El Nino’s effect on CO2 causes confusion about CO2′s role for climate change

Fig. 4: A trivial demonstration with two similar sinusoinds with noise super-imposed, one shown in black and one in red plotted on top of each other - differencing methods fail to pick up the longer signals. The lower panel shows a lag-correlation based on a differenting method that fails to pick up the signal we know is present. In other words, the method is inappropriate for the task. Also reproduced with the on-line R-script.

Simply by choosing a method that ignores trends associated with human activity, they argue that atmospheric CO2 is ‘largely independent of humans‘. This logic is circular reasoning, but since they did not specify time scales, their argument gets a bit fuzzy (the argument is true on short time scales but not on decadal time scales – just see the trends in the figure).

The set-up with ‘DIFF12′ also does a strange mix to the data and gives weird results as already noted on Troy’s scratchpad. It is indeed easy to show that the ‘DIFF12′ methods fail to pick up long-term co-variations, as done in Fig. 5:

Fig. 5: A demonstration with the DIFF12 method used in Humlum et al. The method fails to pick up the long-term co-variations. The correlation peak should be at zero lag, but the method mixes scrambles time steps. Also reproduced with the on-line R-script.

Contrast these results with the claim made by Humlum et al. (2012):

the association between periods of maximum DIFF12 CO2 increase and no or negative ocean surface temperature change … is difficult to reconcile with the notion of atmospheric CO2 changes controlling changes in ocean surface temperature.

Choosing a method discarding the trends may get one into trouble, however, one would also expect a consistent picture in terms of physics. Here is another glitch, and the weak-on-physics aspect is due to the failure to appreciate what implications their claims would have for the climate sensitivity.

They implicitly expected that minute and almost invisible similarities between the inter-annual variations in CO2 would have a visible effect on inter-annual variation in the global mean temperature (this is their hidden “strawman” argument; see curves in Fig. 1). Yet they ignored the CO2‘s steady march upwards.

Furthermore, they failed to note that the forcing is proportional to the natural logarithm of CO2ln(conventration in volume). Such are omissions expected from novices (my speculation: this is somehow related to the composition of the author team consisting of a professor in geology, a statistician from a telephone company, and an astrophysicist).

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  1. O. Humlum, K. Stordahl, and J. Solheim, "The phase relation between atmospheric carbon dioxide and global temperature", Global and Planetary Change, vol. 100, pp. 51-69, 2013. http://dx.doi.org/10.1016/j.gloplacha.2012.08.008