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:
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.
The pattern of warming and residual differences between models and observations indicate a role for circulation changes as a contributor to early 20th cenury warming (Figure 10.2), and the contribution of internal variability to the early 20th century warming has been analysed in several publications since the AR4. Crook and Forster (2011) find that the observed 1918–1940 warming was significantly greater than that simulated by most of the CMIP3 models. A distinguishing feature of the early 20th century warming is its pattern (Brönnimann, 2009) which shows the most pronounced warming in the Arctic during the cold season, followed by North America during the warm season, the North Atlantic Ocean and the tropics.
Some studies have suggested that the warming is a response to the AMO (Schlesinger and Ramankutty, 1994; Polyakov et al., 2005; Knight et al., 2006; Tung and Zhou, 2013), or a large but random expression of internal variability (Bengtsson et al., 2006; Wood and Overland, 2010)
Nonetheless, these studies do not challenge the AR4 assessment that external forcing very likely made a contribution to the warming over this period. In conclusion, the early 20th century warming is very unlikely to be due to internal variability alone. It remains difficult to quantify the contribution to this warming from internal variability, natural forcing and anthropogenic forcing, due to forcing and response uncertainties and incomplete observational coverage.
In essence, what the IPCC said was that they could not attribute early 20th century warming (approx. 0.5-0.6C) to internal variability alone, that it was likely a result of some external forcing and some of that external forcing was likely anthropogenic, though they could not specify the exact contribution. Why, I wonder, did SR15 not mention this paper, also published after AR5, which attributes the early 20th century warming to a combination of internal variability/circulation changes (about half) and natural and anthropogenic external forcing.
These anomalous events occurred during a period of strong global‐scale warming, which can be attributed to a combination of external forcing (particularly, greenhouse gas increases, combined with a hiatus in volcanic events) and internal decadal variability. The exact contribution of each factor to large‐scale warming remains uncertain, largely due to uncertainty in the role of aerosols in the cooling or stabilization of climate following the middle of the 20th century.
This basically echoes the earlier conclusions of AR5 WG1. The contribution from early 20th century warming to total post industrial warming up to 2010 is not insignificant, being roughly equal to the rapid warming which occurred 1976-1998, each period contributing about 0.6C. Mid-20th century cooling (which climate science likes to attribute mostly to anthropogenic aerosols masking ongoing warming) was about 0.2C, which gives the 1.0C net post industrial rise in global mean temperature. So obviously, if say, anthropogenic warming only contributed about a quarter to the early 20th century warming (which is not an unreasonable guesstimate given the available scientific literature on this subject), natural forcings then contributed approximately 0.6-0.15=0.45C to the total post industrial warming, i.e. nearly half. SR15 reduces this to zero in favour of anthropogenic warming!
So the very first sentence of the SR15 Summary for Policy Makers, after the Introduction, consists of a statement which is not well supported by the totality of the available scientific literature and which is at odds with the IPCC’s own findings in the AR5 Working Group 1 Report of just 5 years ago! Not a good start.