Observational confirmation of aerosol impact on clouds

17 July 2020

A team from Oxford’s Department of Physics has observationally confirmed a long-standing hypothesis that aerosols can extend the lifetime of low-level cloud, thereby cooling the earth.

The observational results suggest that the enhanced aerosol cooling potential from longer-lived clouds is essential to include in climate models in order to accurately predict global mean temperatures as greenhouse gases and aerosol emissions continue to change in the future.

'This is an important finding for climate modelling as the underlying fine-scale cloud processes are currently not correctly represented,' explains Professor Philip Stier.

Time-resolved observations of evolving weather systems

The results have been obtained combining trajectory modelling with imagery from satellites in geostationary orbit. These satellites observe the earth from a fixed relative position thereby providing time-resolved observations of continuous evolving weather systems. Clouds are tracked from thousands of Lagrangian trajectories spanning several days along the classic stratus-to-cumulus cloud transition zones. The trajectories are then sorted by the amount of aerosols to reveal that, compared to unpolluted trajectories, relatively polluted trajectories tend to have: lighter precipitation rates, longer average lifetimes (by 2.5 hours), higher cloud albedo and cloud fraction.

The work was led by Matthew W. Christensen together with William K. Jones and Philip Stier from the Climate Processes group, part of the Atmospheric, Oceanic and Planetary Physics sub-department and was supported by funding from the European Research Council.

Aerosols enhance cloud lifetime and brightness along the stratus-to-cumulus transition’, Proceedings of the National Academy of Sciences (PNAS)

Image © William K Jones