More on the Limited Evidence for Global WarmingMike Rappaport

I have frequently questioned the Al Gore as well as the UN view on global warming.  This article in National Review does a good job of pointing out the problems with these views.

The U.N. Intergovernmental Panel on Climate Change (IPCC) . . . summary indicates that the IPCC is 90% confident that we have caused global warming.  The summary further implies that if wedouble the concentration of carbon dioxide (CO₂) in the atmosphere, the IPCC is 90 percent onfident that we will cause further warming of 3° C /- 1.5° C.

But what do these statements of confidence really mean? They are notderived mathematically from the type of normal probabilitydistributions that are used when, for example, determining the marginof error in a political poll (say, /- 5%). IPCC estimates of confidence are really what we would mean by this word in everydayconversation a subjective statement of opinion. This is a very bigdeal, since bounding the uncertainty in climate predictions is centralto deciding what, if anything, we should do about them.

Three key assertions in the global-warming debate are supported by a very large body of evidence. One, the concentration of CO₂in the atmosphere has increased by approximately 35 percent overroughly the last 150 years. Two, global temperatures have risensomething like 0.6° C over this period. Three, CO₂ is agreenhouse gas, meaning simply that it absorbs and redirects infrared(longer-wavelength) radiation but not shorter-wavelength radiation.

It s highly plausible to proceed from these three points to a fourth assertion, namely, that the increase in CO₂ caused the increase in temperature.

The problem lies in the next step.  While increases in CO2 would lead to increases in global temperature, the IPCC report relies on feedback effects to do the bulk of its “work.”

The entire legitimate scientific debate is really about these feedbackeffects. Feedbacks are not merely details to be cleaned up in a picturethat is fairly clear. The base impact of a doubling of CO₂ in the atmosphere with no feedback effects is on the order of 1° C. The IPCC estimate of the impact of doubling CO₂ is about 3° C. So the feedback effects in the IPCC scenario dominate the prediction.

When evaluating model reliability, the second test can it predictaccurately? is the acid test. We can debate all day about whether amodel is complete enough, but if it has correctly predicted majorclimate changes over and over again, that is pretty good evidence thatits predictions should be taken seriously. There are plenty of studiesthat show what is called hindcasting, in which a model is built onthe data for, say, 1900-1950, and is then used to predict the climatefor 1950-1980. Unfortunately, it is notoriously common for simulationmodels in many fields to fit such holdout samples in historical datawell, but then fail to predict the future accurately. So the crucialtest is actual prediction, in which a model is run today to forecastthe climate for some future time-period, and then is subsequentlyvalidated or falsified. No global climate model has ever demonstratedthat it can reliably predict the climate over multiple years ordecades never.

(emphasis added).  My understanding of the debate is that all of this is uncontroversial.  Why there should be a consensus in favor of the stronger predictions, not to mention the extremism of Al Gore, would seem to be a mystery. 

Let me put the point differently.  Scientists traditionally have a norm of claiming less for  what their theories hold than one might think justified.  This enhances their objectivity and guards against overconfidence.   In this area, presumably for obvious political reasons, scientists have abandoned that posture.  (It seems like a good area for the behavioral economics people to study overconfidence.)

If there was no effort to push this consensus idea — if the problems with the theory were acknowledged forthrightly — I would be much more likely to accept the claims than in the current politicized environment.