The Unasked Question

My old acquaintance (and brother Knight of the SCA), David Friedman, is the son of Nobel Laureate Milton Friedman. He has a question that I haven't seen anyone else ask, nor try to answer:

The argument for large and expensive efforts to prevent or reduce global warming has three parts, in principle separable: Global temperature is trending up, the reason is human activity, and the consequences of the trend continuing are very bad. Almost all arguments, pro and con, focus on the first two. The third, although necessary to support the conclusion, is for the most part ignored by both sides.

The usual argument to show that an increase in global temperatures by a few degrees centigrade over the next century would be a catastrophe, or at least a very bad thing, consists of pointing out specific bad effects: rising sea level increasing the risk of flooding in very low lying areas, rising temperature making particular areas less suited to growing the crops they now grow. But an increase in global temperature would also have good effects, as should be obvious to anyone who has ever spent a winter in Chicago, not to mention Alaska or Siberia. The question is not whether there are any bad effects but whether there are net bad effects, whether the increased risk of flooding in Bangladesh does or does not outweigh the opening of a sea route north of Asia and the increase in the habitable area of Canada and Siberia.

The answer, I think, is that nobody knows if the net effects would be good or bad, and probably nobody can know. [Emphasis added.]