Climate Sensitivity: Nic Lewis Responds To Drew Shindell
As with most papers by establishment climate scientists, no data or computer code appears to be archived in relation to the paper.
This new Nature Climate Change paper[i] by Drew Shindell claims that the lowest end of transient climate response (TCR) – below 1.3°C – in CMIP5 models is very unlikely, and that this suggests the lowest end of model equilibrium climate sensitivity estimates – modestly above 2°C – is also unlikely. The reason is that CMIP5 models display substantially greater transient climate sensitivity to forcing from aerosols and ozone than to forcing from CO2. Allowing for this, Shindell estimates that TCR is 1.7°C, very close to the CMIP5 multimodel mean of ~1.8°C. Accordingly, he sees no reason to doubt the models. In this connection, I would note (without criticising it) that Drew Shindell is arguing against the findings of the Otto et al (2013) study,[ii] of which he and myself were two of the authors.
As with most papers by establishment climate scientists, no data or computer code appears to be archived in relation to the paper. Nor are the six models/model-averages shown on the graphs identified there. However, useful model-by-model information is given in the Supplementary Information. I was rather surprised that the first piece of data I looked at – the WM-GHG (well-mixed greenhouse gas) global forcing for the average of the MIROC, MRI and NorESM climate models, in Table S2 – is given as 1.91 W/m², when the three individual model values obviously don’t average that. They actually average 2.05 W/m². Whether this is a simple typo or an error affecting the analysis I cannot tell, but the apparent lack of care it shows reinforces the view that little confidence should be placed in studies that do not archive data and full computer code – and so cannot be properly checked.
The extensive adjustments made by Shindell to the data he uses are a source of concern. One of those adjustments is to add +0.3 W/m² to the figures used for model aerosol forcing to bring the estimated model aerosol forcing into line with the AR5 best estimate of -0.9 W/m². He notes that the study’s main results are very sensitive to the magnitude of this adjustment. If it were removed, the estimated mean TCR would increase by 0.7°C. If it were increased by 0.15 W/m², presumably the mean TCR estimate of 1.7°C would fall to 1.35°C – in line with the Otto et al (2013) estimate. Now, so far as I know, model aerosol forcing values are generally for the change from the 1850s, or thereabouts, to ~2000, not – as is the AR5 estimate – for the change from 1750. Since the AR5 aerosol forcing best estimate for the 1850s was -0.19 W/m², the adjustment required to bring the aerosol forcing estimates for the models into line with the AR5 best estimate is ~0.49 W/m², not ~0.3 W/m². On the face of it, using that adjustment would bring Shindell’s TCR estimate down to around 1.26°C.
Additionally, the estimates of aerosol forcing in the models that Shindell uses to derive the 0.3 W/m² adjustment are themselves quite uncertain. He gives a figure of -0.98 W/m² for the NorESM1‑M model, but the estimate by the modelling team appears to be -1.29 W/m². Likewise, Shindell’s figure of -1.44 W/m² for the GFDL-CM3 model appears to be contradicted by the estimate of -1.59 W/m² (or -1.68 W/m², dependent on version), by the team involved with the model’s development. Substituting these two estimates for those used by Shindell would bring his TCR estimate down even further.
In any event, since the AR5 uncertainty range for aerosol forcing is very wide (5–95% range: -1.9 to -0.1 W/m²), the sensitivity of Shindell’s TCR estimate to the aerosol forcing bias adjustment is such that the true uncertainty of Shindell’s TCR range must be huge – so large as to make his estimate worthless.
I’ll set aside further consideration of the detailed methodology Shindell used and the adjustments and assumptions he made. In the rest of this analysis I deal with the question of to what extent the model simulations used by Shindell can be regarded as providing reliable information about how the real climate system responds to forcing from aerosols, ozone and other forcing components.