On June 22, 2011 the post
was presented which commented on an earlier post by Tim Curtin titled
Tim has provided a response to Jos’s post which is reproduced below.
Reply By Tim Curtin
I am very glad to have Jos de Laat’s comments on my paper, not least because I know and admire his work. I agree with much if not all of what he says, and fully accept his penultimate remark: “estimating the effect of anthropogenic H2O should include all the processes relevant to the hydrological cycle, which basically means full 3-D climate modelling”. I begin by going through his points sequentially.
1. Jos said “in the past I had done some back-of-the-envelope calculations about how much water vapour (H2O) was released by combustion processes. Which is a lot, don’t get me wrong, but my further calculations back then suggested that the impact on the global climate was marginal. Since Curtin  comes to a different conclusion, I was puzzled how that could be”. Well, using my paper’s equation (1) and its data for the outputs from hydrocarbon combustion, I found that combustion currently produces around 30 GtCO2 and 18 GtH2O per annum. Given that the former figure, with its much lower radiative forcing than that from H2O, is considered to be endangering the planet, I would have thought even only 18 GtH2O must also be relevant, not necessarily in terms of total atmospheric H2O (which I henceforth term as [H2O]) but as part of the global warming supposedly generated by the 30 GtCO2 emitted every year by humans, to which should be added, as my paper notes, the 300 GtH2O of additions to [H2O] from the water vapor generated by the cooling systems of most thermal and nuclear power stations.
2. The next key point is not how much [H2O] there is across the surface of the globe, but how much at the infrared spectrum wavelengths, and how much of that varies naturally relative to the incremental annual extra fluxes generated by the total H2O emissions from hydrocarbon combustion and the cooling process of power generation.
3. Then, if we do accept de Laat’s claim that the quantity of [H2O] per sq. metre is relevant, then that also applies to the annual NET increase in atmospheric [CO2] in 2008-2009 of just 14 GtCO2 (from TOTAL emissions, all sources including LUC, of 34.1 GtCO2) and that is much less than the total 33 GtH2O from just hydrocarbon combustion. How much is the net increase in [CO2] per square metre? See Nicol (2011: Fig. 6, copy attached below).
4. Pierrehumbert’s main omission is the [H2O] emitted during the cooling process. Let us recall what that involves, namely collection of water from lakes and rivers, using it to cool steam-driven generators, which produces emissions of steam (Kelly 2009), which is then released to the atmosphere through the cooling towers at the left of the photograph Roger put at the head of de Laat’s post, and it soon evaporates to form [H2O] and then precipitates back to earth after about 10 days, as de Laat notes. What is significant is the huge acceleration of the natural flux of evaporation of surface water to the atmosphere and then back again as rain after 10 days. Natural evaporation is a very SLOW process, power station cooling towers speed that up enormously. As my paper footnoted, cooling the power stations of the EU and USA would need at least 25% of the flow of the Rhine, Rhone and Danube rivers, but how much do those rivers contribute to ordinary evaporation over a year? For another order of magnitude, average daily evaporation in Canberra is around 2 mm, rather more than its annual mean rainfall of 600 mm. That is why we have to rely on dams for our water needs!
5. My paper cites Pierrehumbert at some length, but I regret that his recent uncalled for attack on Steve McIntyre and Ross McKitrick has led me to change my opinion of him.
6. The graph below is from John Nicol (with his permission); he’s an Australian physics professor (James Cook University). It shows how indeed [CO2] like [H2O] operates at close to the surface of the globe, not at the stratosphere or upper troposphere as perhaps de Laat would have it.
Caption to Figure 6: John Nicol’s diagram shows the power absorbed by carbon dioxide within a sequence of 10 m thick layers up to a height of 50 metres in the troposphere. The five curves represent the level of absorption for concentrations of CO2 equal to 100%, 200% and 300% of the reported current value of 380 ppm. As can be seen, the magnitude of absorption for the different concentrations are largest close to the ground and the curves cross over at heights between 3 and 4 metres, reflecting the fact that for higher concentrations of CO2, more radiation is absorbed at the lower levels leaving less power for absorption in the upper regions.