The Weekly Standard has an excellent overview by Steven F. Hayward on where we stand in the climate change debate:
How do you go about sorting out sense from nonsense? Very few people who follow closely the subject of climate change argue that there's nothing to it. There is unanimity that the planet has warmed by about 1 degree over the last century. Just about everyone agrees that the growth of greenhouse gas emissions from fossil fuels cannot continue forever. That's where the agreement ends. The range of possible temperature increase over the next century is fairly wide in the official forecasts, from 1.4 degrees Celsius on the low side, which might not be difficult to cope with, to 5.8 degrees Celsius on the high side, which would mean major environmental problems for the planet. How probable is any point along the distribution? For reasons having to do with the cascading statistical uncertainties of the thousands of variables in computer climate models, we can't assign a probability to any narrower range of temperature forecasts, though very clever people are trying.Do read the whole thing, which is quite thorough and level-headed.
Ultimately, policymakers will have to exercise their best judgment rather than wait for oracular scientific conclusiveness, which will never come. Notwithstanding the relentless drumbeat of studies offered as proof of onrushing catastrophe, policymakers are rightly wary of handing over the keys of the economy to the very same people who brought us the population bomb that turned out to be a wet firecracker, predicted imminent resource scarcity, which also fizzled, and even, in the 1970s, hyperventilated that our greatest climate risk was a new ice age. (The ice age scare was not the tiny sideshow climate action advocates today try to claim that it was; the EPA in the early 1970s thought one reason to reduce sulfur dioxide emissions was that "aerosols" like SO2 were reflecting too much sunlight and increasing the risk of cooling the planet.) The suspicion of hidden agendas is buttressed by the default position of the most vocal environmentalists and the front-page-seeking reporters who cover the climate beat: They greet with complete credulity the most extreme forecasts and portents, whether it is melting ice, boiling oceans, or expiring frogs.
The case of David Henderson and Ian Castles is a good example. Henderson, the former chief economist of the OECD, and Castles, a highly regarded Australian economist, noticed three years ago a serious methodological anomaly in the IPCC's 100-year greenhouse gas emission forecasts, which are the primary input for the computer climate models. Henderson and Castles made a compelling argument that the forecasts were unrealistically high. Everyone recalls the first day of computer science class: garbage in, garbage out. If future greenhouse gas emissions are badly overestimated, then even a perfect computer climate model will spit out a false temperature prediction. If Henderson and Castles are right, it means we may have more time to address even the most alarmist global warming forecasts. Since Henderson and Castles opened the debate, the IPCC's emissions forecasts have been subject to withering criticism from dozens of other reputable economists, including from a number of climate alarmists who, to their credit, argue that this crucial question should be got right.
The IPCC's reaction to Henderson and Castles was startling. The panel issued a vituperative press release blasting the two men for peddling "disinformation." A few scientists and economists connected with the IPCC had the decency to say publicly that the press release was a regrettable error. But it is typical of the increasingly arrogant IPCC leadership. The IPCC's chairman, Dr. Rajendra Pachauri, compared Danish eco-skeptic Bjorn Lomborg to Hitler because of Lomborg's wholly sensible and well-founded calculation that near-term emissions reductions make no economic sense. "What is the difference between Lomborg's view of humanity and Hitler's?" Pachauri told a Danish newspaper in 2004. "If you were to accept Lomborg's way of thinking, then maybe what Hitler did was the right thing." It is hard to have much confidence in an organization whose chairman can say this and keep his job. (The reductio ad Hitlerum is contagious: Two weeks ago NASA's James Hansen compared having a Bush political appointee listen in on his media phone calls--an obnoxious but routine practice in the federal government--to Nazi Germany or the Soviet Union, eliciting rapturous applause from an audience in New York. And Hansen wonders why people call him an alarmist.)