I thought I would answer a question that several people have posed to me recently. When determining whether a scientific consensus exists, whose opinion counts? My answer is: look for consensus in the peer-reviewed literature.
As I discussed in a recent post, the scientific enterprise determines whether to accept or reject a claim by repeatedly testing it, and publishing the results of those tests in the relevant journals. Claims that have been repeatedly tested are accepted, with the confidence given that claim proportional to how numerous and rigorous the tests have been. One can determine which claims have been repeatedly tested in the "crucible of science" and therefore accepted, as well as how confident we are in our acceptance of that claim by reading the peer-reviewed literature.
Viewed through my definition, there is clearly a strong consensus that 1) the Earth is warming and 2) that humans are likely to be the dominant cause of the recent warming.
Might a "consensus" position be wrong? Of course. All knowledge is provisional and subject to future revision as new data comes in. However, as policymakers, the scientific consensus is the position most likely to be correct. In particular, strongly held consensus positions (e.g., smoking causes cancer, the Earth is warming) are verly unlikely to turn out to be wrong. Policymakers can do no better than to follow the scientific consensus in formulating their policy.