By the time the yellow Haze had reached southern Utah, Marith Reheis was already camped out in Canyonlands National Park and trapping local desert dust for the U.S. Geological Survey.
“The weather had been absolutely perfect—not a cloud in the sky,” she recalls. “But that third morning, we saw a patch of sky that just looked weird. It looked like a dust cloud. But there hadn’t really been any change in the wind, and it wasn’t the right color.”
The right color would have been grayish white or red, hues typical of dust clouds churned up in the Mojave Desert to the west. Instead, the cloud Reheis saw that morning in May 1998 was the color of mustard. And it wasn’t from the Mojave: When Reheis returned to her office in Denver, she tracked down satellite images on the Internet and learned that the yellow haze had erupted several days earlier over a broad swath of the Gobi Desert in China. It had swept east across 5,000 miles of ocean. Some of it had landed on the West Coast, from British Columbia to California; the rest moved inland in a slow crawl across the continent.
Reheis wasn’t the only scientist to notice. Meteorologists, oceanographers, remote-sensing specialists, and air-quality experts—not to mention a few air traffic controllers—had all gotten their first overhead view of an Asian dust storm moving across the Pacific Ocean to North America. The phenomenon itself isn’t new. Massive clouds of sediment have been blowing around Earth ever since there was earth to blow and wind to blow it. But using sophisticated Earth-watching satellites launched in the 1990s, scientists around the world have gained a new appreciation for the global scale of dust movement. For example, NASA’s Total Ozone Mapping Spectrometer, or TOMS, launched in 1996, is the first instrument that can track dust storms over both land and sea. Its sensors can distinguish a dust cloud from a rain cloud, and it operates in real time. The Sea-viewing Wide Field-of-view Sensor, or SeaWiFS, another satellite-borne NASA instrument, transmits images in true, vibrant colors and provides global coverage every 48 hours.
These instruments reveal the movement of windborne sediments for thousands of miles. Soft-focus plumes sweep west from North Africa across the Atlantic and north into Europe; they trail from stark expanses of South America; they travel from Asia halfway around the globe. Dust clouds are bigger, faster, and more far-reaching than anyone suspected, and they constitute a truly international geophysical phenomenon—one that has stayed hidden in plain sight all this time.
With a new grasp of global dust traffic, scientists are now working to understand the often subtle effects that dust has on climate, human health, and the biosphere.
Dust storms play a vital role in global ecology. Long before satellite sensors filled the sky, scientists like Reheis sought to identify the provenance of widely spread sediments by collecting samples of airborne dust particles and analyzing their mineral content. The studies show that winds from the Mojave, for instance, supply as much as half of the fine soils on the Colorado Plateau and add nutrients and trace metals that are essential for plant life. Global storms supply fertile topsoil to rocky islands in the Caribbean and the Hawaiian archipelago. Iron-rich sediment from deserts feeds plankton blooms in the ocean and plants in the upper canopy of tropical rain forests.
“Certainly they’re benefiting from it,” says Dale Griffin, a microbiologist with the U.S. Geological Survey station in Saint Petersburg, Florida. “But we’re not.”