There is twice as much desert dust in the atmosphere now than a century ago. Particles of dust in the air affect climate and ocean ecology. A better understanding of changing dust levels should help scientists make more accurate climate predictions.
The amount of dust in the atmosphere has doubled over most of the planet since the last century, finds a new study.
As wind blows through the world's deserts, it whisks dust up into the air and down into the oceans and can significantly affect climate and the environment in all sorts of ways.
Understanding the changing patterns of dirt particles in the atmosphere could help scientists improve the accuracy of climate predictions. Tracing swirls of dust to their roots could also lead to better land management practices that might mitigate the flow of dust from Earth to sky and sea.
But first, researchers need to figure out why dust levels are rising in the first place.
"We don't know," said Natalie Mahowald, an atmospheric scientist at Cornell University in Ithaca, N.Y. "It's probably a combination of agriculture and pasture-usage as well as climate change because a lot of regions are getting drier, and that would increase desert dust."
"We put big uncertainty bars on everything," she added. "We need more data."
Climate researchers have spent a lot of time worrying about the effects of particles that human actions release into the atmosphere. Known as anthropomorphic aerosols, these include sulfates from coal-fired power plants and nitrogen oxides from automobile exhaust.
Mahowald is more interested in desert dust, which can also affect climate in a number of ways. For one thing, particles of soil that are suspended in the air alter the way the atmosphere absorbs and reflects energy from the sun. Dust also changes the properties of clouds, which play a big role in climate patterns. Overall, rising levels of dust tend to cool the atmosphere down.
Dust also affects the chemistry of the oceans. That's because dust contains iron, which boosts growth of plankton, allowing the oceans to pull a little more carbon out of the air.
To piece together a history of Earth's blowing dust, Mahowald and colleagues compiled a wealth of published data, which included analyses of layered ice cores and sediment samples taken from more than a dozen sites around the world. Most of these cores held specks of dust that had blown in from somewhere else.
In ice cores from Antarctica, for example, scientists identified soil from South America. Lake sediments in Colorado's San Juan Mountains contained soil particles originally from the Mojave Desert in California, 1,000 kilometers (600 miles) away. Each layer was dated so the researchers could tell how the levels of blowing dust changed over time.
For the 100-year span from about 1900 to 2000, levels of dust fluctuated quite a bit and patterns differed in different regions, the researchers reported in the journal Atmospheric Chemistry and Physics. Overall, though, levels of dust around the globe doubled everywhere except above North America, where levels dropped a little bit.
By comparing dusty periods with periods that weren't so dusty, the researchers were able to show that dusty skies lead to lower temperatures -- masking some of the warming effects of greenhouse gasses. As dust accumulates in the air, it also affects clouds enough to move storms away from desert areas, possibly propelling droughts that, in turn, lead to even more dust. In the oceans, dust boosts productivity and sucks up more carbon from the air, which can also cool the climate. But that may be offset by erosion and the loss of plant cover on land.
In satellite images of Earth, you can see three obvious colors, said Joseph Prospero, an atmospheric scientist at the University of Miami in Florida: The blue of oceans, the white of clouds and the brown of massive desert dust storms.
Those images, Prospero said, clearly show that dust is a major player that needs to play a more significant role in climate models as one of a multitude of fluctuating and complicated factors.
"When climate changes, you get a tremendous amount of variability in dust output," he said. "There is a strong possible loop where the climate becomes drier and windier causing more dust, and more dust affects radiation, so it feeds back on climate."
"There is a lot of uncertainty about how that works," he added. "That's why we focus on dust."
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