Each year, on average, a dizzying 182 million tons of dust departs from the western Sahara, enough to fill 689,290 semitrucks. These clouds of dust make up one of the greatest annual migrations on the planet—not animal, but mineral. It begins in the Sahara, where wind storms levitate enormous plumes of desert dust thousands of feet above the surface of the Earth. There, in camel-colored wisps thousands of miles long, the dust hitchhikes on trade winds traveling west, across the Atlantic Ocean and Caribbean Sea.
Saharan dust clouds make this transcontinental trek all the time, and on the way, the dust falls and settles in the ocean, in rain forests, and, occasionally, on the windshields of unsheltered cars. But the gargantuan plume currently making its way across the sky, over the Caribbean and heading toward the United States, is unusual for a Saharan dust cloud, both in volume and density. “It’s definitely a very significant amount of dust,” says Hongbin Yu, an atmospheric scientist at NASA’s Goddard Space Flight Center.
A historic amount of dust, some say. “In terms of concentration and density and size, it is the most dust we’ve seen in 50 or 60 years,” says Pablo Méndez Lázaro, who researches environmental health at the University of Puerto Rico.
At first glance, a migration of brown dust—even one leaping across continents and oceans—might seem uninspiring compared with, say, the murmurations of monarch butterflies or stampedes of wildebeests. Admittedly, there is no “Planet Earth: Dust,” but maybe there should be. The Saharan dust cloud is a billowing ribbon of life-giving minerals such as iron and phosphorus that fertilize the most biodiverse oases on the planet, including the lush menagerie that is the Amazon rain forest. Phosphorus, a vital nutrient for plant growth, drains quickly from Amazonian soil after rainfall or flooding, escaping into waterways, and in the Amazon, abundant rain is a given. Luckily, a yearly windfall of 22,000 tons of Saharan dust delivers approximately enough phosphorus to replace the minerals leached by rainfall, according to a study Yu published in Geophysical Research Letters in 2015. And high within the jungle canopy, air plants depend on the mineral dust for nutrients as it falls slowly to Earth, according to Dale Griffin, an environmental and public-health microbiologist with the U.S. Geological Survey.