A few weeks ago on the MasterResource blog, Paul Knappenberger noted that, in light of generally stagnant global average temperatures over the last decade (and falling temperatures over the last half dozen years), we might justifiably begin to doubt the reliability of the models on which much of the concern over global climate change is based. On the basis of this concern and a few allusions to other lines of evidence which seem to cast doubt on the case for alarm, Knappenberger concludes that:
While coming years may or may not continue the cooling trend of the past several, they will almost assuredly continue to add to the growing evidence that our coming climate will likely be far less detrimental than the popular projections of it to which we are often exposed.
In writing this response, I don't want to create the impression that I think that Knappenberger does a bad job in his post. I think he built a very reasonable case for concern over the reliability of climate models, and to a large extent I agree with him. About a year ago, I wrote a post on this blog discussing some of my reservations about climate modeling, noting that models raise uncertainty in at least five important ways: There may be problems resulting from "tuning" models; the IPCC uses averages from multiple models, potentially distracting us from serious and different flaws in each model; the models cannot effectively model small-scale phenomena which could be important in determining the future state of the climate system; they make use of proscribed variables; and they cannot effectively capture unprecedented, game-changing possibilities without opening themselves up to radical uncertainty. Knappenberger discusses a number of these issues in his post, and I think he does a good job.
But I do have one objection to his handling of the issue with which he concerned himself in his post: his conclusion was not at all supported by his argument. I believe that Knappenberger has mistaken the absence of evidence to be the evidence of absence. He coherently argued that climate models had made a vague prediction about what would happen in the future, and that since the prediction has not obtained as expected, we have reason to doubt that the climate models are reliable. However, he then appears to jump from the claim that climate models (which predict warming) are unreliable to the claim that warming is unlikely. But this jump cannot be sustained without further evidence.
To illustrate the problem, we might imagine that we are on our way to the mall with a particularly superstitious friend when she suddenly exclaims, "I bet we find a parking spot right away; we've gotten green lights the whole way so far, and that usually means I'm going to find a parking spot!" We might be skeptical of this claim, and regard her thesis as entirely unsupported. But what does that mean about the prediction that we'll find a parking spot? It simply doesn't tell us anything. We'd need to throw out our friend's claim entirely and appeal to entirely different lines of evidence to discuss the truth of the matter. In the same way, showing that climate models' predictions of future warming are unreliable does not suggest that there will not be warming. It simply suggests that we should place less emphasis on their predictions in forming our outlooks, or discard them completely.
The reason I belabor this point is that I think there's a broader point to be made here. There is a fundamental and important difference between an argument that says, "The state of scientific knowledge is not advanced enough that we can make a reliable prediction that distressing or catastrophic global climate change will occur in the relatively near future as a result of human activities," and an argument that says, "The state of scientific knowledge is advanced enough that we can make a reliable prediction that distressing or catastrophic global climate change will not occur in the relatively near future as a result of human activities." In his post, Knappenberger supports (effectively!) the first kind of argument. But in his conclusion, he advances the second kind, and without justification. I think it's critical that in thinking about climate change, we make sure to keep in mind this distinction and frame our skepticism in a way that we can defend.