Three miles above the United States, the atmosphere is filled with dust that has blown thousands of miles from Asia and elsewhere.
The discovery, which used data from a new satellite that provided a three-dimensional view of the air from above, suggests that the transport of tiny dust particles around the world may be having an unexpectedly large impact on storms, temperatures and other weather patterns.
Future climate models may need to factor in dust transport to more accurately predict what's to come and what we can do about it.
"Half the particles in North American air come from somewhere else," said Lorraine Remer, an aerosol scientist at the University of Maryland, Baltimore County. "That was news to me. I had no expectations of that."
"It's telling us in a lot of ways how small the world is and how interconnected the airspace is," she added. "So much of what we see in our skies comes from someplace else. And if we're working towards solutions, there has to be global engagement to do so."
Along with ozone, carbon dioxide and other gasses, aerosols hang in the air. These particles come from coal-burning and other manmade sources of pollution as well as from natural events, including forest fires and volcanic eruptions.
Close to the surface, aerosols pose threats to human health, raising the risk of heart attacks, respiratory problems, and other issues. Higher up in the atmosphere, particles absorb heat and reflect sunlight. Because particles provide surfaces for the formation of ice crystals, aerosols can also affect cloud formation and precipitation. Another concern is that aerosols will settle on the high mountains of the western United States, causing snow to melt more quickly.
Plenty of researchers have attempted to use modeling studies to estimate the concentration of aerosol particles in the atmosphere, but results have varied widely. To get the first accurate measurement of what kinds of particles are up there and how they're moving, Remer and colleagues used data from two satellites.
One satellite showed pictures of cloud and aerosol patterns over Earth from above. The other, a new satellite called Calypso, offered a three-dimensional view of how high up the aerosols were, kind of like a CT scan of the atmosphere from the surface to space. The researchers combined data over a several years to get a seasonal picture of where the particles were and how they moved.
Together, the images showed that the concentration of particles that moved from Asia to North America equaled the total concentration of particles produced by North America, the researchers report today in the journal Science.
The good news, at least for our health, was that the aerosols that blow in from Asia stayed high up in the atmosphere -- between about four kilometers (2.5 miles) and six kilometers (3.7 miles) above the surface.
Satellite images also showed that the majority of high-flying aerosols were made up of desert dust, which can affect climate patterns in a number of ways. And while experts continue to debate the effects of climate change and development on the conversion of fertile lands to deserts in many parts of the world, most of the dust in the atmosphere probably comes from the natural lifting of desert sands, said Daniel Jacob, an atmospheric chemist at Harvard University in Cambridge, Mass.
"In terms of the effect of aerosols on climate, we tend to be fixated on human activity," Jacob said. "But we have this big dust haze layer that's several miles over our heads. This study nicely reminded us of that."
The next step is to figure out exactly how all that dust is contributing to shifts in global climate patterns. It could go either way. Dust scatters solar radiation back into space, potentially creating a cooling effect. But with their yellow-brown color, dust particles also absorb radiation, potentially contributing to atmospheric warming.
"Right now, we don't even know what the feedback is for dust on climate impacts, but we know it's potentially big," Jacob said. "Satellite observations like this are really going to be helpful to better represent dust in our climate models."