The massive dust cloud arrived in Puerto Rico the Sunday before last, sweeping in and staining the evening sky a milky white. Monday and Tuesday, the air grew heavier with dust by the hour. Late on Tuesday, Olga Mayol-Bracero, an atmospheric chemist at the University of Puerto Rico, Rio Piedras, checked the readings at the atmospheric observation station she runs on the northeastern tip of the island. The numbers were higher than anything she’d seen in the 16 years she’s been taking measurements at the station.
“I couldn’t see the sky or clouds, just a grayish layer,” she says. “Definitely, we have never seen something like this.”
The dust over Puerto Rico represented the leading edge of a giant plume that had traveled more than 5,000 miles from the Sahara desert across the Atlantic Ocean, spreading into the skies above North America and beyond.
This particular dust plume is at once remarkable and totally ordinary. Each year, these kinds of plumes sweep off the Sahara, carrying some 180 million tons of mineral-rich dust from its dried-out pans. Thousands of miles downwind, the fine-grained dust shapes both the ecology of the places it lands and the climate as a whole.
But this year’s plume is remarkable because it appears to be the thickest and densest to cross the since the start of satellite monitoring, in 1979. Also, it’s carrying tons of lung-irritating fine particles toward North America, where chronic respiratory conditions are significant causes of disability and death.
What’s a normal plume like—and why is this one different?
The expanses of dry desert in northern Africa are the biggest, most consistent sources of dust in the world. Sand dunes don’t usually provide the dust; only the fiercest winds can lift such heavy particles. But fine dust particles often collect into hollows or flats in the desert landscape that at some point in the past held water. Year round, just a strong wind running across the surface of these dust-rich places can loft tons of dust into the air.
Under the right conditions, which typically align between late spring and early fall, great amounts of dust are swept up into the “Saharan air layer,” a pancake of dry, hot air usually a mile or more above Earth’s surface that can be two miles thick.
In the summer, a pulse of dust rolls off the continent every few days. Once cooler air masses from the ocean propel it high into the atmosphere, the dust can float for days if not weeks, depending on how high and how dry the air gets. East-to-west trade winds sweep it across the Atlantic toward the Caribbean and U.S. in a few days. As the dust plume moves, bits of it fall in a steady rain of particles.
Usually, the dust sparkles thousands of feet above Earth’s surface. But this plume is not only much bigger than usual; it’s much lower, too. By the time the plume made it to land near the Caribbean and southern U.S. last week, that dust-rain was closer than it usually is to where people live and breathe.
“The parameters we’re looking at have reached values we’ve never seen before, in terms of particulate matter,” says Mayol-Bracero. The air quality across Puerto Rico leapt to “hazardous” conditions as the plume settled over the island. Even with windows closed, the dust crept in; it settled on surfaces and was inhaled into people’s lungs.
Dust hurts health
The dust in the Sahara plume is made up mostly of tiny bits of minerals that used to be rock. Typically, as a plume passes the Spanish Canary Islands, which sit a few hundred miles downwind of the desert sources, most of the dust that’s falling out is smaller than 20 microns in diameter, half as big as a particle that can be seen by the naked human eye. By the time a plume makes it across the ocean to the Caribbean, the falling dust is even finer—less than 10 microns across—and many of the remaining bits are even smaller.
Scientists have long known that breathing in fine particles isn’t good for lungs. There are many sources of unhealthy fine particles: Burned-up fossil fuels and agricultural pollutants load the air with tiny flecks of the lung-irritating stuff. And dust, as it is pushed along, can cause significant damage to the health of communities downwind.
In a study published this week in Nature Sustainability, scientists tracked the effects of dust plumes flowing out of Chad’s Bodèle depression, one of the biggest, most prominent dust sources in the world. Dust from that depression has been found as far away as Greenland and South America, but its plumes are thickest and most damaging in West Africa and sub-Saharan Africa. The air there holds so much dust at times that it’s hard to breathe.
The scientists traced 15 years of records of the dust’s impact on air quality on downwind communities on the African continent. They found that how densely laden the air was with dust was intimately and devastatingly related to whether a newly born baby would survive one year. In West Africa, if the dust thickened the air by about 25 percent—an extra 10 micrograms of dust in each cubic meter of air—the likelihood of that baby surviving dropped by 18 percent.
“It’s one thing to say, ‘Breathing in dust is bad,’” says Jen Burney, an environmental scientist at the University of California, San Diego, and an author of the study. “But now we can say—we can say clearly—when these plumes were in slightly different places, there was a real impact. Babies died here, and not there,” because of the extra dust load.
