In IPCC land “very likely” means 90% – 100% probability, while extremely likely means 95% – 100% probability. In light of that, I was wondering if anyone had any insights as to why, in Chapter 10 of the IPCC’s WGI report, it says
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 (GHG) concentrations.
with respect to anthropogenic GHGs, but says
It is extremely likely that human activities caused more than half of the observed increase in GMST from 1951 to 2010.
when it comes to human activities overall.Judging from the figure on the right, it seems that if you combine all anthropogenic influences then there is only a small chance (< 5%) that non-anthropogenic influences could have produced more than 50% of the observed warming since 1950. If, however, you consider anthropogenic GHGs alone, then the chance that other factors (OA, NAT, Internal Variability) could produce more than 50% of the observed warming goes up slightly, and hence one can't reject the null at the 95% level.
The odd thing is that it seems likely that anthropogenic GHGs alone would have produced much more warming than that observed and that other anthropogenic influences would have had a cooling influence. Therefore it seems slightly strange that it's extremely likely (> 95%) that anthropogenic influences overall caused more than 50% of the observed warming, but only very likely (> 90%) that anthropogenic GHGs caused more than 50% of the observed warming. Maybe I’m missing something but this seems to be a consequence of sticking rigidly to a frequentist statistical analysis, rather than following a Bayesian approach, as many seem to think would be better.
So, if anyone else has any insights as to why there is this apparent discrepancy between anthropogenic influences overall and anthropogenic GHGs only, feel free to point it out in the comments. FWIW, this post was motivated by Fabius Maximus’s most recent post in which he appear to be arguing that because the confidence about the anthropogenic GHGs is only very likely (> 90%) that it is insufficient for policy purposes.
Paul S, in the comments, points to Figure 10.4 which I’ve included below. The left-hand panels shows the trends due to anthropogenic GHGs, other anthropogenic influences, and natural influences, while the right-hand panel shows the trends due to all anthropogenic influences combined and natural influences. It seems clear that when you combine all the anthropogenic influences, you get a much clearer attribution then when you consider anthropogenic GHGs and other anthropogenic influences in isolation.