Some interesting ideas that are surfacing at Judith Curry’s “Cloud wars” blog post, leading to the conclusion that there might as well be no such a thing as a “pure” climate forcing.
A “pure” climate forcing (i.e. one that occurs only as a forcing to the climate system, not also as a feedback) is of course an independent input to the system. IOW it is an independent variable that will provide its “push” in a specific direction whatever the value of all other variables.
It sounds obvious (maybe not to the average Climate Believer) that a great deal of so-called forcings aren’t: clouds of course (both a forcing and a feedback, perhaps on different timescales); but also CO2 emissions (as noted by commenter Eric Ollivet), water vapor, and pretty much anything that happens in the atmosphere.
One is left with the influence of other planets, of volcanoes, and of course of the Sun. But are those true and “pure” forcings, really?
For example, who’s going to demonstrate that the atmosphere will respond predictably and progressively if the Sun input to it varies, and everything else remains equal? For all we know, the Sun could be a positive forcing up to a point, then negative, then positive again, or simply positive but by different amounts following a complex multi-step function that moves up and down, all according to the atmosphere’s initial status.
Every reduction and simplification seems poised to destroy our ability to understand the climate itself. We might be ending up trying to apply statistics and/or computer models simply to distract us from the underlying truth: perhaps, in climate science, the only good map IS the territory. And the only hope to understand the climate, is by considering it whole.