The IPCC AR5 attribution statement

Last year I discussed the basis of the AR4 attribution statement:

Most of the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations.

In the new AR5 SPM (pdf), there is an analogous statement:

It is extremely likely that more than half of the observed increase in global average surface temperature from 1951 to 2010 was caused by the anthropogenic increase in greenhouse gas concentrations and other anthropogenic forcings together. The best estimate of the human-induced contribution to warming is similar to the observed warming over this period.

This includes differences in the likelihood statement, drivers and a new statement on the most likely amount of anthropogenic warming.

It is useful to remind ourselves that these statements are addressing our confidence in the characterisation of the anthropogenic contribution to the global surface temperature trend since the middle of the 20th Century. This contribution is unavoidably represented by a distribution of values because of the uncertainties associated with forcings, responses and internal variability. The AR4 statement confined itself to quantifying the probability that the greenhouse-gas driven trend was less than half the total trend as being less than 10% (or alternately, that at least 90% of the distribution was above 50% based on the IPCC definition of “very likely”):

Figure 1: Two schematic distributions of possible ‘anthropogenic contributions’ to the warming over the last 50 years. Note that in each case, despite a difference in the mean and variance, the probability of being below 50, is exactly 0.1 (i.e. a 10% likelihood).

In AR5, there are two notable changes to this. First, the likelihood level is now at least 95%, and so the assessment is for less than 5% probability of the trend being less than half of the observed trend. Secondly, they have switched from the ‘anthropogenic greenhouse gas” driven trend, to the total anthropogenic trend. As I discussed last time, the GHG trend is almost certainly larger than the net anthropogenic trend because of the high likelihood that anthropogenic aerosols have been a net cooling over that time. Both changes lead to a stronger statement than in AR4. One change in language is neutral; moving from “most” to “more than half”, but this was presumably to simply clarify the definition.

The second part of the AR5 statement is interesting as well. In the AR4 SPM, IPCC did not give a ‘best estimate’ for the anthropogenic contribution (though again many people were confused on this point). This time they have characterised the best estimate as being close to the observed estimate – i.e. that the anthropogenic trend is around 100% of the observed trend, implying that the best estimates of net natural forcings and internal variability are close to zero. This is equivalent to placing the peak in the distribution in the above figure near 100%.

The basis for these changes is explored in Chapter 10 on detection and attribution and is summarised in the following figure (10.5):

Figure 2. Assessed likely ranges (whiskers) and their mid-points (bars) for attributable warming trends over the 1951–2010 period due to well-mixed greenhouse gases (GHG), other anthropogenic forings (OA), natural forcings (NAT), combined anthropogenic forcings (ANT), and internal variability. The HadCRUT4 observations are shown in black with the 5–95% uncertainty range due to observational uncertainty.

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