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.
Scientists have known about the long-distance celebrity relationship between the Sahara and the Amazon for more than a decade, giving researchers such as Yu years to analyze the exchange. But Saharan dust storms are driven by the seasonal migration of the winds themselves. The storms that head south to land directly in the Amazon occur in the winter and spring, Yu says. The current supersize Saharan plume is a summer storm, which means it’s heading northward, forecast to dump its swirling mass of particles into iron-limited waters of the tropical Eastern Pacific Ocean. If the forecasts are true, the impact on the ocean could be big, Yu says. The fertilizing effects of African dust on this particular oceanic biosphere are less studied than their effects on the Amazon.
Sea creatures are also thirsty for the precious minerals of Saharan dust, for better or for worse. “If you’re an alga, then you love dust,” Griffin says. A 2001 study in Limnology and Oceanography suggested that the seasonal windfalls of iron-rich Saharan dust become a banquet for red tides, blooms of algae that spill into the ocean like dye, deplete it of oxygen, and release toxins. Dust clouds can also host unwelcome stowaways. “These dust clouds carry a diverse community of microorganisms, some of which have the potential to be plant pathogens or human pathogens,” Griffin says. Some scientists suggest that the dust storms can carry fungal spores or bacteria that spread diseases in corals and encourage algae blooms.
The dust may be responsible for more alluring ocean phenomena too. Some scientists theorize that millions of years of this seasonal sprinkling of Saharan dust fed the cornucopia of corals encrusting the Bahamas, which are surrounded by waters that lack many of the nutrients required to create such an oasis. Like any kind of dust, it’s a mixed bag.
Humans, however, do not thrive in enormous dust plumes. To anyone living in its direct path, the dust cloud poses a significant hazard to public health. In one sense, there’s never been a better time to ask people to wear a mask—everyone has one. But there’s also never been a worse time to be surrounded by a miasma of particulate pollution that could trigger preexisting or new respiratory conditions. People living in areas with high levels of air pollution are more likely to die of COVID-19, according to a recent study.
Still, large brown dust storms have a silver lining. Though the Saharan clouds may look disastrous, they suppress hurricanes in several ways. The dry, dusty storms soak up moist, hurricane-friendly air like a sponge and can create sinking air and changing winds that tear apart baby hurricanes before they get big. Dust storms can also paint sunsets with sherbet-colored streaks. “When I drive down to St. Pete to go to work, I can see the dust—big orange streaks in the atmosphere,” Griffin says. “Though I haven’t actually been outside because of, well, the virus.”
When Méndez Lázaro looked out his bedroom window in downtown San Juan Tuesday morning, though, the sky was filled with dust, an impermeable, jaundiced gray. “It’s almost impossible to see the shoreline from my house,” he says, noting that he lives just a little more than a mile away. “I don’t remember ever seeing the sky like this in Puerto Rico.” He, along with everyone else on the island, is waiting for rain to wash away the far-flung dust from the air, the windows, and car windshields: a souvenir that is kind of impressive but that no one asked for.