Anti-lukewarmers/anti-skeptics have a longstanding challenge to lukewarmers and skeptics to demonstrate that low-sensitivity models can account for 20th century temperature history as well as high-sensitivity models. (Though it seems to me that, examined closely, the supposed hindcast excellence of high-sensitivity models is salesmanship, rather than performance.)

Unfortunately, it’s an enormous undertaking to build a low-sensitivity model from scratch and the challenge has essentially remained unanswered.

Recently a CA reader, who has chosen not to identify himself at CA, drew my attention to an older generation low-sensitivity (1.65 deg C/doubling) model. I thought that it would be interesting to run this model using observed GHG levels to compare its success in replicating 20th century temperature history. The author of this low-sensitivity model (denoted GCM-Q in the graphic below) is known to other members of the “climate community”, but, for personal reasons, has not participated in recent controversy over climate sensitivity. For the same personal reasons, I do not, at present, have permission to identify him, though I do not anticipate him objecting to my presenting today’s results on an anonymous basis.

In addition to the interest of a low-sensitivity model, there’s also an intrinsic interest in running an older model to see how it does, given observed GHGs. Indeed, it is a common complaint on skeptic blogs that we never get to see the performance of older models on actual GHGs, since the reported models are being constantly rewritten and re-tuned. That complaint cannot be made against today’s post.

The lower sensitivity of GCM-Q arises primarily because it has negligible net feedback from the water cycle (clouds plus water vapour). It also has no allowance for aerosols.[Update: July 22. Aerosols impacted the calculation shown here as the RCP4.5 update column is CO2 equivalent, which included aerosols though they were not listed as a separate column.]

In the graphic below, I’ve compared the 20th century performance of high-sensitivity HadGEM2 RCP45, the UK Met Office contribution to CMIP5 (red), and low-sensitivity “GCM-Q” (green) against observations (HadCRUT4-black). In my opinion, the common practice of centering observations and models on very recent periods (1971-2000 or even 1986-2005 as in Smith et al 2007; 2013) is very pernicious for the proper adjudication of recent performance. Accordingly, I’ve centered on 1921-1940 in the graphic below.

On this centering, HadGEM2 has a lengthy “cold” excursion in the 1960s and too rapid recent warming, strongly suggesting that aerosol impact is overestimated and that this overestimate has disguised the effect of too high sensitivity.



Figure 1. Black – HadCRUT4 plus (dotted) decadal HadGEM3 to 2017. Red – HadGEM2 CMIP5 RCP45 average. Green – GCM-Q average. All centered on 1920-1940. 25-point Gaussian smooth.

Although the close relationship between GCM-Q and observations in the above graphic suggests that there has been tuning to recent results, I have verified that this is not the case and re-assure readers on this point. (I hope to be able to provide a thorough demonstration in a follow-up post.)

I hope to provide further details on the model in the future. In the meantime, I think that even this preview shows that GCM-Q shows that it is possible to provide a low-sensitivity account of 20th century temperature history. Indeed, it seems to me that one could argue that GCM-Q actually outperformed HadGEM2 in this respect.

Update July 22: Forcing for GCM-Q was from RCP4.5 (see here zip). The above diagram used CO2EQ (column 2) defined as:

CO2 equivalence concentrations using CO2 radiative forcing relationship Q = 3.71/ln(2)*ln(C/278), aggregating all anthropogenic forcings, including greenhouse gases listed below (i.e. columns 3,4,5 and 8-35), and aerosols, trop. ozone etc. (not listed below).

Column 4 is CO2 only. The CO2EQ and CO2 column 4 forcings are compared in the diagram below in the same style.



