Aerosol Pollution Warming Effects on Climate Due to Their Impacts on Cold Icy Clouds
Because ice clouds are nucleated by aerosol particles, changes to the aerosol composition may alter the ice crystal properties, ice-cloud reflectivity of incoming sunlight, and absorption of outgoing long-wave radiation. This aerosol-ice-cloud effect is poorly understood, largely unconstrained, but potentially quite significant. U.S. Department of Energy researchers, including at Pacific Northwest National Laboratory, quantified these aerosol “indirect” effects (AIE) on high-altitude cirrus clouds, using several different ice nucleation formulations in two different advanced General Circulation Models (GCMs): the Community Atmosphere Model version 5 (CAM5), and the European Center Hamburg model version 5 with the Hamburg Aerosol Model. They investigated (a) the climate states simulated by different ice nucleation schemes, (b) anthropogenic effects on ice clouds, and (c) the role of black carbon (soot) as ice nuclei in ice clouds. Different ice nucleation formulations in the two climate models result in different balances between “homogeneous” nucleation (freezing of cold sulfate droplets) and “heterogeneous” nucleation (cloud particle freezing enhanced by contact against a solid sulfate-coated dust particle). However, the magnitude of AIE on ice clouds is remarkably similar with the total ice AIE estimated at 0.27 ±0.10 W m-2, a warming effect. This warming effect represents a 20 percent offset of the simulated total shortwave scattering of incoming radiation (cooling) AIE of -1.6 W m-2. Black carbon (soot) aerosols have a small AIE (-0.06 W m-2) for the ice nucleation efficiencies within the range of laboratory measurements. This study is one of the first to estimate the warming effect of aerosols on high-altitude ice clouds.
Gettelman, A., X. Liu, D. Barahona, U. Lohmann, and C. Chen. 2012. “Climate Impacts of Ice Nucleation,” Journal of Geophysical Research 117, D20201. DOI: 10.1029/2012JD017950.