Richard Lindzen’s HoL testimony

That leads in to Lindzen’s main theme in his evidence – how sensitive climate will be to increasing CO2. He starts off by giving the standard Stefan-Boltzmann no-feedback value for the climate sensitivity: “A doubling of CO2 should lead (if the major greenhouse substances, water vapour and clouds remain fixed), on the basis of straightforward physics, to a globally averaged warming of about 1°C”. But he couples this with an extremely misleading statement: “The current increase in forcing relative to the late 19th Century due to man’s activities [by which he means greenhouse gases alone] should lead to a warming of about 0.76°C, which is already more than has been observed, but is nonetheless much less than current climate models predict.” He repeats this point in the Q&A session as well. However, Lindzen is undoubtedly well aware (having written papers on the subject i.e. Lindzen, GRL, 2002) that lags in the surface temperature due to ocean thermal inertia imply that the transient response is always smaller than the equilibrium response, and that additionally, there are other forcings in the system (specifically land-use change and aerosols) that counteract the forcing from greenhouse gases alone. Since he does not mention these two factors in connection with this statement, a listener could be left with a rather misleading impression. He combines this with a (deliberate?) overstatement (Q130) of the ‘consensus’ value for the sensitivity as being 4 to 5°C (while the actual consensus is between 1.5 to 4.5°C, best guess around 3°C), misleadingly giving the impression that the mainstream is way off. Similarly, his claim that models overpredict the 20th Century temperature rise is easily shown to be false.

Later he states: “Attempts to assess climate sensitivity by direct observation of cloud processes, and other means, which avoid dependence on models, support the conclusion that the sensitivity is low. More precisely, what is known points to the conclusion that a doubling of CO2 would lead to about 0.5°C warming”. One wonders which attempts he is referring to, since it can’t be Lorius et al (1991), or Forrest et al (2004) or Andronova and Schlesinger (2002), given that they give ranges that are all significantly higher than this, and indeed, Gregory et al (2002) specifically rules out anything less than 1.6°C. A more recent estimate (Annan and Hargreaves, in press) using multiple lines of observational constraints places the sensitivity well within the value estimated by the models (i.e. around 2 to 4°C).

Actually, I think it is quite easy to rule out a sensitivity as low as 0.5°C by considering the last glacial period about 20,000 years ago. At that time the temperatures were globally around 5 or 6°C colder than the pre-industrial, and the forcings (from ice sheets, vegetation, greenhouse gases and dust) are estimated to be around 6 to 11 W/m2 (a slightly broader range than I previously quoted, updated from some of the PMIP2 results). This implies a sensitivity of between 1.8 and 4°C for a doubling of CO2, with a most likely value of around 3°C. If however, the sensitivity really was as low as 0.5°C, that would imply that either the forcings estimates are 3 to 8 times too low, or the temperature changes are 3 to 8 times too high. Since around 3 W/m2 of the ice age forcing is directly related to greenhouse gases and is well accepted (even by Lindzen), it would require the ice sheets to impart an enormous forcing even to get anywhere near a level consistent with his sensitivity estimate. That does not appear even remotely plasuible. On the other hand, it is unlikely that we have mis-interpreted the proxy evidence for temperature since it comes from very many different sources – snow lines, foraminefera, alkenones, Mg/Ca, pollen records, ice core isotopes, speleothems, faunal assemblages etc. To be sure, some of these data do not completely agree, but none would imply that global temperatures were only 1.5°C cooler (which is the minimum that would be required).

In summary, Lindzen’s testimony regarding on climate sensitivity is idiosyncratic at best, and certainly not supported by the literature.

Aerosols

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