AR5 inquiry followup
Mar 27, 2014
Bishop Hill in Climate: Allen, Climate: sensitivity

This is a guest post by Nic Lewis, describing the flurry of activity since he appeared before the Energy and Clmate Change Committee.

My comments on Myles Allen's oral evidence to the ECCC, and his response have been published.

Some things in Myles' response that might be worth pointing out:

1. Under Point 1:  "The IPCC Summary for Policymakers does not give “best estimates” of 2100 temperature, largely because they would not be policy relevant: the one thing that can be said with confidence about best estimate predictions is that the real world will not follow them. A best estimate of a strongly skewed distribution is particularly misleading".

and under Points 4 & 5: "The IPCC does not give a best estimate of 2100 climate: it gives a range."

It may not call them "best estimates", but Table SPM.2 of the IPCC Summary for Policymakers (SPM) gives "mean projected changes" in global mean surface temperature (GMST) for 2081-2100 on each RCP scenario. And Figure SPM.7 graphs the CMIP5 multi-model [mean] simulated  GMST to 2100 on the lowest and highest scenarios, as lines with a measure of uncertainty shaded. The means are implied to be best estimates by virtue of being highlighted as lines within the shaded range and by being the only central measures given. Moreover, Myles statement 'A best estimate of a strongly skewed distribution is particularly misleading' is very strange, since for every scenario the uncertainty distribution for GMST given in the SPM is symmetrical. The IPCC AR5 report does of course not give a point estimate for GMST in 2100 across all scenarios, which would indeed be misleading. Nor have I ever done so.

2. "Mr. Lewis argues that the IPCC’s ranges should be compared to his 17-83% ranges, which would suggest a larger discrepancy, but this would mean placing greater faith in his own analysis than any other study assessed by AR5. … If current evidence gives a 5-95% range for a forecast quantity, the IPCC typically assigns a lower than 90% confidence to the real world response falling inside that range".

I didn't actually make any such claim in my comments on his oral evidence or in any other evidence I provided to the ECCC. And TCR is not a 'forecast quantity' in this context. It is a property either of climate models, measurable with little uncertainty, or an estimate derived by scaling the historical temperature response of the climate system to increased forcing, based on uncertain estimates of both quantities.

3. "It is a common mistake to interpret these simple models too literally, such as arguing there is a “correct” heat capacity of the box representing the near-surface ocean."

What I objected to was Myles using the IPCC's 2-box model time constants, which given short and long term heat capacity elements of any fixed values are a function of ECS and TCR, with quite different ECS and TCR values from those used by the IPCC. Sure, the model is simple, but one should ensure consistency in using even a simple model. And while I agree there is no "correct" heat capacity of the box representing the near-surface ocean, if the combined heat capacity of the two boxes falls far short of the total ocean heat capacity – as it does in Myles' 2-box model – then the model cannot be considered physically sensible.

4. "Mr. Lewis argues that CMIP5 models are already converging towards equilibrium by 2100, making their 2100 temperatures more dependent on ECS than TCR."

I argued no such thing. I simply pointed out that the rises in CMIP5 model temperatures from now to 2091-2100 were actually more highly correlated with the model ECS values than TCR values, contrary to what one would have expected.

5. "The actual impact of prior choice on ECS and TCR inferred from the energy budget for the 2000s is straightforward to calculate from the numbers provided in Otto et al (2013), and clearly supports the view that prior choice is more of an issue for ECS than TCR."

I pointed out that prior choice was more of an issue for ECS than for TCR in my comments on Myles evidence; my objection was that he had said that the issue only applied to ECS.  What I wrote was that the statistical controversy:

...particularly using uniform priors – applies to estimating TCR as well as ECS, albeit the effects are somewhat lesser for TCR. … Accordingly, Professor Allen's claim that the controversy about statistical methods revolving around ECS does not apply to TCR is wrong.

The example in my Appendix was as stated simplified and approximate, in that it only reflected the largest source of uncertainty. But it was not at all – as Myles claims – "entirely meaningless".

6. "The model Mr. Lewis is using assumes that future warming is given by future forcing multiplied by the TCR plus a small (0.15°C) adjustment to past forcing that is independent of both ECS and TCR. This is both physically incoherent (the adjustment to past forcing must also depend on ECS and TCR) and inconsistent with the results of more complex models."

The claim that I use an adjustment for past forcing that is independent of ECS and TCR is untrue. My warming projections were based on fixed best estimates for ECS and TCR, so there is no reason why the adjustment for past forcing (i.e., for emerging warming-in-the-pipeline) shouldn't be fixed.

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