A plume like this current one, says Burney, is a concentrated delivery system for fine particles that harm human health. Respiratory diseases are leading causes of death and disability throughout the world, including in the United States. The dangers of exposure to poor air quality are well known, and scientists link long periods of exposure with higher risks of dying from COVID-19.
A rain of fertilizer
Normally, the plumes that sweep off northern Africa are not as densely loaded, or low to the ground, as this one. But every pulse of dust influences biology and climate in places often many thousands of miles away from the source.
“One misunderstanding I’m seeing is this idea that Oh, there’s dust coming from the Sahara, it’s an apocalyptic sign!” says Geeta Persad, a climate scientist at the University of Texas, Austin. “This is an unusual version of this kind of an event because it’s so big, but this happens every year.”
The mineral fragments that make up the Sahara dust plume are often rich in iron and phosphorus; both plants on land and phytoplankton in the sea need those nutrients to grow. As the dust falls from the traveling plume and lands on the sun-soaked surface of the ocean, it fertilizes the photosynthesizing creatures that live there, which are often starved for the elements. More than 70 percent of the iron available to the ocean-bound photosynthesizers in the Atlantic comes from Saharan dust, a 2014 study found.
Dust does the same kind of work for the Amazon. The rainforest is one of the most biologically productive places in the world, but the soil anchoring the forest’s trees in place is notoriously low in some of the elements crucial to growth—particularly phosphorus. Much of the basin’s dirt doesn’t have enough of it to support the abundance of life growing out of it, and a key characteristic of rainforest habitat—the rain—strips away any unused phosphorus nearly as quickly as it appears.
How could the incredible richness of Amazonian biology emerge from such nutrient-poor soils? An answer, some teams of scientists suspected, could be in the tiny flecks of dust they knew had floated across the Atlantic for millions of years. That dust, they knew, contained phosphorus. In 2015, a team calculated that the phosphorus from that dust could essentially fill the gap between how much the rainforest needed and how much it appeared to have in its heavily depleted soils.
The storm suppressor
Dust traveling in the high atmosphere plays another role in the Atlantic basin, many scientists think: It helps to suppress the formation and strengthening of tropical cyclones.
The dusty air layers, like the one carrying the current massive plume, are often bone dry—and that’s a death knell for tropical storms, which feed off of humid heat. So if a developing storm that’s growing upward in the atmosphere hits the dusty layer, the dry air helps snuff it out, like a flame robbed of the oxygen it needs to keep burning.
The dusty layers are often propelled by fast-moving winds, which is why they can cross the ocean in just a few days. A storm growing into a towering whirlwind can get its top knocked off by those winds, preventing it from getting bigger.
That’s good news for now, since this hurricane season is predicted to be more active than usual. Emergency managers are deeply concerned about the effects of a major hurricane hitting North America while COVID-19 would complicate disaster recovery efforts. But the dust season is strongest in June and July, while hurricane season peaks in August and September, so any potential dampening effect happening now is likely temporary, meteorologists say.
What humans do to put a check on climate change might determine whether there will be more or fewer dust plumes in the future, and whether they’ll be thicker or sparser.
“It does look like there’s been an increase in dustiness over the 20th century,” says Natalie Mahowald, a climate scientist at Cornell University. About half of the increase, she says, is probably because of climate change; the other half is likely influenced by how land use in the Sahara has changed, as agricultural and human pressures have increased.
But it’s not entirely clear what effect a dustier future would have on climate writ large, because dust can both warm and cool the planet. When light-colored dust plumes flow over the ocean, they reflect back the sun’s incoming heat, which the dark ocean surface would otherwise absorb. But when dust collects on otherwise bright surfaces like snow or ice, it does the opposite, sucking up solar heat and hastening ice melt.
Dust also changes the way different types of clouds form. Sometimes dust seeds a bank of reflective clouds that can deflect extra heat—and sometimes it builds clouds that trap warmth near Earth’s surface.
And there are yet more complicated ways that dust can interact with climate. For one, its ability to fertilize photosynthesizing organisms is so powerful that under the right conditions, it can spur population explosions that result in carbon dioxide getting pulled out of the atmosphere. Dust’s fertilizing force may have driven at least a quarter of the change in atmospheric carbon dioxide that tipped Earth into the last ice age.
”These kinds of events are so powerful,” says Mahowald. “I just find it so remarkable that the atmosphere can transport and transfer—well, really, it’s Earth itself—so far and with such huge effect.”