Many interesting points in this mix.
Let me start by emphasizing where I think I agree with James. We must
begin by thinking rationally, and in order to think rationally we must
ultimately think quantitatively. Costs must be reduced to some sort of
numerical quantity (although I am not happy with how this is normally
done), and must be risk-weighted to the best of our abilities.
Nevertheless, and despite my admiration for James, I find the question
he is raising quite shockingly wrongheaded.
The main reason the greenhouse issue is not trivial is because of the
enormous benefits of cheap energy. This is obvious and I am not one to
minimize its complexity.
However, that isn't the question asked. James asked what the optimum change is.
All else equal (essentially, ignoring the benefits of cheap energy) it
seems to me almost certain that the optimum rate of change of climate
is zero or near enough zero as not to matter for practical purposes.
Historical and prehistoric civilization collapses have often been
triggered by climate change. (See Jared Diamond's book _Collapse_ for
a remarkable exposition on this and related points.) One never hears
about abrupt declines caused by excessive climate stability or about
abrupt improvements in fortune caused by climate change.
This is not accidental. If, under global warming, the fortunes of
Canada improve and the fortunes of India decline, the net effect
area-weighted might be for increased biomass, but the net effect on
human well-being will be highly negative. To a very significant
extent, this is because India is more hospitable than Canada. This is
at least one reason why the former is overcrowded and the latter
largely uninhabited.
Similar arguments apply to ecological risk.
Consider that it would not take a huge change to make the planet
essentially uninhabitable; say 20 C in either direction would do the
trick. That we are talking in terms of a significant fraction of 20 C
seems to me, therefore, completely and utterly beyond the remotest
justification.
It is conceivable that our troubles are smaller than many of us
suspect, but it is also possible that they are as large or larger. In
the former case, we may be missing out on a lovely opportunity to
increase the distribution of fine vineyards and pineapple plantations.
In the latter case, it is possible that, early in our mad trajectory
to devastation and absurd tragedy, we may briefly pass by a climate
that is slightly more benign than the one provided by nature. It is
the latter case that dominates the risks. I would rather make every
possible effort not to tempt the fates in that way.
James' question seems to me oddly disproportionate to our actual circumstances.
mt
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