The emission of greenhouse gases over the past 150 years is driving global warming, which scientists warn may soon pass 1.5°C relative to preindustrial temperatures. But rising mineral dust levels in the atmosphere are counteracting that warming to some extent, according to a study published in Nature Reviews Earth and Environment.
“Models really struggle with getting the changes in dust,” said University of California, Los Angeles atmospheric physicist Jasper Kok. “Part of that is just the physics of it is so complicated. It’s also happening on very small scales that we don’t resolve in models, and we don’t have data for in models.”
To reconstruct the history of mineral dust in the atmosphere, Kok and his colleagues compiled 25 records of dust deposition from around the world, including ice core samples from Antarctica and lake bed sediments from North America. Their reconstruction also factored in the locations contributing the most dust, including Africa, the Middle East, central and eastern Asia, Australia, and North and South America.
Examining 150 years of atmospheric dust data, Kok and his coauthors found that airborne dust levels increased by around 55% between the mid-19th century and the 1980s and have largely held steady since.
To estimate mineral dust’s net effect on climate, the researchers made use of climate and dust interaction models built on existing data from both satellites and ground-based photometers, which measure how much the Sun is blocked by airborne aerosols, including dust. That modeling is complicated and uncertain, Kok said, because of the complex ways in which dust appears to interact with other Earth systems, such as clouds.
Floating dust grains act as seeds for ice crystals around which clouds can form. But different clouds have varied effects on cooling. Mixed-phase clouds, which are composed of both supercooled water droplets and ice crystals, are more likely to produce a cooling effect, whereas high, wispy cirrus clouds cause warming.
And then there are other aerosols in the atmosphere, particularly sulfates and nitrates from human-caused pollution. Previous research has shown that these pollutants have an even greater cooling effect than mineral dust, but dust helps pull many of those pollutants out of the atmosphere, Kok explained, creating a warming effect that most climate models do not account for.
Kok and his colleagues pegged the overall cooling effect of mineral dust at 0.25–0.07 watt per square meter. Because previous research has suggested that the atmosphere has warmed by 3–4 watts per square meter from the preindustrial period, dust has “masked” up to 8% of the global warming. “That implies that the climate is just a little bit more sensitive to greenhouse gas warming than we thought,” Kok said.
“If dust were to decline, it would speed up the warming further,” he said.
Limited Dust Data Continue to Fuel Uncertainty
The uncertainty in dust’s effect due to so many factors is made worse by the generally poor data Kok’s study has had to rely on, according to Gisela Winckler, a paleoclimatologist at Columbia University. The study relied on 25 records of historic dust levels, and “that’s very poor in coverage for when you try to do a global view of mineral dust,” she said. “I have no doubt that this [analysis] is robustly done, but still, the underlying data sources are poor.”
A major question is whether mineral dust levels will hold steady or decline, but answering it would require understanding what is driving changes in dust levels. Changes in land use are known to create atmospheric dust problems on human timescales, such as the Dust Bowl that struck the United States in the 1930s, according to Paul Ginoux, a climate modeler with NOAA’s Geophysical Fluid Dynamics Laboratory. But the data available are often too spaced out in time, he said. In the new study, the researchers “show an increase [in dust], but they totally missed the Dust Bowl.”
Climate change itself may affect dust levels in multiple ways, such as through shifting wind patterns. “Wind strength has a lot to do with actually lofting that dust into the atmosphere,” Winckler said. Climate scientists expect a warming climate to result in stiller winds, which could reduce the levels of dust blown aloft into the atmosphere.
The Future of Dust and Climate Studies
Kok, Ginoux, and Winckler each said that more data are needed to improve how climate models account for dust, and there are new opportunities to do just that. Ginoux is part of the science team for the Earth Surface Mineral Dust Source Investigation (EMIT), an instrument recently installed on the International Space Station to monitor dust levels in the atmosphere.
One of the takeaways of the study is the need to research mineral dust more locally, in the regions where it originates or could originate, because climate change itself changes local conditions, Kok said. “In California, this might be quite an important discussion, because the U.S. Southwest in general is expected to get drier” and therefore dustier. Kok and his colleagues recently began early work on a study of dust in California to report on “how that might change and what the implications could be. And maybe what policymakers could do about it.”
—Jon Kelvey (@jonkelvey), Science Writer