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Posted on 10/08/2001 4:51:53 PM PDT by blam
Out Of Africa
Dust, the Thermostat
How tiny airborne particles manipulate global climate
On April 15, 1998, Mongolia's Gobi Desert lay between an area of low atmospheric pressure on the eastern end of the country and a zone of high pressure to the west. As swift winds rushed across the desert floor, they lofted sand and dust into the heart of a storm system racing southward into China. During the next 2 days, a yellow, muddy, acidic rain fell in a wide swath that covered Beijing and the Korean peninsula.
On April 16, 1998, a strong storm system passing over the Gobi Desert sent a massive cloud of yellow dust southward into China (center of left image). The dust from a subsequent storm (arrows) crossed the Pacific and reached the western coast of North America on April 25. On April 19, another low-pressure system blasted across the same region and boosted yet more particles into the sky, this time toward the east. In this case, there was no rain to rinse the air clean. The thick, dusty cloud swept across the Pacific Ocean and reached North America 6 days later. It whitened the sky from British Columbia to California and dropped more dust along the coast than the region had received from any single storm in a decade. Particles from the cloud passed over Minnesota and probably onward to the Greenland ice cap, where future climatologists might detect the event as a stripe of grit in an ice core.
Dust provides one of the most critical influences on Earth's climate, some scientists say. In some cases, dust particles serve as nuclei for condensing raindrops, but in others, those particles stifle precipitation. Dust cools Earth by blocking sunlight from reaching the ground but absorbs some of the sunlight and so warms the atmosphere directly. Although dust's cooling effect on Earth seems to predominate today, high amounts of dust ironically may have triggered the ends of the ice ages.
It's no wonder then that dust science is on the rise. As some scientists drill ice cores from Greenland and Antarctica in search of dusty clues to ancient climates, other analysts collect their dust samples by swooping into storms with aircraft, sieving powdery particles from the winds, and viewing the clouds from satellites. Researchers are just beginning to assimilate enough data to map out how the type and amount of dust in the atmosphere vary according to latitude, longitude, altitude, and time of year. They intend to unravel dust's profound, complex, and far-reaching effects on the planet's climate system.
Plenty of dust
There is no shortage of dust in the atmosphere. Small particles that abound include soot, salt, bacteria, fungal spores, and interstellar dust. Geophysicists focus on deciphering the effects on climate of the dust particles made up of minerals, such as oxides and phosphates, less than 62 micrometers in diameter.
One major supply line for mineral dust begins when wind, water, and severe changes in temperature join forces to erode large stones into smaller ones. Every time loose pieces of rock bang into or scrape across one another, yet smaller chunks can break off and join the fray. Eventually, all that's left are micrometer-size bits of mineralthe dandruff of the Earth.
Glaciers are major dust makers, too. As they grind their way along the stony terrain, they produce prodigious amounts of rock flour. Overly dry soil is yet another large source of mineral dust.
Winds at speeds no greater than 5 meters per second can boost dust to substantial heights. The wind often blows dust, sand, silt, and other mineral detritus into a thick, unconsolidated deposit called loess, which takes its name from the German word for loose. These ingredients may instead be washed or blown into rivers, lakes, and oceans where they eventually settle out of the water into mud and ooze that may dry.
If loess or these sediments don't compact into rock, they can become a major source of dust in the atmosphere, says Joseph M. Prospero, an atmospheric scientist at the University of Miami.
With imagery from satellites, scientists can track dust plumes back to their origins. They find that many dust springs have a common feature: They're arid basins in areas that were flooded during the recent ice ages. In other words, today's dust was yesterday's mud.
One of the most prolific spawning grounds for dust today is the Bodele depression, a low-lying area that was once part of Lake Chad. Although this lake on the southern edge of the Sahara Desert was about the size of Lake Erie in the mid-1960s, the recent lack of rain and the increased demand for water for irrigation has caused Lake Chad to shrink to 5 percent of that area.
A tan, mushroom-shape cloud of mineral dust lofted west from Africa surges past the Canary Islands in this Feb. 26, 2000, image. Dust-laden winds bring iron and other soil-fortifying nutrients to the Amazon basin and islands in the Caribbean, and they stimulate algae blooms in the North Atlantic.
