Bishop Hill Another paper finds that climate sensitivity is low
Apr 16, 2013 Climate: sensitivity Nic Lewis has had a paper published in Journal of Climate. It's a reworking of the Forest et al 2006 paper on climate sensitivity, but removing the warm bias of Forest's uniform prior, as well as dealing with some data issues.
A detailed reanalysis is presented of a ‘Bayesian’ climate parameter study (Forest et al., 2006) that estimates climate sensitivity (ECS) jointly with effective ocean diffusivity and aerosol forcing, using optimal fingerprints to compare multi-decadal observations with simulations by the MIT 2D climate model at varying settings of the three climate parameters. Use of improved methodology primarily accounts for the 90% confidence bounds for ECS reducing from 2.1–8.9 K to 2.0–3.6 K. The revised methodology uses Bayes’ theorem to derive a probability density function (PDF) for the whitened (made independent using an optimal fingerprint transformation) observations, for which a uniform prior is known to be noninformative. A dimensionally-reducing change of variables onto the parameter surface is then made, deriving an objective joint PDF for the climate parameters. The PDF conversion factor from the whitened variables space to the parameter surface represents a noninformative joint parameter prior, which is far from uniform. The noninformative prior prevents more probability than data uncertainty distributions warrant being assigned to regions where data responds little to parameter changes, producing better-constrained PDFs. Incorporating six years of unused model-simulation data and revising the experimental design to improve diagnostic power reduces the best-fit climate sensitivity. Employing the improved methodology, preferred 90% bounds of 1.2–2.2 K for ECS are then derived (mode and median 1.6 K). The mode is identical to those from Aldrin et al. (2012) and (using the same, HadCRUT4, observational dataset) Ring et al. (2012). Incorporating forcing and observational surface temperature uncertainties, unlike in the original study, widens the 90% range to 1.0–3.0 K.
1.6°C, again and again - there really is quite a lot of evidence for low climate sensitivity now, isn't there?
[Full disclosure - I made some (relatively minor) editorial suggestions in the drafts.]
Reader Comments (51)
'Martin A (2:46 PM)
"Do you have a reference for the log formula?"
Try Myhre et al. (1998), "New estimates of radiative forcing due to well mixed greenhouse gases", in particular their figure 1.
Apr 16, 2013 at 4:58 PM HaroldW
Harold thank you. That paper has been referred to before as the source of the climate sensitivity logarithm rule. Yet it involves nothing more fundamental than fitting a log formula to numerical results over a limited range from a "radiative forcing" model, not something derived from first principles. From there, it seems to have acquired the status of a physical law without, so far as I know, any further investigation.
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Perhaps you need to understand the radiative transfer codes being used.
to calculate the effect of various gases on the transmission of radiation physicists and engineers use
tested and validated codes. So, for example, if you are building an Ir sensor or a radar or any kind
of transmitter/receiver you would use one of these transfer codes to determine the effects of the atmosphere on your transmission. These codes come in various levels of fidelity depending on the task at hand. For example to calculate IR transmission for the YF-23A ( back in 1985 ) We used LOWTRAN and MOTRAN. These codes are validated and tested. They will tell you how transmission changes with changing
composition of the atmosphere. These codes are also used in the processing of Satellite data. When
a sensor in space looks at "brightness" or temperature at 10km it has to calculate the effects of the atmosphere above 10km.. Doing that requires using Radiation transfer codes. When ou use a satelleit to estimate the ground temperature (LST) you also have to use radiative transfer codes to account for the
effect of the atmosphere between the ground and your sensor. So, the effectiveness and accuracy of these codes has been tested and verified. Our national defense depends on it being correct. UAH and RSS depend on it. The highest fidelity codes are LBL codes. These were used in Myhre. They accurately predict the effects of atmosphere on the transmission of radiative energy. These codes are used by engineers. It is not open to question by people who actually build working devices.
The effect of doubling c02 can be calculated from first principles ( these codes are first principles ) by running the LBL code. or if you want a quick and dirty approximation 5.35ln(co2a/co2b)
Go ahead and show that the LBL codes are wrong. win a nobel.