what blows The Sahara does not stay in the Sahara. Vast African deserts often spew clouds of dust that fly into Europe, turning snow-capped mountains orange. They clearly cross the Atlantic Ocean to fertilize the Amazon rainforest. These things can even reach the United States.
But despite their hype, dust emissions from the Sahara—and dirt from any other desert region—are not well explained in climate models. While satellites can track plumes as they move through the atmosphere, scientists don’t have enough data to show definitively how the dust cools or warms the planet, accelerating or slowing human-caused climate change.
“Our dataset is based on 5,000 soil samples, which is not nearly enough coverage,” said Natalie Mahowald, an earth system scientist at Cornell University. “Nobody wants to go to the middle of the desert to figure out what the soil is.” So Mahowald has been working with NASA on the Earth’s Surface Mineral Dust Source Survey Mission (EMIT), which will launch to the International Space Station next month. Their instruments will use a powerful technique called spectroscopy, which astronomers have used for decades to determine the composition of distant objects, but turn it to Earth to analyze our own land. This will eventually give scientists a global picture of where the dust comes from, what it is made of and how these particles affect the climate. “It makes more sense to sense it remotely,” Mahowald said.
The molecules of any material absorb and then emit electromagnetic radiation in unique ways. As a result, astronomers can use spectrometers to analyze light from distant planets to separate out individual elements, such as hydrogen or carbon, based on their unique signatures. That planet may be billions of miles away, but the composition of its atmosphere is exposed by the light it reflects. It’s a bit like being able to take someone’s fingerprint, even if you can never touch them.
The EMIT spectrometer, which will be attached to the bottom of the International Space Station, will image the Earth 50 miles wide, looking for unique signatures of specific minerals. Iron oxide, for example, looks different from clay on a spectrometer, even though to the human eye, the surface of one desert area might look similar to another. “We need to measure the fingerprints of minerals in arid regions,” said EMIT principal investigator Robert O. Green, a researcher at NASA’s Jet Propulsion Laboratory. “We’ll have enough mineral maps within a year to start feeding climate models with new initialization information.”
Plugging this new data into existing models will give climate scientists a better understanding of dust’s role in Earth’s temperature. Traditionally, researchers have represented dust as a simplified average, a yellow haze. “But if you look at the soil, they’re probably all different colors: black, red, white — a very reflective color,” said EMIT associate principal investigator Mahowald. “Anything darker absorbs more radiation and warms us, while anything lighter reflects radiation and cools.”