One of the main clubs with which the very vocal pro-greenhouse camp repeatedly beat their opponents is the assertion that there is a “consensus” of scientific opinion that temperatures are set to increase dramatically in coming decades.
This club is wielded again and again in response to almost every counter argument. Pro-greenhousers repeatedly state that “every scientist” agrees or that there is a “steady accumulation of scientific evidence” in favour of warming, and so on.
Some of this is due to public confusion over the object of the debate. There is no doubt that temperatures have increased by about a degree or so since 1860, but greenhousers seem to be arguing as if the sceptics are questioning that basic point. In fact, the sceptics are usually questioning the value of the temperature forecasts made by the Intergovernmental Panel on Climate Change, which is most certainly up for debate.
All that said, however, there is no doubt that many scientists, rightly or wrongly, support the IPCC forecasts: however, to claim there is a consensus about forecasts is plainly silly. It is like claiming a consensus for a long-range weather forecast. But there is at least a large and vocal minority who seem to regard them as a form of holy writ. Even before ABC television recently screened the program The Great Global Warming Swindle distinguished scientists with undoubted credentials in climate studies were organising seminars to debunk it. Plenty of others started tapping out articles at their computers.
We will discuss the program itself only in passing here. Instead this article will look at the question of whether this support among experts for the IPCC forecasts of global warming, constitutes a validation of those forecasts. In fact it is, at best, a very weak argument.
For we are not dealing with testable scientific propositions: we are dealing with a bunch of forecasts, issued earlier this year, that average temperatures will be anywhere between 1.1C to 6.4C higher than now in 100 years or so, with a best guess around the 3C mark. The IPCC scientists also state that they had a “very high confidence” (which translates to 90 per cent confident) that human activity would be responsible for anywhere between 0.6C to 2.4C of that increase, depending on what happens with emissions.
The first point to note is that all of this is completely unprecedented. It is difficult to think of any other forecasting exercise of this magnitude, or one apart from astronomy, that deals in such a long period. The limit for serious general forecasting in business and economics is five years, at a stretch. Forecasts for specific markets to be serviced by very large projects, such as power stations, may go out to 40 years. But the models used are almost trivial compared to the climate systems under discussion, and even then a single wrong assumption can make nonsense of the results.
As for trying to make usable forecasts - or say anything useful at all - about a complex system a full century out, anyone with knowledge of forecasting would refer those involved to the divination department at Hogwarts.
Forecasting has a long history which mostly involves failure, but by trial and error forecasters have learned what they can and cannot do; what works and what does not. Unfortunately, the IPCC seems to be repeating many basic mistakes.
This point was made with some force by the publication in late June of the paper Global Warming Forecasts by Scientists versus Scientific Forecasts (PDF 236KB). The authors are J.Scott Armstrong of The Wharton School, University of Pennsylvania and Kesten C.Green, of the business and economics forecasting Unit at Monash University in Melbourne.
Neither author is a climatologist - Armstrong is a professor of marketing - but then, as they point out in the paper, the IPCC climatologists are not forecasters. Readers can look at the paper, which is clearly written, for themselves. It makes a number of points in what amounts to an audit of chapter eight of the IPCC’s recent report which have already been made by myself and others, but with considerably more authority.
One such point involves the use of the technique of tweaking a computer model to fit historical data and then using the model to make forecasts: this is the IPCC approach. It is known to be of little use in forecasting. The paper also counts as the second independent review of aspects of the panel operations where the reviewers have been incontestably independent (the other was the Wegman report (PDF 1.41MB) on the hockey stick graph, involving the use of statistics).
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