clock menu more-arrow no yes mobile
A man walks near Morro Castle as a vast cloud of Sahara dust blankets the city of Havana on June 2.
Yamil Lage/AFP via Getty Images

Filed under:

The “Godzilla” Saharan dust cloud over the US, explained

The giant dust cloud is part of a system that feeds the ocean, fertilizes the rainforest, and suppresses hurricanes.

Umair Irfan is a correspondent at Vox writing about climate change, Covid-19, and energy policy. Irfan is also a regular contributor to the radio program Science Friday. Prior to Vox, he was a reporter for ClimateWire at E&E News.

Clouds of dust that arose in Africa are wafting over parts of the Americas, tinting skies brown, creating shimmering sunsets, and suppressing hurricanes in the Atlantic Ocean. And more dust is on its way.

The Saharan dust storm is part of a regular meteorological phenomenon that sends dust from the Sahara Desert to the Gulf Coast of the United States, the Caribbean, and South America. But the 2020 cloud that recently shrouded cities like Houston, Miami, and New Orleans in a shadowy haze was one of the most intense on record. Satellite instruments showed that the cloud was far denser with dust particles than previous events.

“That one was really for the books,” Jason Dunion, a meteorologist at the University of Miami and a researcher at NOAA’s Atlantic Oceanographic and Meteorological Laboratory, told Vox. “We were seeing numbers that were twice as large as some of the storms we were seeing last year ... This was an extreme outlier.”

This “true-color” composite animation of visible satellite imagery shows the movement of the Saharan Dust plume from June 15 to 25, 2020.
This animation shows a brown swirl of dust starting in Africa and moving toward the Americas.
Colin Seftor/NASA-NOAA

On the ground, the dust cloud, dubbed Godzilla, is triggering air quality alerts for millions of people. The first intense plume is now on its way out of the United States, but another, thinner cloud of Saharan dust is on the way this week.

While the dust poses a serious health hazard, it’s also linked to vital climate mechanisms that nourish the oceans, fertilize the rainforest, and quash tropical storms. The Saharan dust is a prime example of the complicated forces that tie our planet together and how the things we experience at home can start from far away.

Massive dust clouds are normal, but scientists aren’t sure why it was so bad this year

The dust behind the recent clouds originates at the convergence of two ecosystems: the Sahara and the Sahel.

The hot, dry Sahara in North Africa is the largest desert in the world outside of the poles. The Sahel is the stripe of land just south of the Sahara with a more tropical climate. It’s also hot and dry, but has a rainy season and can develop dense vegetation.

“Mother nature has really set up things in an interesting way over Africa,” Dunion said. “You’ve got the largest [hot] desert in the world, and then just south of that, you’ve got the hurricane nursery for the Atlantic. Over half the named storms that we get each year are coming from this nursery over the Sahel, just south of the Sahara.”

An archaeologist studies rock engravings in the Sahara Desert in Mauritania. Dust clouds originate in the Sahara, the largest desert in the world outside the poles, and the Sahel, just south of the Sahara.
John Wessels/AFP via Getty Images

Much of the dust originates in the Bodélé Depression in Chad, an ancient dry lake bed at the threshold of the Sahara and the Sahel. There, convective storms in the early summer whip the dry ground and loft particles of silica, iron, and phosphorous as high as 20,000 feet into the sky.

When this airborne dust and dry air floats off the coast of West Africa, it forms the Saharan Air Layer, a segment of the atmosphere that moves across the North Atlantic Ocean every three to five days from the late spring through the early fall.

“It’s not that unusual for these dust storms to develop and cross the Atlantic to South America and North America,” said Colin Seftor, an atmospheric scientist and contractor at NASA. “What’s unusual about this one was this is an extremely large one, number one. And number two, it held together all the way across [the Atlantic Ocean].”

Ordinarily, this dust would start to thin as it approached the Americas, with some falling into the ocean along the way. That’s why there was less noticeable haze in previous years. But there was so much dust picked up recently that even after this thinning process, a lot made it across the Atlantic.

And the Saharan dust doesn’t usually reach from the sky all the way down to the surface like it did this week, creating fog-like conditions in some areas and drastically reducing visibility. The bottom of the Saharan Air Layer typically starts about a mile above the Earth’s surface.

“As you saw in the Caribbean, it seriously affected these people. It turned day into night,” Seftor said. “Everything was multiplied here.”

Researchers are still unclear as to why the recent Saharan dust cloud was so intense since there are so many factors at play. Seftor said it may have to do with the intense rainfall in the Sahel region in May and June. That in turn may have fueled local weather to whip up more dust. But he cautioned that this is just speculation at this point.

Another culprit could be strong tropical waves. These are bands of low air pressure that move from east to west across the tropical Atlantic Ocean. This atmospheric perturbation may have kicked up an exceptional amount of dust from the Sahara.

A cloud of dust from the Sahara covers the lighthouse of Havana on June 25.
Guillermo Nova/Picture Alliance via Getty Images
A woman meditates as a vast cloud of Sahara dust covers San Juan, Puerto Rico, on June 22. Intense rainfall in the Sahel region in May and June may have contributed to the intensity of this year’s dust cloud.
Ricardo Arduengo/AFP via Getty Images

Scientists also haven’t detected an overall trend in the pattern of Saharan dust in recent years, and it’s hard to anticipate what will happen as the climate changes. “If the meteorological conditions of the Sahel or even the Sahara change, and there’s more surface dust, it could change,” Seftor said. “How it will change, I don’t know.”

