Schmidt: “+++ The two pieces [Monckton in the Daily Telegraph, November 5, 2006, and Milloy at www.junkscience.com] both spend a lot of time discussing climate sensitivity +++ . We have often made the case here that equilibrium climate sensitivity is most likely to be around 0.75 +/- 0.25 C/(W/m²) (corresponding to about a 3°C rise for a doubling of CO₂).”

Comment: The UN’s 2001 report mentions climate sensitivity of 0.5C per watt per square meter. Schmidt’s estimate is higher by half.

Schmidt: “Both these pieces instead +++ show +++ that climate sensitivity must be small (more like 0.2 W/m², or less than 1°C for 2xCO₂).”

Lord Monckton’s detailed calculations showed that with climate sensitivity of 0.3C per watt per square meter the entire observed temperature increase since 1900 could be accounted for.

Schmidt: “Our previous posts should be enough to demonstrate that this can't be correct, but it [is] worth seeing how they +++ get these answers. +++ . Any temperature change (in °C) divided by any energy flux (in W/m²) will have the same unit and thus can be compared. +++ ”

Comment: Lord Monckton’s article was explicit that the units for what the UN calls “lambda” are degrees C per watt per square metre. If Schmidt dislikes the use of these units, he should take the matter up with the UN.

Schmidt: “Readers need to be aware of at least two basic things. First off, an idealised 'black body' (which gives of radiation in a very uniform and predictable way as a function of temperature - encapsulated in the Stefan-Boltzmann equation) has a basic sensitivity (at Earth's radiating temperature) of about 0.27 °C/(W/m²). That is, a change in radiative forcing of about 4 W/m² would give around 1°C warming.”

Comment: No, not 0.27C. If emissivity is unity the climate sensitivity in the climate-relevant temperature range is 0.22C per watt per square meter. This is a matter of elementary calculation.

“The second thing to know is that the Earth is not a black body!”

Comment: My article and the supporting calculations took full and explicit account of the fact that Earth/troposphere emissivity is not 1 (for a blackbody) but ~0.6 (the Earth being a badly-behaved greybody). Schmidt had seen the supporting calculations, because he later mentions the “M climate model”, to which my article did not refer by name. Schmidt ought to have known that the Stefan-Boltzmann radiative-transfer equation, often miscalled the “blackbody” equation, is in fact capable of representing not only blackbodies (emissivity 1) that absorb and, by Kirchhoff’s law, emit all radiation, but also whitebodies (emissivity 0) that reflect all radiation, and all graybodies in between. Schmidt here erroneously implies that this fundamental climate equation applies to blackbodies only. A fourth-rate zoologist in the UK lifted this unfortunate implication from Schmidt’s blog without checking it, and repeated it in a UK newspaper, which was obliged to print an article correcting this and other schoolboy errors arising from Schmidt’s blog on the following day.

Schmidt: “On the real planet, there are multitudes of feedbacks that affect other greenhouse components (ice albedo, water vapor, clouds etc.) and so the true issue for climate sensitivity is what these feedbacks amount to.”

Comment: Climate feedbacks are of course explicitly mentioned in my article.

“ +++ . Ignore all the feedbacks - then you will obviously get to a number that is close to the 'black body' calculation. +++ Any calculation that lumps together water vapour and CO₂ is effectively doing this +++.”

My article, far from ignoring feedbacks, demonstrated that, even if positive and negative feedbacks cancel, the entire 20^th-century mean surface air temperature increment can be accounted for. Also, in the accompanying document the official explanation for the discrepancy between observed and projected temperatures – namely, climate feedbacks – is explicitly stated. Schmidt appears to believe that climate feedbacks are what prevent the Earth from being a blackbody. In fact, emissivity and climate feedbacks are independent climate processes each of which must be taken separately into account.

Schmidt: “As we explain in our glossary item, climatologists use the concept of radiative forcing and climate sensitivity because it provides a very robust predictive tool for knowing what model results will be, given a change of forcing.”

Comment: My article was concerned with what real world results would be.

Schmidt: “The climate sensitivity is an output of complex models (it is not decided ahead of time) and it doesn't help as much with the details of the response (i.e. regional patterns or changes in variance), but it's still quite useful for many broad brush responses. Empirically, we know that for a particular model, once you know its climate sensitivity you can easily predict how much it will warm or cool if you change one of the forcings (like CO₂ or solar).”

Comment: My article was concerned with what real world temperature changes would be.

Schmidt: “We also know that the best definition of forcing is the change in flux at the tropopause, and that the most predictable diagnostic is the global mean surface temperature anomaly.”

Comment: No. It is not clear that “the best definition of forcing is the change in flux at the tropopause”. Dr. Hugh Elsaesser, in a paper submitted to the IPCC, said there is likely to be a pronounced difference between tropopausal and surface temperature response. Observation has proven him right. The UN’s models only achieve high climate sensitivity because they posit a significantly faster rate of increase in temperature in the high troposphere than at the surface, particularly in the tropics and sub-tropics. However, the observed rate of upper-troposphere increase is far too low to match the UN’s predictions. Professor Richard Lindzen of MIT concludes that the UN’s climate-sensitivity projection is overstated by at least a factor of three, concurring with my own calculations.

Schmidt: “Thus it is natural to look at the real world and see whether there is evidence that it behaves in the same way (and it appears to, since model hindcasts of past changes match observations very well).”

Comment: The “hindcasts” do not match observations well: as the article pointed out, there is a large shortfall between observed and projected temperatures, which led the Hadley Centre – proprietor of one of the UN’s models – to divide its projections by three. The article also mentioned evidence on both sides of the case for the oceans acting as a heat-sink to account for the shortfall, and it mentioned Stern’s 84% of projected forcing taken up by the oceans.

“So +++ try dividing energy fluxes at the surface by temperature changes at the surface. +++ , this isn't the same as the definition of climate sensitivity – it is in fact the same as the black body (no feedback case) discussed above - and so, again it's no surprise when the numbers come up as similar to the black body case.”

Comment: Here again Scmidt confuses emissivity and climate feedbacks. Precisely to avoid such confusion (understandable among non-scientists), my calculation methodology was identical to that of the models on which the UN relies. I used the UN’s definition of radiative forcing, the UN’s CO2 forcing equation and the UN’s all-ghg-to-CO2 ratio, all of which are explicitly quoted in my calculations, which Schmidt has seen.