Key takeaways
- New UCLA research finds that airborne dust traps about twice as much heat as climate models have estimated, even though dust likely cools the planet overall.
- The effect of dust retaining heat is equal to roughly 10%, not the previously calculated 5%, of the warming effect of human-emitted carbon dioxide, one of several greenhouse gases.
- The overall global climate effect of dust remains uncertain because the reflection of sunlight partly offsets this warming.
- These new findings could increase precision in climate models and weather forecasts.
Atmospheric dust plays a dual role in Earth's climate: it reflects some sunlight back into space while also absorbing and retaining the planet's heat like an insulating blanket. But while dust likely cools the planet overall, that's not the whole story. New UCLA research shows that the heat-trapping effect of airborne desert dust in the atmosphere is about twice as big as previously believed.
Although researchers emphasized that current climate models are performing well, the new findings will further increase precision. Updating climate and weather models to account for the larger heat-trapping power of dust could improve both short-term weather forecasts and long-term climate projections, said lead researcher and UCLA atmospheric scientist Jasper Kok.
Using data from satellites, aircraft measurements and new climate simulations, combined with meteorological data related to temperature, UCLA-led researchers developed a global estimate, shared in a study newly published in Nature Communications. They found that the heat-trapping effect of dust is equal to about 10% of the warming effect of human-emitted carbon dioxide — one of the greenhouse gases responsible for climate change — while most climate models estimate only about 5%.
"Improving how models represent warming caused by dust could therefore help improve both weather forecasts and climate projections," said Kok, a UCLA professor in the department of atmospheric and oceanic sciences. "Regions with more dust will be a little warmer, leading to higher surface temperatures and faster evaporation. This can alter atmospheric motions and shift where and when rainfall occurs — for example, by suppressing precipitation in some regions while enhancing it in others. Such effects are expected to be strongest in regions downwind of major deserts such as in the Sahara, the Middle East and East Asia."
Depending on size, elevation, concentration and other factors, atmospheric dust can have a cooling effect by reflecting sunlight back into space or a warming effect by absorbing and scattering heat radiation back toward Earth, Kok explained. Some of Kok's previous research showed that these opposing effects of dust combine to create a small cooling effect that slightly slows but does not stop the pace of global warming. His team's new research shows that part of dust's overall cooling effect is overcoming an even greater heating effect than climate models previously recognized.
"Atmospheric dust traps about a quarter of a watt per square meter of heat by absorbing and scattering the heat radiation emitted by the Earth, comparable to roughly one-tenth of the warming effect produced by the carbon dioxide emitted from all human activities," Kok said. "Current climate models undercount the heating effect of dust by about half. The climate models remain effective and useful, and this will make them even more precise."
Atmospheric dust increased in the twentieth century, peaking in the 1980s for reasons scientists are still exploring. While dust has declined since the '80s, it's still elevated compared to pre-industrial times, Kok said. Much of the dust today comes from deserts like the Sahara and the Gobi, as well as drying lakebeds where agriculture and other needs led humans to reroute water, such as at the Salton Sea, Owens Valley and the Great Salt Lake, Kok said.
"Climate models are slightly too conservative, because they typically omit the ways dust scatters heat emitted by Earth's surface and atmosphere back downward," Kok said. "Our previous research showed that the models also undercount the amount of very coarse dust particles in the atmosphere, which are especially effective at trapping heat. There are about 20 million metric tons of very coarse dust in the sky — the mass of roughly 4 million African elephants — and models account for only about a quarter of that."
The scientists united multiple sources to reach their conclusions. They obtained satellite observations of atmospheric dust's abundance and its effects on Earth's outgoing heat radiation. Aircraft measurements provided the sizes of dust particles. Climate model simulations showed the spatial pattern of dust in the atmosphere. Meteorological data described the temperature structure of the atmosphere.
"A lot of my work is about fine-tuning climate models," Kok added. "But that doesn't mean the models are wildly wrong, they're just not 100% perfect. There's so much complexity in our climate system, and the models are right on the money about the dangerous warming we're experiencing."
The research was funded in part by grants from the National Science Foundation, with additional funding or assistance from NSF's National Center for Atmospheric Research, NASA and the Department of Energy, among others.
