We have been expecting too much from the IPCC about its confidence level increase: the explanation may actually be simple… and surprising.

Guest essay by Stephane Rogeau

Image: From IPCC FAQs

Many people are wondering what actually made the IPCC raise its confidence level about the fact “human influence has been the dominant cause of the observed warming since the mid-20th century” from 90% to 95%, since its last report in 2007.

Most of the time, it is argued that there has been no warming since 2007 whatsoever, which makes the increase of confidence level in the IPCC’s statement very dubious. But it may be the other way around: because there has been no warming, the IPCC raised its confidence level! And it actually makes sense… at least inside the thought paradigm of the UN organization.

The reason may actually be simple, for one reason: the theoretical warming due to anthropogenic CO2 is, for the IPCC, a given.

It is what makes the climate models “work” (i.e. match more or less historical records). It is actually the basis of the IPCC’s line of reasoning: we cannot find any other way to match our models with the data than by entering feedback assumptions that give climate sensitivity to CO2 the value x… therefore its value has to be around x.

Based on this given assumption that cannot be disproved by facts anymore, and knowing the quantity of CO2 released by human activity, one can easily calculate the theoretical human-induced contribution to global warming since 1951 (let’s call it HIC). Discrepancy with observed global warming (OGW) is, of course, due to natural variability. Therefore, the proportion of human influence in observed warming between 1951 and year “n” is simply p(n) = HIC(n) / OGW(n). If, in year n, the theoretical human-induced contribution since 1951 is for example 0.4°C, and the observed global warming is 0.5°C, then the calculated proportion of human influence is 80%.

Obviously, as we release more and more CO2 into the atmosphere, the human-induced contribution is an increasing function of time: HIC(2012) >HIC(2007). On the other hand, the so-called “hiatus” means OGW(2012) = OGW(2007), as no warming has been observed since 2007. Then it’s just basic arithmetic: p(2012)>p(2007).

Long story short: the proportion of human influence in observed global warming has increased since the last IPCC report because temperatures have leveled off. Translated in terms of confidence level: if the IPCC was 90% certain that human activity was responsible for more than half of the observed warming in 2007, it is not surprising that the confidence level for this same proportion has now risen to 95%.

To conclude: the less warming, the more confident the IPCC about its claims to policy-makers.

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