Guest Weblog By Roy Spencer on Ray Pierrehumbert’s Real Climate Post of May 21 2008
Since Ray Pierrehumbert has decided to critique some of my published work (and unpublished musings) on global warming over at RealClimate.org, I thought I’d offer some rebuttal. The main theme of his objections to our new paper and what it demonstrates is clearly wrong – and leading IPCC experts have agreed with me on this.
But first the big picture. The bottom line of what I try to demonstrate these days is that the claimed high probability for the belief that mankind is responsible for most or all of the warming over the last century is grossly overstated. Is an anthropogenic explanation plausible? Sure. But since virtually no serious work has been done to investigate natural variability on daily to decadal time scales and how it can influence lower frequency climate variability, it is far from ‘very probable’.
Ray’s first objection is to our new paper, now in press in J. Climate (Potential Biases in Feedback Diagnosis from Observational Data: A Simple Model Demonstration, by Spencer and Braswell). I quote:
“In Spencer and Braswell (2008), and to an even greater extent in his blog article, Spencer tries to introduce the rather peculiar notion of “internal radiative forcing” as distinct from cloud or water vapor feedback. He goes so far as to say that the IPCC is biased against “internal radiative forcing,” in favor of treating cloud effects as feedback. Just what does he mean by this notion? And what, if any, difference does it make to the way IPCC models are formulated? The answer to the latter question is easy: none, since the concept of feedbacks is just something used to try to make sense of what a model does, and does not actually enter into the formulation of the model itself.”
Ray is quite simply wrong — and the reviewers of our paper (Piers Forster and Isaac Held) agree with me. It matters a great deal whether radiative fluctuations are the result of feedback on surface temperature, versus the myriad other variables that control cloudiness. Piers Forster was honest enough to admit that their neglect of the internal variability term in Eq. 3 of “The Climate Sensitivity and its Components Diagnosed from Earth Radiation Budget Data” (Forster and Gregory, J. Climate, 2006) was incorrect, and that it indeed can not be neglected in feedback diagnosis efforts using observational data. He also stated that the climate modeling community needs to be made aware of this.
In fact, both Forster and Held had to construct their own simple models of the effect to understand what I was talking about so that they could convince themselves. Now, I am not a modeler – I’m more of an observationalist. Why did it take someone like me to point this out before anyone else in the modeling community discovered it? I’m not funded to do this stuff – they are.
Our paper gives the very simple case of daily random cloud variability over the ocean (does Ray believe there is no such thing as stochastic variability?). As the following figure demonstrates, this random behavior can cause decadal-scale SST variability that looks like positive feedback.
In this simple case, where the model noise and SST forcing matches satellite-observed statistics from CERES (for reflect SW) and TRMM TMI (for SST), a positive feedback bias of 0.6 W m-2 K-1 resulted (the specified feedback, including the Planck temperature effect, was 3.5 W m-2 K-1).
And if daily random cloud variations can do this, what might weekly, monthly, or yearly non-feedback fluctuations do? Any cloud changes resulting from fluctuations in stability, wind shear, precipitation efficiency, etc. accompanying El Niño/La Niña, the Pacific Decadal Oscillation, or any other mode of internal variability will ALWAYS look like positive feedback – even if there is no feedback present. The question of how the neglect of this effect has contaminated observational estimates of feedback has never even been addressed, let alone answered.
I repeat: to the extent that any non-feedback radiative fluctuations occur, their signature in climate data LOOKS LIKE positive feedback. And when modelers use those relationships to help formulate cloud parameterizations, it can lead to models that are too sensitive.
Next, Ray objects to my simple example of using a different non-feedback source of variability: I assumed cloud changes proportional to the SOI and PDO indices as a potential low-frequency example of this behavior. He shows that the resulting yearly radiative forcing would be much larger than what satellite radiative budget data have measured. Well, the 5-year average forcing was only 1 or 2 W m-2, and any higher frequency (e.g., yearly) noise in the relationship could just be chalked up to the fact that something like the PDO index is not likely to be perfectly correlated to a cloud change.
And besides, the SOI/PDO example took me 1 hour on a weekend with a very simple single idea, internet access, and an Excel spreadsheet. In stark contrast, the IPCC work represents many years and hundreds of millions of dollars of effort to connect the few degrees of freedom contained in the last 100 years of global temperature variations to an anthropogenic cause for those low-frequency signals. What might we have learned if we put that kind of money and brainpower into looking for potential natural non-feedback sources of radiative variability?
Finally, Ray continues the popular ad hominem attack and revisionist history when referring to the fact that our (UAH) satellite temperature data dataset contained errors (before Mears and Wentz and RSS developed their own analysis and discovered those errors). Well, contrary to Ray’s claim, we corrected those errors after they were demonstrated. For years now, our decadal temperature trends have been pretty close to those from RSS. This is how science progresses.
If there had been only one climate model up till now, would we be surprised if a new, second modeling group found errors in what the first modeling group had done?
And, Ray might be surprised to learn that we were not the last ones to make such an error. The RSS satellite temperature record recently had spurious COOLING since early 2007 – which we helped RSS find the reason for.
Finally, I want to reiterate that I DO believe that an anthropogenic source for most of the warming over the last century is a plausible theory. But the claim that an anthropogenic source for the warming has been demonstrated to a high level of confidence can not be supported…simply because so little work on potential natural causes has been done.