Muddying the peer-reviewed literature

For reference, the amplification is related to the sensitivity of the moist adiabat to increasing surface temperatures (air parcels saturated in water vapour move up because of convection where the water vapour condenses and releases heat in a predictable way). The data analysis in this paper mainly concerned the trends over land, thus a key assumption for this study appears to rest solely on a personal communication from an economics professor purporting to be the results from the GISS coupled climate model. (For people who don’t know, the GISS model is the one I help develop). This is doubly odd – first that this assumption is not properly cited (how is anyone supposed to be able to check?), and secondly, the personal communication is from someone completely unconnected with the model in question. Indeed, even McKitrick emailed me to say that he thought that the referencing was inappropriate and that the authors had apologized and agreed to correct it.

So where did this analysis come from? The data actually came from a specific set of model output that I had placed online as part of the supplemental data to Schmidt (2009) which was, in part, a critique on some earlier work by McKitrick and Michaels (2007). This dataset included trends in the model-derived synthetic MSU-LT diagnostics and surface temperatures over one specific time period and for a small subset of model grid-boxes that coincided with grid-boxes in the CRUTEM data product. However, this is decidedly not a ‘land-only’ analysis (since many met stations are on islands or areas that are in the middle of the ocean in the model), nor is it commensurate with the diagnostic used in the Klotzbach et al paper (which was based on the relationships over time of the land-only averages in both products, properly weighted for area etc.).

It was easy for me to do the correct calculations using the same source data that I used in putting together the Schmidt (2009) SI. First I calculated the land-only, ocean-only and global mean temperatures and MSU-LT values for 5 ensemble members, then I looked at the trends in each of these timeseries and calculated the ratios. Interestingly, there was a significant difference between the three ratios. In the global mean, the ratio was 1.25 as expected and completely in line with the results from other models. Over the oceans where most tropical moist convection occurs, the amplification in the model is greater – about a factor of 1.4. However, over land, where there is not very much moist convection, which is not dominated by the tropics and where one expects surface trends to be greater than for the oceans, there was no amplification at all!

The land-only ‘amplification’ factor was actually close to 0.95 (+/-0.07, 95% uncertainty in an individual simulation arising from fitting a linear trend), implying that you should be expecting that land surface temperatures to rise (slightly) faster than the satellite values. Obviously, this is very different to what Klotzbach et al initially assumed, and leaves one of the hypotheses of the Klotzbach paper somewhat devoid of empirical support. If it had been incorporated into their Figures 1 and 2 (where they use the 1.2 number to plot the ‘expected’ result) it would (at minimum) have left a somewhat different impression.

For reference, if you plot the equivalent quantities in the model that were in their figures, you’d get this:

(for 5 different simulations). Note that the ‘expected amplification’ line is not actually what you would expect in any one realisation, nor the real world. The differences on a year to year basis are quite large. Obviously, I don’t know what this would be like in different models, but absent that information, an expectation that land-only trend ratios should go like the global ratios can’t be supported.

Since I thought this was very likely an inadvertent mistake, I let Phil Klotzbach know about this immediately (in mid-August) and he and his co-authors quickly redid their analysis (within a week) and claimed that it was not a big deal (though their reply also made some statements that I thought unwarranted). Additionally, I provided them with the raw output from the model so that they could check my calculations. I therefore anticipated that the paper would be corrected at the proof stage since I didn’t expect the authors to want to put something incorrect into the literature. After a few clarifying emails, I heard nothing more.

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