Aerosols Help Clouds Warm Up Arctic
Lubin, D., National Science Foundation
Radiation Measurements
Radiative Processes
Lubin, D., and A.M. Vogelmann, 2006: A climatologically significant aerosol longwave indirect effect in the Arctic, Nature, 439, 26 January, 453-456, doi:10.1038/nature04449
In a process known as the first aerosol indirect effect, enhanced aerosol concentrations cause the droplets in a cloud to be smaller and more numerous within a cloud of fixed water amount. This study found that this process can make many clouds more opaque and emit more thermal energy to the surface.
The warming of the Arctic climate and decreases in sea ice area and thickness observed over recent decades are believed to result from greenhouse gas warming. The Arctic region also experiences significant periodic influxes of anthropogenic aerosols, which originate from the industrial regions in lower latitudes. In the January 26 issue of Nature magazine, scientists supported by the Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) Program found that enhanced aerosol concentrations increase the amount of thermal energy emitted by many Arctic clouds to the surface, which would augment that by greenhouse gas warming. The increase found is comparable to the surface warming effect from established greenhouse gases, suggesting that it is significant to the Arctic energy balance.
In the Arctic where the sunlight is generally weak, the clouds, via their normal emission of thermal energy, exert a net warming on the Arctic climate system throughout most of the year, except briefly during the summer. In a process known as the first aerosol indirect effect, an increase in aerosol amount causes an increase in cloud droplet concentration and a decrease in droplet size within a cloud of fixed water amount. Until now, scientists knew little about how this process would affect the cloud's emission of thermal energy to the surface. The key to understanding this process lay in the long-term measurements made at the DOE ARM Climate Research Facility in Barrow, Alaska, which has an extensive suite of sophisticated instruments for measuring the surface energy balance and atmospheric properties. These data were used in concert with aerosol measurements made next door by the National Oceanic and Atmospheric Administration (NOAA) Climate Modeling and Diagnostics Laboratory (CMDL) facility. Six years of data were used to determine the impact of aerosol on Arctic clouds and the surface thermal energy budget. The study focused on thin, single-layer clouds that are close to the surface, with temperatures that would favor them containing liquid water. Liquid water was recently discovered to largely govern Arctic cloud radiative properties during spring and summer, with liquid water being found in clouds at temperatures as low as -34 C.
The ARM measurements were used to determine that the aerosol first indirect effect operates in these clouds that frequently occur in the Arctic, causing cloud droplets to be smaller when the aerosol concentrations are high. At the same time, there is a significant increase measured in the clouds' downwelling thermal energy. The portion of the energy that can be attributed solely to the systematic changes in the cloud droplet size is an average of 3.4 watts per square meter, comparable to the surface warming effect from established greenhouse gas enhancements. Since the cloud amount during the Arctic spring generally exceeds 80 percent, this implies that the observed enhancement is significant to the Arctic energy balance.