Researchers are starting to get answers, however. NASA and NOAA have a new generation of geostationary satellites that can monitor movement of these dust clouds in real time.

“We have eyes on these storms that we’ve never had in the past,” Dunion said.

Saharan dust is a vital ingredient in ecosystems in the Western Hemisphere

As they sail over the Atlantic, the Saharan clouds sprinkle bits of dust in their wake. The minerals in this dust can then trigger blooms of phytoplankton on the surface of the ocean. These are microscopic organisms that serve as the foundation of many marine ecosystems, becoming food for animals ranging from tiny crustaceans to giant whales. Across the world’s oceans, phytoplankton soaks up a volume of carbon dioxide comparable to all plants on land.

Saharan dust is critical to other ecosystems too. About 27.7 million tons of these particles fall onto the Amazon rainforest basin every year. Of that dust, roughly 22,000 tons is made of phosphorus, a vital nutrient for soil.

Without this replenishment, the Amazon rainforest would wither. With so much dense, fast-growing vegetation, most of the nutrients in the rainforest are sequestered in plants rather than in the soil. What little is left in the ground gets washed away by frequent rainfall and flooding. So a regular deposit of minerals like phosphorous keeps the most biodiverse ecosystem on the planet verdant and lush.

But perhaps one of the more astonishing effects of Saharan dust is that it can suppress hurricanes. To form, hurricanes need warm water at the ocean surface and moist, unstable air. The Saharan Air Layer has about half the moisture that’s expected in the air over the tropics.

A Saharan dust cloud covers Quintana Roo, Mexico, on June 23.
Medios y Media/Getty Images

“You’re injecting this extremely dry air deep into the tropics where hurricanes tend to form,” Dunion said. “That causes thunderstorms and the developing storms, their clouds, to collapse.”

The Saharan Air Layer also moves fast, with winds up to 50 miles per hour. “That acts to rip the storms apart,” Dunion said. And the layer brings high temperatures to the skies above the seas, with warm air reaching as high as 10,000 feet. That combination of hot, dry air creates downdrafts that can prevent clouds from forming, which need cooler air to condense moisture.

This mechanism also disrupts clouds over land in areas like Florida, weakening or preventing the formation of thunderstorms and allowing the area to accumulate more heat.

However, scientists have had to postpone some of their experiments to measure the interaction of dust with hurricanes because of the Covid-19 pandemic, including flights of hurricane hunter aircraft.

Why dust is so bad for your health

In general, breathing in anything that isn’t air isn’t good for you. But small particles tend to be some of the more dangerous things to inhale.

Scientists usually divide small particles into PM10, particles smaller than 10 microns in diameter, and PM2.5, which are smaller than 2.5 microns in diameter. PM10 tends to get trapped in the nasal passages while PM2.5 can go further into airways.

“[I]t’s the smaller particles that will get transported further and will cause the greatest health harms, as they can penetrate deep into the lungs and even enter the bloodstream,” said Ploy Pattanun Achakulwisut, a scientist at the Stockholm Environment Institute, in an email.

Residents wear face masks to protect themselves from the Saharan dust clouds covering Dakar, Senegal, on February 25. N95 masks and even surgical masks can help protect people from getting sick from the dust.
Alaattin Dogru/Anadolu Agency via Getty Images

Breathing dust can trigger problems like asthma attacks and worsen conditions like heart disease. But particles from natural sources can pose some unique threats. “Desert soil can also be contaminated with bacteria and fungal spores or with toxic heavy metal,” Achakulwisut said. “For example, in the US Southwest, dust episodes there have been linked to outbreaks of Valley Fever and arsenic poisoning.”

The particles themselves can be suspended in the air as aerosols, making them easy to inhale and hard to avoid. Together, these factors make dust a potent health hazard. According to the United Nations Environment Programme, dust storms sap $13 billion a year from the global economy because of their health effects.

In the summer months, this dust can coincide with other air quality hazards like ozone, which forms more on hot days, further reducing air quality.

The impact of the recent Saharan dust cloud was likely most severe in the Caribbean. Ordinarily, the concentration of PM10 is 10 to 20 micrograms per cubic meter of air, mostly due to sea salt. During the peak of the Saharan Air Layer, parts of the Caribbean reported particle concentrations above 400 micrograms per cubic meter.

But it will take some time to figure out the full health impacts of the dust. “How bad of a problem it is has really not been determined yet because it takes a specific dedicated program where you get a lot of data from hospitals and clinics and bring it together with the meteorology,” said Joseph Prospero, a professor emeritus of atmospheric sciences at the University of Miami.

As more dust clouds roll in, more people may get sick. But people can protect themselves with the same tactic that they use to control Covid-19: face masks.

“Any N95 mask will protect you, but even the regular surgeon’s mask will do a pretty decent job,” Prospero said. “Because dust particles are so large, it should have a fairly decent efficiency for minimizing inhalation.”

Future Perfect

Why psychedelics produce some of the most meaningful experiences in people’s lives


US life expectancy no longer catastrophic, now merely bad

The Future Perfect 50

Christopher Fuchs is revolutionizing how we understand our quantum reality

View all stories in Science