Geoheresy

The distortion of science for ideological purposes has a long history, and the results are generally ugly.

Saturday, November 18, 2006

 
CLIMATE IN-SENSITIVITY

The Monbiot-Monckton debate continues in splendid fashion and now the climate surrealists are wading in with argumentum ad hominem over climate sensitivity. (One is left to one's own devices to find it in the Surreal Climate site - as the Wandering Hissink does not bother with it).

Preamble

The definition of climate sensitivity is as follows:

In IPCC reports, equilibrium climate sensitivity refers to the equilibrium change in global mean surface temperature following a doubling of the atmospheric (equivalent) CO2 concentration. This value is estimated, by the IPCC Third Assessment Report primarily on the basis of climate models, to be "likely to be in the range of 1.5 to 4.5°C" [1]. More generally, equilibrium climate sensitivity refers to the equilibrium change in surface air temperature following a unit change in radiative forcing, expressed in units of °C/(W/m2). In practice, the evaluation of the equilibrium climate sensitivity from models requires very long simulations with coupled global climate models, or it may be deduced from observations

Source

Climate sensitivity has NEVER been determined experimentally by measurement, so no one really knows what its value is. Considering that this concept is the MOST important issue in the climate debate, one would have thought that its experimental determination would be of paramount importance.


Christopher Monckton has added a second reply to some comments made by one Schmidt on Surreal Climate but with the ad homs deleted as indicated in the following reproduction sent to me by email today.

Chuck it, Schmidt!


Commentary on a posting by one Schmidt on a website run by him with, inter alios, two of the authors of the universally-discredited UN graph purporting to erase the mediaeval warm period.

Passage, some substantial, were deleted as merely ad hominem where marked +++ and are not considered. The passages which appear to contain what looks as though it were science are reproduced in Roman face. Commentary is in bold face.


“+++ The two pieces [Monckton in the Daily Telegraph, November 5, 2006, and Milloy at http://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). Both these pieces instead +++ show +++ that climate sensitivity must be small (more like 0.2 W/m2, or less than 1°C for 2xCO2). 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. +++ ”

My 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.


“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. The second thing to know is that the Earth is not a black body! On the real planet, there are multitudes of feedbacks that affect other greenhouse components (ice alebdo, water vapour, clouds etc.) and so the true issue for climate sensitivity is what these feedbacks amount to.”

Climate feedbacks are of course mentioned in my article. Also, the supporting calculations explicitly state that Earth/troposphere emissivity was taken not as 1 (for a blackbody) but as ~0.6 (for a greybody). Schmidt had seen the calculations, because he mentions the “M climate model”, to which the article did not refer by name. Schmidt knew that the Stefan-Boltzmann equation, often called 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 greybodies in between. Schmidt’s implication is that the equation applies to blackbodies only. A zoologist lifted this unfortunate implication from Schmidt’s blog without verification and repeated it in a UK newspaper, which was obliged to print an article correcting this and other errors on the following day.


“ +++ . 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 20th-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.


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


The “hindcasts” don’t match observations well: as the article pointed out, there is a large shortfall between observed and projected temperatures, which led the Hadley Centre 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.”

To avoid points like this, my methodology was identical to that of the models on which the UN relies. I used its definition of radiative forcing, its forcing equation and its all-ghg-to-CO2 ratio, all of which are explicitly quoted in my calculations, which Schmidt has seen.


“ +++ climate sensitivity is an equilibrium concept. It tells you the temperature that you get to eventually. In a transient situation (such as we have at present), there is a lag related to the slow warm up of the oceans, which implies that the temperature takes a number of decades to catch up with the forcings. This lag is associated with the planetary energy imbalance and the rise in ocean heat content. If you don't take that into account it will always make the observed 'sensitivity' smaller than it should be.”

My article explicitly mentioned, took into account, and gave evidence for and against, the ocean notion.


“Therefore if you take the observed warming (0.6°C) and divide by the estimated total forcings
(~1.6 +/- 1W/m2) you get a number that is roughly half the one expected.”

Using the UN’s CO2 forcing equation, and its all-ghg-to-CO2 ratio, I calculated the 1900-1998 forcing from all ghgs as 1.99wm-2. Using the Stefan-Boltzmann equation for the Earth/troposphere system as a greybody with dT 0.6C, I calculated the 1900-1998 forcings from all sources as 1.98wm-2. The two values are near-identical, suggesting either the cancellation of positive and negative feedbacks or the ocean notion, or some combination of the two.


“ +++ If you ignore the fact that there are negative forcings in the system as well (chiefly aerosols and land use changes), the forcing from all the warming effects is larger still (~2.6 W/m2), and so the implied sensitivity even smaller.”

Again, I used the UN’s assumptions, which is that all non-ghg forcings are not as well understood as ghg forcings, and that – by deduction from its table of forcings – all non-ghg forcings broadly self-cancel. This point was clearly explained, and illustrated with the UN’s forcings table, in the supporting calculations, which Schmidt has seen


“Of course, you could take the imbalance (~0.33 +/- 0.23 W/m2 in Hansen et al., 2005) into account and use the total net forcing, +++ ”

I cited Hansen et al. (2006), an update making a similar point.
“And finally, you can +++ imply that all the warming is due to solar forcing.”

My article correctly showed that, if one takes 1900 and not 1750 as the start-date for solar forcings, so as to coincide with the 20th-century temperature rise of 0.6C relied upon by the UN, its own table of solar-irradiance proxies shows a base 20th-century solar TSI increment of some 4wm-2, equivalent to about 0.7wm-2 outgoing at the tropopause after allowing for albedo and disc-to-sphere. This is before adding climate feedbacks, for which the UN’s current multiple of base forcing is approximately 2.7 according to Sir John Houghton in a reply to a question from me on the subject. Thus one multiplies the solar forcing by the feedback coefficient to get forcings plus feedbacks – i.e. 1.9wm-2, which is close to the 2wm-2 actually observed. However, the accompanying calculations, which Schmidt has seen, made it very clear that I had assumed, as a base case, no solar forcing above that mentioned by the UN, and that I had compared that case with others, some of which had considered 20th-century solar forcings greater than those assumed by the UN.


“ +++. Either there are important feedbacks or there aren't. You can't have them for solar and not for greenhouse gases. Our best estimates of solar are that it is about 10 to 15% the magnitude of the greenhouse gas forcing over the 20th Century. Even if that is wrong by a factor of 2 (which is conceivable), it's still less than half of the GHG changes. And of course, when you look at the last 50 years, there are no trends in solar forcing at all. +++ ”

My article explicitly addressed the need to apply climate feedbacks to all forcings and said the UN had not done so. See Solanki and Usoskin (2005) for a conclusion that the Sun has been hotter, for longer, in the past 50 years than in the past 11,400 years. Recent papers apparently disputing this have failed to take account of Solanki’s point that it is the combination of the amplitude and duration of the recent solar activity that is exceptional, not the amplitude alone.



Links to this post:

Create a Link



<< Home

Archives

May 2006   August 2006   September 2006   October 2006   November 2006   December 2006  

This page is powered by Blogger. Isn't yours?