Anyone who looks seriously at climate issues should recognize that the consequences of climate change are very uncertain. My own view is that they are sufficiently uncertain to raise serious doubts about the sign as well as the size of the effect, that warming due to human production of greenhouse gases might well make us better off rather than worse off. Even if I am wrong and the effect is almost certainly negative, how negative it will be is very uncertain. CO2 emissions might fall sharply due to increases in the cost of fossil fuels or decreases in the cost of alternatives. For a given value of emissions, varying estimates of climate sensitivity imply at least a factor of two range for the resulting temperature. For a given increase in temperature, the effect on humans depends on what humans will be doing for the next century. Diking against a meter of sea level change could be a serious problem for Bangladesh if it happened tomorrow. If Bangladesh follows the pattern of China, where GDP per capita has increased twenty fold since Mao's death, by the time it happens they can pay the cost out of small change.





A possible response to this point is to argue that uncertainty is no argument against action. One simply replaces the uncertain range of outcomes with the best estimate one can provide of its expected value, the average of costs weighted by their probability, and acts as if that were the known consequence of warming. If the estimate of expected cost is ten trillion dollars, then any precaution to prevent it that costs less than ten trillion is worth taking.





It is a possible response and a popular one, but it is wrong for a reason that ought to be obvious to (at least) economists. The question we are answering is not "what should we do?" but "what should we do now?" Waiting may raise the cost of dealing with the problem but it will also provide additional information. The more information we have, the better our ability to decide what precautions are worth taking. Or not worth taking. Uncertainty that will be reduced over time is an argument against immediate action.





The usual rhetorical response is to claim that we barely have time to act at all, that if we wait more than a very short time it will be too late. This claim becomes less persuasive the more times it is made, and it has been made, by various people, quite a large number of times over the past twenty years or so. It largely depends on picking some arbitrary temperature change, most commonly two degrees C, and treating it as if it were the end of the world. As salesmen commonly put it, "Buy Now—This Is Your Very Last Chance To Take Advantage of Our Special Offer."





For a more realistic opinion, consider an estimate of the cost of waiting by William Nordhaus, an economist who has specialized in climate issues. In the course of a piece arguing for immediate action against climate change , he reported his estimate of how much greater the cost of climate change would be if we waited fifty years to deal with it instead of taking the optimal action at once. The number was $4.1 trillion. He took that as an argument for action, writing that "Wars have been started over smaller sums."





As I pointed out in a post here responding to Nordhaus, the cost is spread over the entire world and a long period of time. Annualized, it comes to something under .1% of world GNP.

"Thought before action, if there is time."

(quote from a character in a Dick Francis novel)

And there usually is.