Thanks to Ob and Richard Telford, who in the comments of that last post suggested I try the demonstration using one of the CMIP5 scenario runs as well. Again, this demonstration is to show why the Humlum, Stordahl, and Solheim (2012) DIFF12 method is not useful for determining the long-term cause vs. effect of the increase in CO2 vs. surface temperatures. Other commentators have made better points on exactly why – basically, the DIFF12 method removes the trend, and if ENSO variations dominate for the CO2 rate-of-change fluctuations (NOT changes in CO2 itself, where the long-term trend of prescribed CO2 dominates the ENSO fluctuations) in these models, the HSS2012 method incorrectly diagnoses what is the overall cause.
I ran into a few issues getting the proper model output. I first tried NCAR CESM1 from the Earth System Grid CMIP5 data archive, for the rcp4.5 scenario along with two of the runs from the idealized 1pct CO2 increase experiment. Unfortunately, the output for the “co2” variable appears to be garbled for the 1pct CO2 increase runs (It starts at the wrong level from the experiment, decreases exponentially to what should be the start value, then goes to 0), so I sent an e-mail to the errata admin…and I noticed the errata also notes that the rcp4.5 scenario variables are corrupted, as I experienced as well. I tried to obtain NorESM data through ESG, but kept getting a server (tomcat) error.
Thus, I decided to go back to the GFDL data portal and simply use the GFDL-ESM2M 1 pct CO2 experiment. Oh, and a word of warning for those who want to do something similar: I was tempted by the “co2mass” variable, since it already includes a global sum of CO2 mass in atmosphere, but from what I can tell this may be a prescribed value and thus does not show the ENSO-induced fluctuations of the co2 mole fraction (which I simply averaged here globally at 600mb, just a bit higher than the Mauna Loa station). The script is available here.
Anyhow, without further ado, here are the graphs of CO2 and temperature anomalies:
And our graph of the DIFF12 values:
Again, according to the HSS12 method, this would seem to imply that the changes in surface temperature are causing the increase in CO2, which we know is simply not true for this model (where the increase in CO2 is prescribed). A closer look at the cross-correlation in this model shows:
So one thing of interest here is that this model seems to show a maximum correlation between CO2 and surface temperature diffs at 4 month lags, rather than 9 to 11 months from the HSS12 paper. To me, this might have been a more interesting paper if the authors had compared their results to several of the CMIP5 models to see how well those models reproduced ENSO-induced CO2 changes. This would have spoiled their conclusions that CO2 increases are caused by ocean warming, but it could have been constructive.
P.S. For some more info on carbon cycles in GCMs, this presentation looks interesting.