Richard Lindzen’s HoL testimony

He goes on to describe the attribution study of Stott et al. (2000) who showed that both natural (solar and volcanic) and human-related forcings (GHGs and aerosols) were necessary for a climate model to match the 20th Century temperature changes. This is seen in every model (for instance) and so is not the result of some individual model quirk. Lindzen goes on to claim that uncertainty in the forcings (particular solar and aerosols) imply that the result is somehow ‘fixed’ to give the observed result. Since we all agree that there are uncertainties in the forcings (which preclude strong statements about climate sensitivity being derived from the 20th century records for instance), is this criticism valid?

In the absence of any other constraints on either of these forcings and if their value was being defined a posteriori then he may have had a point. However, timeseries for solar forcing have been produced by groups unaffiliated with any modelling group, and the modellers have simply taken the values from the literature (Lean et al, 1995;2000;2005, Hoyt and Schatten, 1998) – any ‘fudging’ to produce the ‘correct’ answer would be immediately obvious. For aerosols, models are needed to produce the 3-dimensional distribution based on independently-derived emission data sets, but the validation is based not on the transient studies over the last 100 years, but on the satellite data and observations over the last 25 years. Once the various unknown parameters have been constrained as much as possible, they are fixed before the transient runs are started. However, aerosol modelling is indeed fraught with uncertainty, and so no group can claim that their resultant transient forcing is the unique best representation of the value found in the real world. Thus in papers such as Hansen et al (2005), it is clearly stated that the results are merely consistent simulations that match the surface temperature response and ocean heat content changes (as well as many other observations) but that this does not rule out a different combination of climate sensitivity and aerosol forcing having as good a match (see this post for more details). The point that needs to be emphasised is that all of these forcings are all very close to the ‘best guesses’ of the aerosol and solar communities.

Water Vapour

In the question session (Q143), Lindzen goes into more detail on the reason why he feels that climate sensitivity is so low – specifically, he believes that water vapour feedbacks are not only less positive than models suggest, but actually negative. That is he feels that the amount of longwave aborbtion by water vapour will go down as the planet warms due to increasing GHGs. This implies that actual water vapour amounts will decrease with increasing temperature. On the face of it this is a rather odd claim to make in general – the amount of water vapour that can exist in the atmosphere depends on the Clausius-Clapyeron equation that goes up with temperature. However, it is conceivable that convective processes might cause more extensive drying due to increased areas of subsidence (the basis of the so-called Iris effect), but this applies mainly to the upper troposphere and in the tropics only. As a general effect, reductions in water vapour as temperature increases in general seem rather unlikely.

But we can do better than simply speculating on the issue – we can look at the data and compare that to the models. The best examples to test this idea come from large and relatively rapid changes in the climate such as El Nino events, the eruption of Mt Pinatubo and the trends over the last few decades. In each case (Soden 1997; Soden et al 2002; Soden et al 2005), water vapour increases with warming, and decreases with cooling. There is some uncertainty about exactly how much it increases in the very uppermost troposphere (Misnchwaner and Dessler, 2004), but even those results show a positive feedback. So in summary, the data and the models both agree that not only is the water vapour feedback positive, it is quite close to the value suggested by the models – Lindzen’s insistence on the converse (while it has generated increased attention on the subject) seems increasingly perverse.

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