"North Africa is pumping dust everywhere, all year long, almost every day," says Prospero.
Depending on the season, the winds chafing the Sahara and the Bodele depression carry dust to Europe, the Middle East, the North Atlantic Ocean, the Caribbean, and South America. For example, isotopic analysis shows that much of the iron in the rich red soils of the Caribbean came from African dust. This iron and many of the other minerals in Caribbean soil couldn't have eroded from the limestone rocks from which the islands formed, Prospero notes.
Similarly, much of the phosphorus in the soils of the Amazon basin originated in Africa.
Windstorms over North Africa can mobilize up to 700 teragrams (700 trillion grams, or about 770 million U.S. tons) of mineral dust each year. That's a substantial share of the world's total. About 2,150 Tg of dust takes a ride on the wind each year.
About half of that derives from natural sources, but the rest is lofted by agricultural and other human activities, says Joyce E. Penner, an atmospheric scientist at the University of Michigan in Ann Arbor. She and her colleagues report their analysis in the Aug. 1 Environmental Science & Technology.
Just what all that suspended dust is doing to the atmosphere and climate remains an extremely complicated question. The magnitude of its cooling over the ocean surface differs from that over the land. The color, size, shape, and chemical composition of the dust grains also strongly influence their behavior
Mineral dust, for instance, comes in a variety of colors. Darker particles absorb lots of radiation and scatter relatively little, so they tend to warm the air substantially. Brighter particles reflect much of the incoming solar radiation back to space and therefore have a net cooling effect.
Despite dust's daunting diversity, Prospero and his colleagues are quantifying its atmospheric effects. Using ground-based instruments, they have observed clouds of Saharan dust passing over the Canary Islands. They've employed satellites to analyze the clouds as they continue over the North Atlantic. With this information, the researchers estimate that, on a worldwide basis, dust cools the Earth slightly. The team reports this research in the Aug. 27 Journal of Geophysical Research (Atmospheres).
Over land, the analysts found, the cooling effect is approximately 0.57 watt of solar radiation per square meter. The net effect of dust over the ocean, the researchers say, is to scatter back into space about twice that radiation. That's because the ocean typically reflects less of the sun's radiation back into space than does the land, Prospero explains.
The worldwide cooling, he calculates, is about the same magnitude as the warming effect attributed to greenhouse gases that have been added to the atmosphere since the beginning of the Industrial Revolution.
Calculating the effects of aerosols such as dust on Earth's climate is much harder than estimating those resulting from greenhouse gases, says Joel Levy of the National Oceanic and Atmospheric Administration's Office of Global Programs in Silver Spring, Md. Whereas greenhouse gases generally last long enough in the atmosphere for them to diffuse and become well distributed, dust stays in the atmosphere only for a few days or weeks.
"It's very difficult to account for dust storms in long-term climate models because they're a 'now you see them, now you don't' kind of phenomenon," Levy notes. As such, any cooling or warming effect is tricky to simulate, but scientists are making steady progress.
The first estimates of the effect of dust on climate were mere guesses, says Levy. Now, Prospero's group has calculated a worldwide value of dust's cooling power, but it's based on dust from only the Sahara. The next step, Levy says, is to collect information on the optical properties of dust that comes from other places on Earth.
That type of large-scale data gathering is beginning. Research programs have become so massive that they resemble military exercises. This summer, an international effort to observe and collect dust streaming off Asia into the North Pacific included two airplanes, one ship, several satellites, and a ground station in Korea. Even so, Levy notes, "we don't yet have an adequate set of measurement instruments and techniques."
For example, the aircraft-mounted sampling equipment that snatches the dust from the atmosphere pulls in small particles effectively but doesn't do as well with those larger than 2 micrometers. Getting a representative sample of the dust is important because only then can scientists make good estimates of a dust cloud's light-scattering properties.
Furthermore, these properties can change over time. As the dust motes waft through the air, water that condenses on them can influence their shape and how much light they reflect. These changes can be even more drastic if the water freezes. Minerals in the dust can also react with or bind to chemicals emitted by human activities, including industrial or automobile emissions, Levy notes.
The direct effects of dust on the atmosphere may pale in comparison with their indirect effect on precipitation patterns, he adds. Dark particles tend to warm the atmosphere and turn clouds into water vapor, thereby letting more sunlight through to Earth (SN: 1/6/01, p. 15).
