Climate Forcings and Feedbacks
Lumo waxes wroth on the subject of the title here. If you drill down through the imprecations and overheated rhetoric, you will find that his point seems to be that one cannot sharply distinguish between forcings (exogenous factors which increase planetary temperature such as solar output, or CO2 from industrial emissions) and feedbacks (e.g. increased atmospheric water vapor due to increased surface temperature).
To which I say Tru Dat, but...
In particular, at issue seems to be the argument that clouds represent a forcing rather than a feedback. That could be the case, for example, if changing cloud cover is due to say cosmic radiation or some kind of volcanic action. On the other hand,if changing cloud cover is due to the heating of the surface and consequent greater moisture in the air, its a feedback.
In practice, as the Lumonator notes, you can't draw a bright line between feedback and forcing, because there are overlaps. That doesn't mean that it's not useful to make the distinction - consider the scattering type problems Lumo invokes - and it certainly doesn't mean that Spenser and friend have made a convincing argument that clouds are (mainly) a forcing.