Almost every time I post something on climate change policy, the comment thread quickly devolves into a debate over the existence of antrhopogenic global warming at all. (See, for instance, this post on "conservative" approaches to climate change policy.) I have largely refused to engage in these discussions because I find them quite unproductive. The same arguments are repeated ad nauseum, and no one is convinced (if anyone even listens to what the other side is saying). I have also seen nothing in these exchanges that would alter my current assessment of the scientific evidence.
Given my strong libertarian leanings, it would certainly be ideologically convenient if the evidence for a human contribution to climate change were less strong. Alas, I believe the preponderance of evidence strongly supports the claim that anthropogenic emissions are having an effect on the global climate, and that effect will increase as greenhouse gases accumulate in the atmosphere. While I reject most apocalyptic scenarios as unfounded or unduly speculative, I am convinced that the human contribution to climate change will cause or exacerbate significant problems in at least some parts of the world. For instance, even a relatively modest warming over the coming decades is very likely to have a meaningful effect on the timing and distribution of precipitation and evaporation rates, which will, in turn, have a substantial impact on freshwater supplies. That we do not know with any precision the when, where, and how much does not change the fact that we are quite certain that such changes will occur.
So-called climate "skeptics" make many valid points about the weakness or unreliability of many individual arguments and studies on climate. They also point out how policy advocates routinely exaggerate the implications of various studies or the likely consequences of even the most robust climate predictions. Economists and others have also done important work questioning whether climate risks justify extreme mitigation measures. But none of this changes the fact that the cumulative evidence for a human contribution to present and future climate changes, when taken as a whole, is quite strong. In this regard, I think it is worth quoting something Ilya wrote below about the nature of evidence in his post about 12 Angry Men":
People often dismiss individual arguments and evidence against their preferred position without considering the cumulative weight of the other side's points. It's a very easy fallacy to fall into. But the beginning of wisdom is to at least be aware of the problem.The "divide and conquer" strategy of dissecting each piece of evidence independently can make for effective advocacy, but it is not a good way to find the truth.
Don't get me wrong. I believe that there is room to question the global warming "consensus," particularly as represented by activist groups and some in the media, and to challenge various climate scenarios and their policy implications. I am unpersuaded that climate change threatens civilization or justifies truly draconian measures. Nevertheless, I believe climate change is a serious concern. And as much as I wish it were not the case, I believe the threat of climate change justifies some measures that the libertarian in me does not much like. But that's the way it is.