New research from UCLA shows that airborne desert dust traps roughly twice as much heat in the atmosphere as most climate models have estimated, a finding that could sharpen both short-term weather forecasts and long-term climate projections.
Lead researcher Jasper Kok, a UCLA professor in the department of atmospheric and oceanic sciences, said the heat-trapping effect of dust is equal to approximately 10 percent of the warming caused by human-emitted carbon dioxide, one of the principal greenhouse gases driving climate change. Most current climate models put that figure at only about 5 percent.
Despite the elevated heating role, dust likely still exerts a net cooling effect on Earth's climate overall, because it simultaneously reflects a portion of incoming sunlight back into space. Kok noted that the new results do not overturn existing models but will make them more precise.
"Improving how models represent warming caused by dust could therefore help improve both weather forecasts and climate projections," Kok said. "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."
To build a global estimate, the UCLA-led team combined satellite observations of atmospheric dust abundance and its effects on outgoing heat radiation, aircraft measurements of particle sizes, climate model simulations describing the spatial distribution of dust, and meteorological data on atmospheric temperature structure.
"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."
The researchers identified two main reasons models underestimate the warming contribution. First, the models typically do not account for the way dust scatters heat emitted by Earth's surface and atmosphere back downward. Second, models also undercount the quantity of very coarse dust particles suspended in the atmosphere, which are especially efficient at trapping heat. Kok said roughly 20 million metric tons of very coarse dust are aloft at any given time – equivalent in mass to approximately 4 million African elephants – yet models account for only about a quarter of that volume.
Atmospheric dust levels rose during the twentieth century, peaking in the 1980s for reasons scientists are still investigating. Concentrations have declined since then but remain elevated above pre-industrial levels. Major dust sources today include the Sahara, the Gobi Desert, and drying lakebeds where human water diversions have exposed bare sediment, such as the Salton Sea, Owens Valley and the Great Salt Lake.
"A lot of my work is about fine-tuning climate models," Kok said. "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 support from NSF's National Center for Atmospheric Research, NASA and the Department of Energy.
Research Report:Revised global estimate of the direct longwave radiative effect of dust
