Following some vigorous exchanges, we seem, surprisingly enough, to have come close to a consensus about the range of scientific views on global warming. All contributors to the debate, notably including Ken Parish, seem to agree that my proposed statement of the Global Warming hypothesis:
GWH: Human activity has contributed to global warming over the past century, and, in the absence of policy responses, is likely to generate additional warming of at least 1 degree C over the next century
is broad enough to encompass the views of all the serious contributors to the climate science component of the debate, including moderate sceptics like Richard Lindzen. In particular, all serious contributors to the debate agree, at least on the balance of probabilities, that the evidence supports human-induced global warming.
So the big question for debate is how much warming the “business as usual” scenario will lead to. The range
(a) around 1 degree – the skeptical view
(b) 1 to 3.5 degrees – the mainstream IPCC view
(c) 3.5 -5 degrees – the worst-case view (e.g Stephen Schneider)
From what I can see, effects like substantial species extinction, destruction of natural ecosystems, flooding etc are likely to be severe if the warming exceeds 2 degrees, and warming of 5 degrees would produce widespread human catastrophe.
Which of these is right depends on complex arguments about feedbacks and so on, on which aren’t going to be resolved in a hurry. But the existence of uncertainty cuts both ways. Things might turn out better than the IPCC expects, but they could also be worse.
So the question is, what do we do in the face of this kind of uncertainty. There are both philosophical and practical issues here. Philosophically, there’s a choice between ‘precautionary’ and ‘permissive’ principles. The precautionary principle says we should minimise environmental risk in cases of uncertainty, the permissive principle says we shouldn’t restrict economic activity unless it’s been proven to be harmful.
I prefer a practical rather than a philosophical approach. Will it be cheaper, on average, to do nothing now and take really drastic action in a shorter timeframe if the warming turns out to be in the upper range of predictions, or to start now with Kyoto and take the risk that costs will have been incurred for nothing if the sceptics turn out to be right? Given the difficulty of doing anything fast that’s already been demonstrated, I think the latter approach is justified.