, is just not as simple as you might expect," said Steven M. Babin, a researcher at the Johns Hopkins University Applied Physics Laboratory in Laurel.
Dust particles that travel the globe are different from the dust in your home. Household dust consists of fibers from clothing, carpets and upholstery, as well as particles of skin, hair, pollens, mold, wood and paint. Dust that crosses the oceans is made up mostly of tiny sand and soils.
"It can have the consistency of finely milled flour," said Ben Barnum, another Hopkins APL researcher who has developed dust forecasting systems for the military.
But the world's dust problem may be getting worse. As parts of the globe become drier, millions of acres of productive land in Africa and Asia are lost each year, raising more dust as deserts expand, forests disappear and soils erode, according to a United Nations panel.
A U.N. report released last week ranks desert growth and land degradation as one of the world's top environmental problems. Experts say that droughts, population growth and global warming are making things worse, drying up areas and producing more dust.
"While warming might increase rainfall in some parts of the world, areas that are dry lands are getting drier," said Zafar Adeel, an author of the report.
Whether dust speeds up, slows down or even contributes to climate change is still unresolved -- in part because dust is being blown all over the world and comes in different sizes.
Large dust particles reflect sunlight and cool things down. But smaller particles absorb sunlight and radiate it, boosting temperatures, experts say. And dust's effect on the oceans may be different from its effect on various terrestrial environments.
"You can't just say dust causes cooling or dust causes warming. It kind of does both," says Babin. "When you're trying to determine the net effect on climate, I don't think it's been conclusively demonstrated at this point.
What is certain is that millions of tons of dust are lofted each year from Africa's Sahara and China's Gobi deserts, rising 20,000 feet on thermal winds. Once airborne, the dust takes about a week to make its way to the United States.
Some scientists blame dust raised by an asteroid strike with blocking out the sun and causing climactic changes that wiped out the dinosaurs 65 million years ago.
Dust has been migrating around the globe for centuries. Much of the Caribbean's soil is, in fact, dust blown from Africa, researchers say.
"A major source for soils in the Bahamas, the Florida Keys and the Caribbean islands is dust from Africa," said Joseph M. Prospero, a researcher at the University of Miami who has been studying African dust transport for 30 years. He said small amounts of African dust contribute to air pollution in Miami every summer and traces of it have been found as far north as New England.
Dust can be tracked by satellite imagery and traced to its origin by its chemical composition. Dust from Africa has less calcium than dust native to the United States, said Kevin Perry, a researcher at the University of Utah.
"We know that in terms of the dust from Africa, it's not coming from the entire continent, but from some very localized sources. Some dry lake beds and basins," Perry said.
Satellites show that 230 million tons of dust leaves Africa each year. About half of that ends up in the Atlantic. But about 50 million pounds of it winds up clouding the skies each summer over the Caribbean and Florida, said Yoram Kaufman, a physicist and atmospheric scientist at the NASA Goddard Space Flight Center. His findings were recently published in the Journal of Geophysical Research.
Some researchers argue that while dust feeds phytoplankton -- tiny marine organisms that help offset the effects of climate change -- dust from Africa could be killing Caribbean sea corals.
Dust increases the number of updrafts of warm, moist air created just before a thunderstorm. Updrafts which are considered a basic ingredient in a storm's formation. The dust also increases the strength of the updrafts, researchers reported this year.
"The dust essentially invigorates the storm," said Paul DeMott, an atmospheric scientist at Colorado State University.
Most intercontinental dust consists of dried-up soils and mineral particles. Scientists believe that any threatening fungi, viruses and bacteria that travel with dust is likely killed by cleansing ultraviolet light from the sun.
"There's nothing in African dust that looks particularly nasty. It's mostly minerals you'd usually find in your own back yard," says Prospero.
Less is known about dust from outer space. Created by the collision of ancient asteroids, it's too small to see. But millions of tons of it are pulled here steadily by the Earth's gravity.
"It's hard to get a good idea of what its effects might be," said Stanley Dermott, an expert on interstellar dust at the University of Florida. "But it seems to be fairly benign."
There is some evidence that earthly dust can act as a protective shield for at least some of its microscopic traveling companions.
Researchers have linked sugarcane rust, a type of plant disease, in the Dominican Republic to rust spores from the African nation of Cameroon. They also say that last year, Hurricane Ivan's unusually strong winds blew spores of soybean rust from South America to Florida and Georgia, where it has infected kudzu plants.
Effects on oceans
At APL, Babin is seeking grants for a three-year study to analyze how the dust content of hurricanes affects ocean chemistry. He recently analyzed the effects of 13 hurricanes that passed through the North Atlantic between 1998 and 2001. He found that the three hurricanes with the highest dust levels also produced the greatest increases in phytoplankton.
The tiny organisms create oxygen, are a food source for marine life and absorb carbon dioxide, a heat-trapping greenhouse gas. Because dust contains iron, storms that deliver iron into the oceans are thought to feed phytoplankton.
"Basically, anything that's good for the phytoplankton is good for us," Babin said.
Since the 1980s, scientists have been debating whether to dump iron into the oceans to spur the growth of phytoplankton to counter the effects of global warming.
Dissolved iron was dumped in ocean experiments in 1999, 2002 and 2004.
James Bishop, a researcher with the Lawrence Berkeley National Laboratory, found that dust storms in China have a strong -- if short-lived -- effect on the Pacific's phytoplankton. The storms pump tons of iron-rich dust a thousand miles out to sea, increasing phytoplankton populations. But the effect lasts only two weeks, he said.
"A lot of people thought dust was important to the delivery of iron to the oceans, but I think its importance has been overplayed," he said.
Yet Bishop was definitive about the scope of a dust storm he observed in 2001. He had flown over the Pacific near Vancouver, British Columbia, to watch a massive dust storm that originated days before in China's Gobi desert.
"It was one of the most intense dust storms of the past decade, and you could see it from 35,000 feet up," he said. "It was incredible."
"You can't just say dust causes cooling or dust causes warming. It kind of does both."
Beginning each spring, dust raised by windstorms in Africa migrates across the Atlantic and is deposited in the Caribbean and southeastern United States. Winds that sweep out of Asia and across the Pacific bring dust to the western United States beginning in March.
Wind direction (see below) is determined by solar heating and the Earth's rotation. Belts of wind flow in the same general direction year round, but shift with the seasons, taking a northerly track in summer and a southerly one in winter.
The intercontinental transport of dust has been going on for millions of years. One effect is that much of the Caribbean soil originated as dust in Africa. On the East Coast of the United States, small amounts of African dust sometimes carry as far north as New England.
Beginning each spring, African dust contributes to air pollution in Florida, and Asian dust adds to haze as far inland as Colorado.
Microscopic organisms such as bacteria and fungi may be carried on the wind with dust. A few are believed to threaten U.S. crops, but scientists believe most are rendered harmless by the sun's rays.
Thunderstorms gain intensity from dust; the particles increase the number and power of updrafts of warm, moist air.
Iron in airborne dust may buffer global warming by encouraging the growth of phytoplankton, a tiny marine organism that captures and restricts release of carbon dioxide.
Extensive droughts since the 1960s in Africa and Asia are turning millions of acres of previously productive land into dust that is transported elsewhere.