The cooling effect of aerosols may be short-lived, says Franziska Glassmeier

The cooling effect of clouds modified by atmospheric aerosols – particles suspended in the atmosphere – may be overestimated. That is the conclusion of a new international study led by Branco Weiss Fellow Franziska Glassmeier. Cloud decks covering vast stretches of the subtropical oceans have a cooling effect on the planet’s climate because they reflect incoming sunlight back to space. Air pollution in the form of aerosols can increase this cooling effect because it makes clouds brighter. It has been suggested that humans could temporarily offset some of the impacts of climate change by artificially seeding clouds with aerosol particles to brighten them and increase their reflectivity. Glassmeier’s research shows that this is highly uncertain because unintended consequences are possible even on the level of cloud physics.

As an example of clouds becoming brighter as a result of aerosols, “ship tracks” are often mentioned. These are visible as bright lines within a cloud deck that reveal the paths of polluting ships that travel beneath the clouds. But the paper shows that ship tracks are not an ideal object to study the effects of aerosols on clouds and climate. They tend to dim and disappear too quickly.

Instead, the researchers created an extensive data set of detailed cloud simulations. At the heart of their study, they designed a clever new way to compare their simulated cloud decks to satellite snapshots. Such snapshots contain information about aerosol effects on clouds all over the globe but have so far been hard to interpret. And the results show that polluted clouds get brighter at first, indeed. But after a while they start to get thinner and thus less bright again. “Our conclusion is that the cooling effect of aerosols on clouds is overestimated when we rely on ship-track data”, says Glassmeier. “To properly quantify these effects and get better climate projections, we need to improve the way clouds are represented in climate models.”

Read the paper in Science

Read the press release from TU Delft

Read the news on Science News

Read the news on wissenschaft.de (in German)

Read the news on Physik in unserer Zeit (in German, paywall)