Another comment on McLean et al

It's been said before, that there is one way of getting things right, but an unlimited number of ways of getting things wrong. Thus, it is not entirely surprising that there is another comment on the McLean et al paper apparently submitted to JGR (found here on the arxiv). However, this appears to be a rather ham-fisted attempt at rescuing the main conclusion of the original paper (that ENSO, and in particular the SOI time series, can explain a large part of the global climate change signal over the last century) while contradicting the analysis on which it was based.

The manuscript is based on a linear regression in which various forcing time series are regressed against global temperature. The novelty here is that the authors propose to use the cumulative SOI series as well as the ordinary one. Nowhere do they justify this specific choice, (ie using two versions of SOI but only one of each other forcing), but I'll let that pass for now. Here are the relevant results, based on their "T2" equation for the full model (where all forcings are considered):

All I have done here is taken some of the forcing time series that S+C used, and multiplied them by the regression coefficients that they obtained. SOI and cSOI are the plain vanilla and cumulative Southern Oscillation Index data respectively, both with a 12 month smooth. AF is all anthropogenic forcings, which as well as the major contributions from GHGs and tropospheric aerosols, also includes more minor things like land use. Since S+C's paper explicitly discusses the the influence of GHGs alone, I have also separated out the well-mixed GHGs from the other anthropogenic forcings (which they did not show). The numerical values printed beside each line refer to the linear trend of each series over this interval 1960-2008 (which was their choice).

So, their results as plotted above suggest that GHGs are responsible for about 2/3 of the trend, with cSOI contributing about half that much (1/3 of the total trend). I don't believe for a minute that the cumulative SOI is that important, and the authors admit that the formal confidence intervals for their regression coefficients are much too small "due to collinearity" (I'd expect autocorrelation is the real problem here, the regression should already account for collinearity, but I have not checked their calculation). But regardless of that, what I'm really interested in is the bizarre way in which they try to interpret these results. They start by observing that their regression coefficient (of about 0.2) for the instantaneous response to anthropogenic forcing is much smaller than the typical equilibrium response (of about 0.8) of climate models which reproduce the recent warming. Then they speculate that this "difference" might be explained by a response to ENSO, which would mean that ENSO would have to be responsible for most of the warming. Of course, this flies in the face of their own results that reproduce the recent warming with ENSO contributing about 1/3 of the total (and clearly less than the anthropogenic effect). They finally conclude that

these results are clearly inconsistent with the claim that ”[M]ost of the observed increase in global average temperatures since the mid-twentieth century is most likely due to the observed increase in anthropogenic greenhouse gas concentrations” [IPCC , 2007]

Well, there's no accounting for arbitrary numerology and make-believe. Their own estimate, presented right there in the paper, is that despite giving the SOI two bites at the cherry by including two version of it in their regression, GHGs still account for about 2/3 of the total warming (and double the total contribution from ENSO). They might as well have said "we did a regression, but didn't like the results, so here are some conclusions we made up earlier".

I guess that McLean et al will welcome this "proof" that their conclusion was right even though their research was wrong (this new paper attributes a mere 0.02C of warming to the SOI itself). It seems that some people start with the answers they want and then try to find arguments to support them (as mt says "very not the IPCC" is really the ultimate aim).