Not all air pollution contributes to warming our planet. For example, sulfate aerosols, which form from industrial emissions, generally have a significant cooling effect. In some climate models, sulfate and other aerosols offset up to half of the heating from greenhouse gas emissions. This cooling effect occurs both directly, by scattering incoming solar radiation, and indirectly, by affecting cloud properties such as water-droplet size and water content that cause the clouds to reflect more sunlight away from the earth.
Still, the effect of aerosols on clouds remains one of the biggest sources of uncertainty when it comes to predicting future climate. Now, a new paper suggests that these tiny, airborne particles are not cooling the earth as much as climate scientists had previously thought (Nature 2019, DOI: 10.1038/s41586-019-1423-9).
A decrease in the size of water droplets in clouds caused by aerosols, known as the Twomey effect, is known to increase cloud reflectivity, but the effects of the pollution particles on changing water content are less understood. Previous attempts at quantifying cloud water changes triggered by aerosols struggled to disentangle the complexities of clouds. For example, climate scientists have wondered whether changes in aerosol levels directly cause changes in clouds, or whether an external meteorological factor affects both aerosols and clouds separately.
Nicolas Bellouin, a climate scientist at the University of Reading, and colleagues tried to reduce that uncertainty by looking directly at known sources of aerosols—such as industrial plants, wildfires, and ships—and comparing clouds downwind of them with adjacent clouds.
The team used 15 years’ worth of near-infrared imagery from the Moderate Resolution Imaging Spectroradiometer (MODIS) satellite, manually identifying more than 2,000 “pollution tracks” from aerosol sources over that time period. Scientists have used this approach previously to study properties of ship tracks, the clouds sometimes generated in the wake of cargo ships. “The track approach is really powerful,” Bellouin says, “because in the same image, we can view both unpolluted and polluted clouds.”
Averaged over the globe, the Twomey effect is estimated to have a negative radiative forcing, meaning it cools the planet, and current climate models predict that changes in cloud water content will result in a cooling effect of a similar magnitude. However, the new study finds that changing cloud water actually leads to a positive radiative forcing, meaning it warms the planet. The net result is that the cooling effect of aerosols on clouds is nearly 25% lower than previously thought.
People have been trying for decades to resolve the uncertainty associated with cloud-aerosol interactions without much success, says Geeta Persad, a research associate with the Carnegie Institution for Science at Stanford University. This paper, she says, makes a major contribution to the field of climate science by providing a “pretty comprehensive, observationally-based constraint” on the strength of the cloud-aerosol interaction.
One caveat is that clouds with visible pollution tracks make up a relatively small fraction of all clouds, Persad says. However, Bellouin notes, there’s no physical reason why clouds without discernable pollution tracks should behave differently from those with them, and therefore the results can be extrapolated to these other clouds.
Bellouin, who is also a climate modeler, says the next step is to begin to integrate these results into next-generation climate predictions. “It’s clear that some climate models overestimate the cooling by pollution particles,” he says. “The next challenge is to fix the model.”
This story was updated on Aug. 1, 2019, to to correct the description of sulfate aerosols to indicate that they're formed from industrial emissions.