Much has been written about the newly released IPCC Special report on global warming of 1.5C, in the media and on blogs ranging from sceptical, to not so sceptical, to climate consensual, to downright alarmist. Much more will be written I am sure. This is just a short(ish) post on my initial impressions upon reading the Summary for Policy Makers and some of Chapter 1.

After the Introduction, the authors get straight down to business with this statement:

A1. Human activities are estimated to have caused approximately 1.0°C of global warming above pre-industrial levels, with a likely range of 0.8°C to 1.2°C. Global warming is likely to reach 1.5°C between 2030 and 2052 if it continues to increase at the current rate. (high confidence) {1.2, Figure SPM.1}

So straight away, we have an attribution statement with regard to the net warming (approximately 1C) since 1850. SR15 ‘estimates’ that humans caused all of it! Where did this come from? There is no precedent in AR5 WG1, which talks mainly about attribution of warming since 1950. Note that, unusually for the IPCC, there is no level of confidence attached to this statement.

We have to go to the text in Chapter 1 to seek further enlightenment on this somewhat puzzling and out of the blue attribution statement. Under the Section headed 1.2.1.3 Total versus human–induced warming and warming rates, we find the following:

In the absence of strong natural forcing due to changes in solar or volcanic activity, the difference between total and human-induced warming is small: assessing empirical studies quantifying solar and volcanic contributions to GMST from 1890 to 2010, AR5 (Fig. 10.6 of Bindoff et al.,2013) found their net impact on warming over the full period to be less than ±0.1°C. Jones et al. (2016) show (Figure10) human-induced warming trends over the period 1905 –2005 to be indistinguishable from the corresponding total observed warming trend accounting for natural variability using spatio-temporal detection patterns from 12 out of 15 CMIP5 models and from the multi-model average. Figures from Ribes and Terray (2013), show the anthropogenic contribution to the observed linear warming trend 1880-2012 in the HadCRUT4 dataset (0.83°C in Table 1.1) to be 0.86°C.

Haustein et al. (2017) give a 5–95% confidence interval for human-induced warming in 2017 of 0.87–1.22°C, with a best estimate of 1.02°C, based on the HadCRUT4 dataset accounting for observational and forcing uncertainty and internal variability.

Right . . . . so they dug up a few papers mostly published after AR5 and used these to justify their new attribution statement regarding net post industrial warming, so it seems. The official line in 2012, when AR5 WG1 Chapter 10 was drafted was this, as I quoted in an earlier post