What was done
The authors employed five multisensor radiometric data sets from the North Slope of Alaska to study how enhanced concentrations of anthropogenic aerosols originating from industrial regions of lower latitudes alter the microphysical properties of Arctic clouds via a process known as the first indirect effect of aerosols.

What was learned
Lubin and Vogelmann determined that this phenomenon operates in low optically-thin single-layered Arctic clouds, producing an increase in the downwelling flux of longwave (thermal) radiation. More specifically, they found and that under frequently occurring cloud types, anthropogenic aerosols regularly advected into the Arctic lead to an average increase of 3.4 W m^-2 in the downward-directed thermal radiation flux at the earth's surface.

What it means
The two researchers state that "the observed longwave enhancement has climatological significance"... and indeed it has. Charlson et al. (2005), for example, report that the longwave radiative forcing provided by all greenhouse gas increases since the beginning of the industrial era amounts to only 2.4 W m^-2, citing the work of Anderson et al. (2003), while Palle et al. (2004) say that "the latest IPCC report argues for a 2.4 W m^-2 increase in CO2 longwave forcing since 1850." Consequently, if the calculations of Lubin and Vogelmann are correct, the longwave radiative forcing of the anthropogenic aerosols that are advected into the Arctic on a regular basis may well be larger than the combined forcing of all greenhouse gas increases since the beginning of the industrial era, which suggests that recent increases in anthropogenic aerosol emissions could be the primary source of whatever Arctic warming may have occurred in recent years.

References
Anderson, T.L., Charlson, R.J., Schwartz, S.E., Knutti, R., Boucher, O., Rodhe, H. and Heintzenberg, J. 2003. Climate forcing by aerosols - a hazy picture. Science 300: 1103-1104.

Charlson, R.J., Valero, F.P.J. and Seinfeld, J.H. 2005. In search of balance. Science 308: 806-807.

Palle, E., Goode, P.R., Montanes-Rodriguez, P. and Koonin, S.E. 2004. Changes in earth's reflectance over the past two decades. Science 304: 1299-1301.