In work over cities on three continents, cloud physicist Daniel Rosenfeld and his colleagues found that clouds polluted with industrial emissions tend to hold their water rather than releasing it as rain (SN: 3/11/00, p. 164: http://sciencenews.org/20000311/fob1.asp). That's because the moisture in the cloud is distributed among more particles, which makes it more difficult for the drops to grow large enough to fall to Earth. A side effect of a polluted cloud's smaller droplets is that it more efficiently scatters solar radiation back into space than does a cloud made of fewer, larger drops. Therefore, it has a greater cooling effect on Earth's surface.
Now, Rosenfeld and other researchers have shown the same phenomenon among clouds tainted by Saharan dust. Moreover, this rain-stifling effect could be self-reinforcing, the team suggests. Less rain means drier soil, which leads to more dust, which means even less precipitation. Rosenfeld and his team published their analysis in the May 22 Proceedings of the National Academy of Sciences.
Dust and ice
Regardless of the effect of dust kicked up by people's activities, dust may have played a major role in the end of the recent ice ages. During glacial periods, there seems to have been from 10 to 100 times more dust in the atmosphere than there was during intermediate warm spells, says Petr Chylek, an atmospheric scientist at Dalhousie University in Halifax, Nova Scotia.
Of the six dusty periods recorded in Antarctic ice cores that cover the last 420,000 years, four immediately preceded the end of the four glacial periods recognized during that time. Chylek and his colleagues propose in the Aug. 27 Journal of Geophysical Research (Atmospheres) that a small but significant change in the way the atmosphere circulates in the tropics may explain the increased dustiness during glacial times.
Warm temperatures near the equator cause air near Earth's surface to rise to high altitudes and then stream toward higher latitudes, cooling as it travels. When this air reaches the edge of the tropicslatitudes of 30°N and 30°Sit falls back to low altitudes and then heads for the equator.
This so-called Hadley circulation ensures that most dust lofted into the atmosphere in the tropics doesn't travel to other parts of the world. Airborne dust at higher latitudes stands a much better chance of reaching the polar regions and being preserved in ice sheets there, Chylek explains.
He and his colleagues suggest that the average temperatures at the equator were lower in glacial periods than during the interglacial warm spells. This provided a weaker driving force for the Hadley circulation, and Chylek's calculations show that the warm, high-altitude tropical air may have reached latitudes of only 27° before it fell back to Earth.
Using current size and locations of arid regions, further analysis indicates that just moving that boundary 3° toward the equator could place larger areas of many major dust sources in a position to spew dust poleward rather than into the tropics.
This explains a large share of the dust trapped in polar ice, Chylek argues. However, a drier climate during glacial periods also might have led to expansion of the deserts, and lower sea levels would have exposed great expanses of mud and ooze that could dry up and blow away, he notes.
Wallace S. Broecker, a climatologist at Columbia University's Lamont-Doherty Earth Observatory in Palisades, N.Y., disagrees with Chylek's scenario. He thinks it's much more likely that the large amounts of dust billowing to Greenland in glacial times were a result of strong storms.
Because the edge of the sea ice extended farther south during glacial times than it does now and even surrounded Greenland, temperatures on that ice sheet then were as much as 23°C colder than they are today. That, in turn, intensified the temperature gradient between northern regions and the tropics, thereby strengthening the storm systems that could lift dust from arid areas. Broecker's analysis appears in the August 2000 Earth-Science Reviews.
Of all the climatic factors that seem to vary between glacial and interglacial periods, the amounts of dust trapped in polar ice sheets underwent the largest change, Broecker notes. Previous analyses estimate that there's a 10-to-50-fold greater amount during glacial times.
Broecker also points out that loess, which was generated in copious amounts during the peaks of the glacial periods, wasn't produced during interglacials.
Several scientists have suggested that dust falling on glaciers made them darker, causing them to absorb more solar radiation and thereby heating them up and accelerating their demise. This could explain why a period of intense dust rain immediately preceded the termination of each of the recent ice ages.
Whether or not dust triggered the ends of the ice ages, scientists hold that it plays a significant role in controlling today's global climate. Says Broecker: "We're just starting to understand the effects of dust."