I finally watched the BBC special Global Warming Swindle and have to say that it presents a pretty good counter-hypothesis to the prevailing theory of anthropomorphic CO2 production to explain recent global temperature changes. It also hits some good points on what might be motivating the hard core of the environmental movement beyond just concern about global warming, and why the costs of CO2 control are so high.
I have historically accepted the basic hypothesis of anthropomorphic global warming but have been skeptical of the exaggerated outcomes (Al Gore's 26 foot sea-level rise, for example, which is 17 times more than even the IPCC predicts over the next century) and have posited that a warmer but richer world may well be better than a cooler but poorer one. I have also pointed out the uncertainties in the IPCC analysis that never get mentioned in the press, like the huge uncertainty in the feedback loops that drive much of the temperature change in current models. For example, the IPCC admits they don't even know the sign of the largest feedback loop (clouds), which is a big uncertainty since about 2/3 or more of the warming in the models come not directly from CO2 but from these feedback loops.
Anyway, most of my past skepticism has been within the framework of these IPCC studies. However, this documentary casts off the whole framework, offering a counter-hypothesis of solar activity to explain temperature variations. I thought the most interesting part of the documentary was when they showed Al Gore from An Inconvenient Truth with a multi-thousand year plot of temperature and CO2. The chart certainly looks compelling, but this movie makes the point that while the two lines move together, the CO2 line is lagging the temperature line by five hundred years. Meaning that CO2 levels may be linked to temperature, but the causality may be opposite of that implied by Gore.
The documentary goes on to offer solar activity as an alternative explanation, with graphs of moving curves of solar activity and temperature that seem to show at least as much correlation as Gore's CO2 graphs. They hypothesize that rising temperatures driven by changes in solar
activity heat up oceans over time and cause them to release CO2 into
the atmosphere. I don't think the evidence is definitive, but it certainly casts doubt as to whether we really know what is going on. I always thought it a bit odd that people would search for the causes of changing temperatures without first checking out the sun, sortof like walking in a room that is too hot and trying to fix it without first checking the thermostat. This is particularly true given new evidence that other planets are warming, presumably due to solar activity (unless, of course, it's an Exxon plot).
By the way: Advocates of the anthropomorphic theory are criticizing this movie in part because it does not use Mann's hockey stick temperature chart. Sorry, but if they want to claim the scientific high ground, I think they need to stop tying their argument to this weak study. Statisticians have dumped on it repeatedly (apparently random white noise fed into their model produces a hockey stick) and the evidence for eliminating the Medieval warm period is based on the rings in one or two trees.