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Pollution, Aerosols and Cloud-Top Temperatures
Devasthale, A., Krüger, O. and Grassl, H.  2005.  Change in cloud-top temperatures over Europe.  IEEE Geoscience and Remote Sensing Letters 2: 333-336.

In a previous study, Ghan et al. (2001) opined that "the great uncertainty in the radiative forcing must be reduced if the observed climate record is to be reconciled with model predictions and if estimates of future climate change are to be useful in formulating emission policies."  They added, however, that accomplishing this goal will "require profound reductions in the uncertainties of direct and indirect forcing by anthropogenic aerosol."  In the present study, Devasthale et al. move us in this direction by evaluating the potential influence of aerosols on cloud-top temperature (CTT).

What was done
The authors examined the possible influence of aerosols on CTTs using long-term satellite brightness temperatures from the Advanced Very High Resolution Radiometer onboard the NOAA satellite series over eastern and central Europe.  In order to identify and quantify possible aerosol-induced changes, CTTs were compared for two four-year periods of distinctively higher (1985-1998) and lower (1997-2000) anthropogenic pollution loads.

What was learned
The results of the analysis, according to Devasthale et al., were nothing short of "astonishing."  For all types and levels of clouds analyzed, over both land and sea, lower CTTs were observed during the higher pollution levels of the late 1980s, such that the tops of low- and medium-level clouds were more than 2°C colder during that earlier period, and about 4°C colder if convective clouds were included.  What is more, regions of greater CTT variability coincided with metropolitan areas and industrial centers that contained higher concentrations of particulate matter and black carbon inventories.

What it means
One implication of these findings, in the opinion of the researchers involved, is that the "observed indirect aerosol effect in the thermal infrared on regional scale could be equally important if not dominant over the solar radiation changes and thus needs to be further investigated for other regions."  We agree, and add that this effect may also be large enough to have had a significant influence on the near-surface and tropospheric temperature histories of the past few decades.  Also like the authors, we too urge that more research be conducted in this area.

Ghan, S.J., Easter, R.C., Chapman, E.G., Abdul-Razzak, H., Zhang, Y., Leung, L.R., Laulainen, N.S., Saylor, R.D. and Zaveri, R.A.  2001.  A physically based estimate of radiative forcing by anthropogenic sulfate aerosol.  Journal of Geophysical Research 106: 5279-5293.

Reviewed 23 November 2005