What was done
Motivated by the fact that the Arctic "experiences significant periodic influxes of anthropogenic aerosols, which originate from the industrial regions in lower latitudes (Barrie, 1986)," the authors used multisensor radiometric data to show that "enhanced aerosol concentrations alter the microphysical properties of Arctic clouds, in a process known as the 'first indirect' effect (Twomey, 1977; Garrett et al., 2002)."

What was learned
From their analyses of a number of data sets obtained from the North Slope region of Alaska, Lubin and Vogelmann determined that "under frequently occurring cloud types" the first indirect effect of the real-world anthropogenic aerosol pollution experienced there "leads to an increase of an average 3.4 watts per square meter in surface longwave fluxes."

What it means
The two researchers say the increased flux of longwave radiation received at the surface of the earth as a result of the increased presence of anthropogenic aerosols in the Arctic atmosphere "is comparable to [the] warming effect from established greenhouse gases and implies that the observed longwave enhancement is climatologically significant." And it truly is! Hansen et al. (2000), for example, have calculated that the radiative forcing of the atmospheric CO2 increase experienced between 1850 and 2000 was 1.4 W/m², and that the combined concomitant radiative impact of all other greenhouse gases was about the same, making their total radiative impact at the turn of the last millennium approximately 2.8 W/m², which is actually just slightly less than the radiative forcing calculated by Lubin and Vogelmann to have been provided by the real-world increase in aerosol pollution experienced in the Arctic over the same period.

These observations raise some interesting questions. Do they imply that essentially all anthropogenic-induced warming in the Arctic may be attributed to the first indirect effect of anthropogenic aerosol pollution alone? Do they imply that earth's climate sensitivity to radiative perturbations is only half as great as what scientists typically assume it to be? Do they imply that Lubin and Vogelmann's results are wrong? Clearly, "something's got to give" in order to make everything we think we know mesh with everything else we think we know.

References
Barrie, L.A. 1986. Arctic air pollution: an overview of current knowledge. Atmospheric Environment 20: 643-663.

Garrett, T., Radke, L.F. and Hobbs, P.V. 2002. Aerosol effects on cloud emissivity and surface longwave heating in the Arctic. Journal of the Atmospheric Sciences 59: 769-778.

Hansen, J., Sato, M., Ruedy, R., Lacis, A. and Oinas, V. 2000. Global warming in the twenty-first century: An alternative scenario. Proceedings of the National Academy of Sciences USA 97: 9875-9880.

Twomey, S. 1977. The influence of pollution on the shortwave albedo of clouds. Journal of the Atmospheric Sciences 34: 1149-1152.