On the day the SPM was published, I wrote about the key IPCC claim that it was “extremely likely” (i.e. at least 95% certain) that human influence had caused most of the warming since 1950. This was an increase on the AR4 attribution statement of 90%, which seemed odd in the light of another six years with no warming, contrary to climate science predictions, and increasing number of papers from mainstream climate scientists acknowledging a discrepancy between models and observations. However, a direct comparison is not appropriate since the IPCC moved the goalposts from greenhouse gases to all human influences. At the time of that post the justification for the 95% claim was not available, but the SPM referred to Chapter 10 of the main report, “Detection and Attribution of Climate Change” in particular section 10.3.
A post at Realclimate tries to address the attribution question but does little more than re-state the IPCC claim, based on another IPCC figure and its error bars.
A post at the manicbeancounter blog notes that the uncertainty levels for CO2 forcing has increased greatly between between AR4 and AR5, which seems inconsistent with the claim of increased certainty.
Apart from these two, I can’t find much detailed investigation of the 95% claim and how it arises from the main report. Are there other blogs I missed?
Chapter 10 starts off with the statement that “More than half of the observed increase in global mean surface temperature (GMST) from 1951 to 2010 is very likely due to the observed anthropogenic increase in greenhouse gas concentrations.” This is almost exactly the same claim as made in AR4. So it seems that the inflation from 90% to 95% results from the switch from greenhouse gases to total anthropogenic contribution.
The second paragraph states the 95% claim in the SPM, with slightly different wording: “It is extremely likely that human activities caused more than half of the observed increase in global mean surface temperature from 1951–2010.”
Here is figure 10.5, quoted at RC. (My screen-grabber has again garbled the colours, so apologies for any confusion)
It shows the observed warming trend at the top, followed by the trend attributable to greenhouse gases, the trend attributed to total anthropogenic factors, and the (cooling) trend due to man-made aerosols. Each of these has error bars associated with it. There is a very obvious problem with this figure, pointed out by Clive Best in his comment at RC:
“Can you explain why in figure 10.5 the error bar on ANT is so small? Naively I would expect this to be the sum of GHG and OA. This would then work out to be an error on ANT of sqrt(2*0.36) = 0.8C. This is also not explained in chapter 10.”
I had a look through the SOD version of Chapter 10, the version reviewed by scientists, to see if this strange figure was there – it isn’t.
The only way the figure makes any sense is if the IPCC has decided a priori that almost all the warming must be anthropogenic, because they can’t think of anything else, but are uncertain how to divide that warming into greenhouse and aerosol effects.
But even this explanation does not make sense, in view of this figure from Chapter 8, showing the total “effective radiative forcing” of GHGs, aerosols and the total anthropogenic.
In this case the error bars add up in a sensible way, as suggested by Clive – the error bar for total anthropogenic forcing is greater than that either GHGs or aerosols.
So the 95% claim seems to be based on a nonsensical figure that is contradicted by another graph elsewhere in the IPCC report.
[Culture spot: “A question of attribution” is a play by Alan Bennett about Anthony Blunt, custodian of the Queen’s pictures and Russian spy. According to Wikipedia, Bennett described the piece as an “inquiry in which the circumstances are imaginary but the pictures are real.” There is a great TV version with James Fox and Prunella Scales.]