Richard Lindzen’s HoL testimony

Prof. Richard Lindzen (MIT) is often described as the most respectable of the climate ‘sceptics’ and is frequently cited in discussions here and elsewhere. Lindzen clearly has many fundamentally important papers under his belt (work on the QBO and basic atmospheric dynamics), and a number of papers that have been much less well received by the community (the ‘Iris’ effect etc.). Last year, he gave evidence to and answered questions from, a UK House of Lords Committee investigating the economics of climate change, in which he discoursed freely on the science. I’ll try here to sort out what he said.

Firstly, it is clear that Lindzen only signs up to the first point of the basic ‘consensus’ as outlined here previously, that the planet has indeed warmed significantly over the 20th century. While he accepts that CO2 and other greenhouse gases have increased due to human activities, and that this should warm the planet, he does not accept that it is necessarily an important component in the 20th century rise. His preferred option (by process of elimination) appears to be intrinsic variability, but he provides no support for this contention.

In terms of scientific content, his testimony covers a few basic topics: the greenhouse effect, climate sensitivity, aerosol forcing and water vapour feedbacks. We have discussed these topics previously (here, here and here), and so my critique of Lindzen’s comments will come as no surprise. He intersperses his comments with references to ‘alarmism’ which I will get to at the end.

Greenhouse Effect

Lindzen accepts the main principle of the greenhouse effect, that increasing greenhouse gases (like CO2) will cause a radiative forcing that, all other things being equal, will cause the surface to warm. He uses an odd measure of its effectiveness though, claiming that a doubling of CO2 will lead to a ‘2%’ increase in the greenhouse effect. How has he defined the greenhouse effect here? Well, a doubling of CO2 is about a 4 W/m2 forcing at the tropopause, which is roughly 2% of the total upward longwave (LW) (~240 W/m2). But does that even make sense as a definition of the greenhouse effect? Not really. On a planet with no greenhouse effect (but similar albedo) the upward LW would also be 240 W/m2, but the absorbed LW in the atmosphere would be zero, so it would make much more sense to define the greenhouse effect as the amount of LW absorbed (~150 W/m2). In which case, doubling of CO2 is initially slightly more*, but as soon as any feedbacks (particularly water vapour or ice albedo changes) kick in, that would increase. Due to the non-linearities in the system, you certainly can’t multiply the total greenhouse effect of ~33 C by 2% to get any sensible estimate of the climate sensitivity. So it’s not clear what relevance the ‘2%’ number has except to make the human additions to the greenhouse effect seem negligible.

*Update: The initial post had an arithmetic error which I have excised (see comment 76 below).

Climate sensitivity

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