Background
In setting the stage for their study, the authors state that "present-day radiative forcing by anthropogenic greenhouse gases is estimated to be 2.1 to 2.8 Wm^-2; the direct forcing by anthropogenic aerosols is estimated to be -0.3 to -1.5 Wm^-2, while the indirect forcing by anthropogenic aerosols is estimated to be 0 to -1.5 Wm^-2," so that "estimates of the total global mean present-day anthropogenic forcing range from 3 Wm^-2 to -1 Wm^-2."  Now let's see, that would be somewhere between a warming and a cooling, right?  And that would seem to be rather shaky justification for the worldwide institution of draconian measures to fight potential global warming, which could well turn out to be potential global cooling, right?  Right; for as the authors themselves say, "clearly" - and we love that word clearly, for it is obviously most appropriate - "clearly, 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."  Doing so, however, will, as they say, "require profound reductions in the uncertainties of direct and indirect forcing by anthropogenic aerosol," which is what they thus set out to do, i.e., reduce the uncertainties.

What was done
In the words of the authors, they employed a strategy that consisted of "a combination of process studies designed to improve understanding of the key processes involved in the forcing, closure experiments designed to evaluate that understanding, and integrated models that treat all of the necessary processes together and estimate the forcing."  For more details - and there's lots of them - we recommend direct consultation of their paper.

What was learned
At the end of their laborious investigation, Ghan et al. come up with some numbers that considerably reduce the range of uncertainty in the "total global mean present-day anthropogenic forcing," but it still stretches from a small cooling influence to a modest impetus for warming.  Hence, they present a long list of other things that must be done in order to obtain a more definitive result, after which they acknowledge that "this list is hardly complete."  Indeed, they conclude their treatment of the topic by saying "one could easily add the usual list of uncertainties in the representation of clouds, etc."

What it means
The bottom line, in the words of the authors, is that "much remains to be done before the estimates are reliable enough to base energy policy decisions upon," to which we can only say "Amen!"