15 May 2017

Hope for Constraining Atmospheric Particles’ Effects on Clouds

A new view of the cloud droplet number-aerosol number relationship offers hope for constraining estimates.

Science

Much of the uncertainty in estimating the human influence on the Earth’s energy imbalance comes from the interaction between atmospheric aerosol and clouds. Researchers, including scientists at the U.S. Department of Energy’s Pacific Northwest National Laboratory, showed that if the human contribution to the aerosol is known, the energy imbalance from aerosol-cloud interactions can be calculated to within 20 percent of its actual value.

Impact

This study offers hope that modern observations can constrain estimates of the influence of human activity on cloud droplet number and climate.

Summary

Uncertainty in the strength of aerosol-cloud interactions drives the uncertainty in the human-caused energy imbalance. Previous studies highlighted shortcomings in using satellite data to determine the imbalance (forcing) because the data underestimated the strength of the aerosol forcing. As cloud droplets form on aerosol particles, changes in the aerosol number concentration can change the cloud droplet number concentration and lead to an instant energy imbalance. A research team, including scientists from Pacific Northwest National Laboratory, considered both the cloud droplet number and aerosol number from global model simulations for preindustrial (PI) and present-day (PD) aerosol emissions. Comparing frequency distribution graphs, they found little difference between the PI and PD emissions, and that the PD frequency distribution and the change in the aerosol can lead to an accurate estimate of the anthropogenic emissions' change impact on the cloud droplet number. These findings suggest that the PD relationship between the cloud droplet number and aerosol number can constrain the estimated impact of human-caused emissions on the cloud droplet number. The work shows that by combining model results and satellite data, more accurate estimates of the aerosol influence on climate can be made.

Contact
Steven J Ghan
Pacific Northwest National Laboratory
Publications
Gryspeerdt, E., Quaas, J., Ferrachat, S., et al. "Constraining the Instantaneous Aerosol Influence on Cloud Albedo." Proceedings of the National Academy of Sciences of the United States of America 114, 19 (2017). [10.1073/pnas.1617765114].