Another recurring theme from people who argue against the fact that humans are changing the climate is a big focus on uncertainty. They are critical of forecasts of future temperature increases from complex climate models run on massive computer arrays. The models require simplifying assumptions to be computable, and they argue these assumptions could be "rigged" to make the models artificially confirm global warming.
Yet somehow, each of the rival proposals that they latch onto are excused from this scrutiny. They talk about galactic cosmic rays (GCRs) or solar variations as though we had clear, indubitable data showing these are the "real reason" behind recent temperature trends. In fact, neither of these external forcings is at all well characterized; we are far from having any clear basis for projecting long term trends in either one.
Then there are economic forecasts. Critics of the Kyoto accord regularly project "catastrophic" economic harm if the CO2 reduction targets are implemented. They are quite adamant about this; they seem certain of these projections. Yet the academic literature on this question tells a different story. The recent Stern Report is only the latest instance. Quite a while earlier there was Robert Repetto and Duncan Austin, The Costs of Climate Protection: A Guide for the Perplexed [World Resources Institute, 1997. 60 pages. ISBN 1-56973-222-1, 60 pages] link.
This document looks at projections made with several different economic models, and how much impact on GDP growth there could be if greenhouse gas reductions are implemented. They focus on seven key assumptions, and assess how strongly the outcome depends on each one. For each issue, they look at a "worst case" and "best case" assumption, and work through how much GDP growth would be impacted if all seven are for the worse, or if some or all are the better case. If all seven are in the negative--a "worst worst" case--the models project a significant hit to GDP, something like 7% less GDP growth over the time span needed to reach a 60% cut in emissions. That's not minor, but looked at in context, it is a reduction in foreseen economic growth (something on the order of a doubling of GDP over several decades.) If this takes 30 years, that's the difference between 2.1% per year and 1.9% per year of growth. The growth that would have taken 30 years might now require 33. Catastrophic?
But that's only the "worst worst" case; if even two of the seven factors turn out to the better, the projected impact shrinks to just a 2% hit to GDP over the whole interval. Then the annual growth is 2% vs. 2.1%.
If four of the seven factors turn out favorably, the impact is projected to be very near zero. Once five go favorably, mitigation turns out to be a net benefit to the economy! Some catastrophe.
Of course there are probably many serious studies proposing that greenhouse gas mitigation will be terribly costly to the economy. I won't try to claim that economists can give us a single, persuasive forecast of what's going to happen. There's plenty of room for debate over the economics. Repetto and Austin is already dated, but the Stern Report brings the issue up to the present with much the same import.
It's only the science that's so clearly worked out. We really can't say for sure what will develop economically over the next century. But some critics seem to want to have the reverse: the economics is settled, on the side of catastrophe if Kyoto is implemented; but the science is still cloudy. So we show forget the "Precautionary Principle" and keep stalling. They have everything backwards.