More about Fears of Climate Change

In my previous post, I looked at a recent estimate by Lewis of climate sensitivity to CO₂ doubling, pointing out that Lewis, while not explicitly disagreeing with SOD's data, used his own substantially lower estimate for heat uptake.

On the other hand, Lewis does explain at some length why he prefers a lower number to the IPCC's median estimate of -0.9 W m^-2 for (non-volcanic, mainly sulphate) aerosol forcing - what the IPCC calls Adjusted Forcing from Combined Aerosol-Radiation and Aerosol-Cloud Interactions, or AF_ari+aci for short.  He points out that that estimate is "not what the observations indicate: it is a composite of observational, GCM-simulation/aerosol model derived, and inverse estimates."  He wants to use observational data only, so he takes the figure of -0.73 W m^-2  from section 7.5.3 of SOD, and corrects SOD's estimate of total forcing for the difference.  Paul S, commenting here, says that that value wrongly treats estimates in some source papers of the indirect effect only as the whole forcing - the principal aerosol forcing mechanisms are the direct interaction with radiation, and the first indirect effect, which is caused by a change in cloud albedo resulting from droplet nucleation.

I've looked up the five papers listed on page 7-109 as sources for the estimate.  I present a crude summary of their results:

Paper	Direct	Indirect	Total	Note
Bellouin	-0.5	-0.4	-0.9
Lohmann		-0.85		[powerpoint]
Quaas 2006		-0.53		Using LMDZ General Circulation Model
Quaas 2006		-0.29		Using ECHAM4 General Circulation Model
Quaas 2009	-0.4	-0.7	-1.2	See paper for why it doesn't add up
Sekiguchi	-0.4	-0.9	-1.3	Not including cloud fraction change

To get more confidence in the numbers, I've had a look at the difference between Adjusted Forcing and Radiative Forcing.  SOD estimates a centre value for RF_ari of -0.4 W m^-2, and explains (page 7-45)

AF_ari adds the radiative effects from rapid adjustments onto RF_ari...Rapid adjustments are principally caused by cloud changes...Overall a best estimate for the rapid adjustment is taken to be –0.1 W m^-2...[giving] an assessment for AF_ari of –0.5 W m^-2

So its estimate of the direct effect is consistent with the numbers in my table.

SOD's preferred method for evaluating AF_aci is to back it out from AF_ari+aci:

We produce a best estimate ... for AF_ari+aci in the following way. The global CMIP5 models and inverse estimates are grouped together and a bootstrapping method is used to estimate a mean ... of –0.98 W m^-2. Processing the satellite-based estimates in the same way leads to a mean ... of –0.73 W m^-2... We combine these two estimates into a best estimate ... for RF_ari+aci of –0.9 W m^-2...

The AF_aci is estimated as the residual between AF_ari+aci and AF_ari. We further assume that AF_ari and AF_aci are additive ..., which yields to our assessment of AF_aci of –0.4 W m^-2. Models indicate that RF_aci is less than AF_aci, which implies an estimate of –0.3 W m^-2...

[I think the "RF_ari+aci" there is a typo for "AF_ari+aci"]

Comparing the numbers in my table with the scatter plot in SOD Figure 7.10 on page 7-130, which shows seven points with the largest at about -1.0, it's apparent that the SAT points plotted are for indirect forcing only.  The two values in Quaas 2006 seem to have been plotted separately.  I can't see where the seventh point comes from.  Averaging the six points I've got gives -0.6, and adding a direct effect of -0.5 onto that gives a total adjusted forcing of -1.1 W m^-2.  Whence the big difference from SOD's combined estimate of –0.73 W m^-2?   It's hard to be sure, but it looks very much as if SOD has used the satellite estimates for indirect forcing only as if they're the whole forcing effect.  Three of the papers explicitly disagree with that, because they talk about additionally about the direct effect and a total effect.  The other two papers are clear that they're discussing indirect effects only.  If the combined effect is really –0.73 W m^-2 and the estimate quoted above of -0.5 W m^-2 for the direct effect is good, that implies an indirect effect of only -0.23 W m^-2, lower than the estimates in any of the studies referenced.  This seems obviously wrong.

I've also looked up the seven papers listed as sources for the estimate of RF_ari+aci (two of them the same as above)

Paper	Direct	Indirect	Total	Note
Bellouin	-0.5	-0.4	-0.9
Dufresne	-0.5	-0.22	-0.72	Change in sulphate aerosols 1860 to 1995
Lebsock		-0.42	-
Quaas and Boucher		-0.35	-	Average of results for MODIS and POLDER satellite data
Quaas 2008	-0.9	-0.2	-1.1
Quaas 2009	-0.4	-0.7	-1.2	See paper for why it doesn't add up
Storelvmo		-0.94		Mode of four values

Here simply summing the average direct (-0.58) and indirect (-0.46) forcings gives a total forcing of -1.04 W m^-2. (I think the Dufresne result should be corrected for this purpose by adding in the 1860 forcing, but I've not done that.)

So the sources SOD references suggest an RF_ari+aci of about -1.0 W m^-2 and an AF_ari+aci of about -1.1 W m^-2.

In conclusion, SOD mentions the value of -0.73 W m^-2 only in passing, and does not offer it as an estimate. And the source papers it refers to seem not to justify using it.  Lewis might reasonably have take SOD's explicit estimate for AF_ari+aci of -0.9 W m^-2, or he could have gone to the sources using his preferred method and come up with a consensus value of -1.1 W m^-2.  I think he is not justified in using -0.73 W m^-2.

Using a value of -0.9 together with my previous heat uptake estimate increases the median sensitivity to 1.95.  Using -1.1 increases median sensitivity to 2.29 .

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It's very possible that this post could be improved by expert interpretation of the source materials.  I welcome suggestions and corrections, and may undertake extensive revision in their light.

[I've omitted throughout estimates and discussion of uncertainties.  Which is not to say that they're unimportant, but I'm concentrating here on the headline numbers.]