In “Pointless to rush a carbon emissions plan,” the Toronto Globe and Mail’s Neal Reynolds compares Yale Professor William Nordhaus’s “optimal” approach to controlling greenhouse gases and finds it superior to approaches that would impose deeper controls more rapidly, such as those favored by Stern, various EU leaders, and many in the US.

Under the Nordhaus approach, which is also discussed by Keith Johnson at the Wall Street Journal, costs of control would start at 0.3 per cent of global GDP in 2010 (currently around $60,000 billion), increase to 0.5 per cent in 2015, 0.6 per cent in 2020 and peak at 0.9 per cent in 2065. He estimates the net present value (NPV) of climate change damages absent any controls at $22 trillion. Under this so-called “optimal” approach, the NPV costs of controls would be $2 trillion and climate change damages would be reduced by $5 trillion (i.e., the “optimal” policy would provide net benefits of $3 trillion, but residual damages would be $17 trillion). As he explains, “More of the climate damages are not eliminated because the additional abatement would cost more than the additional reduction in damages.”

He also estimates that proposals that emphasize “excessively early reductions [make] the policies much more expensive… For example, the Gore and Stern proposals have net costs of $17 trillion to $22 trillion relative to no controls; they are more costly than doing nothing today.” By his calculations, his proposal is clearly superior to these other reduction proposals.

However, while Nordhaus’s prescription may indeed be the most “optimal economic approach” to slow global warming, it isn’t the optimal approach to addressing global warming. This is because it ignores adaptation. Some adaptations may reduce climate change damages more efficiently than mitigation. Perhaps all or part of the $2 trillion that Nordhaus would spend on mitigation should, instead, be invested in adaptation. That might reduce damages by more than the $5 trillion. In any case, with adaptation in the mix, $5 trillion may well be the lower bound for the optimal reduction in climate change damages. And, of course, emission reductions that seem to be optimal under 0.9 percent of GDP in 2065 in the absence of adaptation may, once adaptation is thrown into the mix, no longer be optimal.

In fact, a recent Cato Policy Analysis indicates that in the short-to-medium term, adaptation — specifically, reducing vulnerability to climate-sensitive problems that might be exacerbated by climate change — would provide greater benefits than mitigation, and at a much lower cost. Most of those benefits come from the fact that one approach to adaptation is to advance adaptive capacity. Significantly, that can help society cope not only with climate change but, more importantly, to other problems that are more important than climate change now and in the foreseeable future. Thus the ancillary benefits of increasing adaptive capacity are very high, higher than climate change damages in the absence of any controls according to the Cato Policy Analysis.

Notably, Nordhaus acknowledges to having “relatively little confidence in our projections beyond 2050.” To his credit, this skepticism informs his recommended approach, but it would probably have been best to avoid stretching the analysis to 2200.

Sometimes such long-range analyses are justified on the grounds that that’s the best that can be done. But even if that’s so, it misses the real issue, namely, whether even the best available analysis is good enough for making trillion-dollar decisions which, moreover, extend out centuries hence. At these temporal distances, Nostradamus may be just as credible as Nordhaus, or Nicholas Stern, for that matter.

Humility isn’t an offense, and it ought to be acceptable for economists and policy analysts—even those whose stock in trade is climate change—to admit that they haven’t a clue what the world will look like beyond 2050 (if then).

Nordhaus’s numbers indicate that estimates of pre-control damages and post-control residual damages frequently are substantially larger than either the costs or benefits of emission controls. But the treatment of damages (i.e., impacts) of climate change in the Nordhaus analysis is somewhat sketchy. As far as I can determine, none of the damage studies properly account for adaptive capacity, particularly considering that that capacity ought to increase if societies accumulate wealth, human capital and technology at rates implied by all the socioeconomic scenarios used to derive future emissions (and climate change). (See, for example, here.) Thus, both pre-control climate change damages and post- control residual damages could be substantially overestimated.

[Some argue that they disbelieve that economic growth will be as high as assumed, but in that case they should also disbelieve estimates of future climate change and impacts predicated on that growth.]

To summarize, the Nordhaus analysis probably overestimates climate change damages. In any case, the Nordhaus approach could be made more optimal by adding to it an adaptation component that would enhance societies’ adaptive capacities (by reducing present day vulnerabilities to climate-sensitive problems and boosting economic development and human capital in developing countries). In fact, optimal carbon taxes (or cap-and-trade approaches) can only be determined after completion of more comprehensive analyses that include full and equal consideration of adaptation and any ancillary (net) benefits.

Of course that still leaves the problem of relying on analyses over time frames that demand, in Coleridge’s words, “willing suspension of disbelief.” Instead of suspending disbelief and succumbing to gullibility, I would recommend a somewhat different approach (see here, p. 37).