More Schwartz

I suppose I should say something about this, though I'm really rather bored by it. The sexy title "Why Hasn’t Earth Warmed as Much as Expected?" rather belies the content, which is just a simple energy balance of the earth system, with no new observational analyses or theoretical insights (contrast Murphy et al for a real job).

The "as Much as Expected" in the title is interpreted as the equilibrium response for GHG forcings alone, given an assumed sensitivity of 3C per CO2 doubling. The obvious answer is that (a) of course the system isn't in equilibrium, and (b) there are other forcings too, notably (but not solely) tropospheric aerosols. In fact, current GCMs which account for these factors do an excellent job of reproducing the observed warming, but the more honest and straightforward title of "Why Has Earth Warmed Just as We Expect" would not have been so catchy.


That picture is from Wikipedia (click for page with caption), based on data from the PCM ("Parallel Climate Model" of NCAR and collaborators, equilibrium sensitivity 2.1C). Just to show that this simulation does not indicate the IPCC's 3C sensitivity estimate is too high, here is HadCM3 (sensitivity 3.4C) doing the same thing 10 years ago in the IPCC TAR (again, click for the captioned version):

I've seen equivalent figures for MIROC3.2 (sensitivity 4.0C) too but don't have them to hand. Although one might argue that the models are over-tuned to the past, and thus this agreement doesn't prove anything, blah blah, one can hardly pretend that there is a puzzling discrepancy between the energy balance of the planet (which of course these models simulate) and the observed warming. So the premise of the paper is a big fat straw man at the outset.

The calculations presented are rather trivial zero-dimensional energy balance estimates. Reasonable enough for a blog post I suppose, I'm surprised that they alone are adequate content for publication in JClim. And anyway, when these two big missing factors are included (and set to reasonable values), the paper's conclusion is that the observed warming is entirely compatible with the simple energy balance theory:

Thus countervailing aerosol forcing could account for much or all of the discrepancy between the expected and observed increase in GMST over the industrial period.

There is no useful uncertainty analysis, just a repetition of the well-known observation that these data on 20th century warming do not tightly pin down the equilibrium sensitivity, and overall, it doesn't seem to me that this paper adds much to the debate. Climate scientists have been well aware of such simple energy balance calculations (and their limitations) for some decades now. So despite the misleading title which has predictably resulted in an equally misleading press release and coverage, there is really nothing for anyone (not even the sceptics!) to get excited about.