A throw of the D.I.C.E. will never abolish chance

In Building a Green Economy, NYT Magazine, April 7, 2010, Paul Krugman wrote,

As Harvard’s Martin Weitzman has argued in several influential papers, if there is a significant chance of utter catastrophe, that chance — rather than what is most likely to happen — should dominate cost-benefit calculations. And utter catastrophe does look like a realistic possibility, even if it is not the most likely outcome. 

Weitzman argues — and I agree — that this risk of catastrophe, rather than the details of cost-benefit calculations, makes the most powerful case for strong climate policy. Current projections of global warming in the absence of action are just too close to the kinds of numbers associated with doomsday scenarios. It would be irresponsible — it’s tempting to say criminally irresponsible — not to step back from what could all too easily turn out to be the edge of a cliff.

... 

So what I end up with is basically Martin Weitzman’s argument: it’s the nonnegligible probability of utter disaster that should dominate our policy analysis. And that argues for aggressive moves to curb emissions, soon.

So far, so good. But Krugman's conclusion produced this pretzel of cognitive dissonance:

...there has to be a real chance that political support for action on climate change will revive. 

If it does, the economic analysis will be ready [no, it isn't]. We know how to limit greenhouse-gas emissions [no, we don't]. We have a good sense of the costs [nope] — and they’re manageable [how could we know?]. All we need now is the political will.

Krugman apparently assumed that the cost estimates developed, for example, in Nordhaus's "dynamic integrated climate-economy" (DICE) analyses are independent of when the greenhouse gas abatement actions are taken. But the rationale for delaying abatement is that the discount rate assumed in the model makes it cheaper to wait to do the abatement. Weitzman's critique doesn't present cost estimates. Contra Krugman, there is not even a consensus about what needs to be done or how to do it, let alone "a good sense of the costs" of doing... it? Whatever "it" is.

The bottom line (literally) is that a key consideration of the structure and assumptions of the conventional models was facilitating economic growth and relying on that economic growth to finance the costs of abatement. The DICE were loaded for growth! To put it somewhat crudely, delaying abatement was supposed to make a large part of the cost "pay for itself" through the dividends earned on the money saved by not doing it now. You cannot have your cake and eat it too. Nor can you finance a current expenditure from revenues you would have earned if you hadn't made the expenditure.

The excerpts and abstracts below are not from people Krugman would ridicule as "degrowthers." There seems to be a dawning awareness that the assumptions of the conventional integrated assessment models need to be, at the very least, radically revised, which is essentially the point those silly anti-capitalist degrowthers on the left (going back to that silly anti-capitalist leftist Nicolaus Georgescu-Roegen)  have been making all along.

"Climate Change Policy: What Do the Models Tell Us?" Robert S. Pindyck

Very little. A plethora of integrated assessment models (IAMs) have been constructed and used to estimate the social cost of carbon (SCC) and evaluate alternative abatement policies. These models have crucial flaws that make them close to useless as tools for policy analysis: certain inputs (e.g., the discount rate) are arbitrary, but have huge effects on the SCC estimates the models produce; the models’ descriptions of the impact of climate change are completely ad hoc, with no theoretical or empirical foundation; and the models can tell us nothing about the most important driver of the SCC, the possibility of a catastrophic climate outcome. IAM-based analyses of climate policy create a perception of knowledge and precision, but that perception is illusory and misleading.

"Climate Policy: Science, Economics, and Extremes," Anthony C. Fisher and Phu V. Le

Economic models help illustrate the links between the climate and the economy, and they are an important component of the multidisciplinary analysis that is needed to address climate change. However, there are major problems with the estimates of potential damages in the IAMs... First, damage functions and estimates appear to have little connection to the empirical findings from econometric studies of sectoral impacts, particularly on agriculture, as we discuss later. More generally, economy-wide damage functions are simply not known, especially at the global level. Thus, as Pindyck (2013b) argues, there is little empirical, or for that matter theoretical, foundation for the specification of functional forms and parameters in the models. This suggests that their quantitative results and policy prescriptions are somewhat arbitrary. 

We agree with Stern (2013) that there are gross underestimations of damages in economic impact models and IAMs, and we discuss some additional issues that are not adequately addressed in the models including the importance of nonlinearities, environmental impacts, extreme events, and capital losses.

"Endogenous growth, convexity of damages and climate risk: how Nordhaus' framework supports deep cuts in carbon emissions." Simon Dietz and Nicholas Stern

'To slow or not to slow' (Nordhaus, 1991) was the first economic appraisal of greenhouse gas emissions abatement and founded a large literature on a topic of great, worldwide importance. In this paper we offer our assessment of the original article and trace its legacy, in particular Nordhaus' later series of 'DICE' models. From this work many have drawn the conclusion that an efficient global emissions abatement policy comprises modest and modestly increasing controls. On the contrary, we use DICE itself to provide an initial illustration that, if the analysis is extended to take more strongly into account three essential elements of the climate problem -- the endogeneity of growth, the convexity of damages, and climate risk -- optimal policy comprises strong controls. To focus on these features and facilitate comparison with Nordhaus' work, all of the analysis is conducted with a high pure-time discount rate, notwithstanding its problematic ethical foundations. [We have argued elsewhere that careful scrutiny of the ethical issues around pure-time discounting points to lower values than are commonly assumed (usually with little serious discussion).]

"Climate Risks and Carbon Prices: Revising the Social Cost of Carbon," Frank Ackerman and Elizabeth A. Stanton

Once the social cost of carbon is high enough to justify maximum feasible abatement in cost-benefit terms, then cost-benefit analysis becomes functionally equivalent to a precautionary approach to carbon emissions. All that remains for economic analysis of climate policy is to determine the cost-minimizing strategy for eliminating emissions as quickly as possible. This occurs because the marginal damages from emissions have become so large; the uncertainties explored in our analysis, regarding damages and climate sensitivity, imply that the marginal damage curve could turn nearly vertical at some point, representing a catastrophic or discontinuous change.  

The factors driving this result are uncertainties, not known facts. We cannot know in advance how large climate damages, or climate sensitivity, will turn out to be. The argument is analogous to the case for buying insurance: it is the prudent choice, not because we are sure that catastrophe will occur, but because we cannot be sufficiently sure that it will not occur. By the time we know what climate sensitivity and high-temperature damages turn out to be, it will be much too late to do anything about it. The analysis here demonstrates that plausible values for key uncertainties imply catastrophically large values of the social cost of carbon.
…
Our results offer a new way to make sense of the puzzling finding by Martin Weitzman: his “dismal theorem” establishes that under certain assumptions, the marginal benefit of emission reduction could literally be infinite (Weitzman 2009). The social cost of carbon, which measures the marginal benefit of emission reduction, is not an observable price in any actual market. Rather, it is a shadow price, deduced from an analysis of climate dynamics and economic impacts. Its only meaning is as a guide to welfare calculations; we can obtain a more accurate understanding of the welfare consequences of policy choices by incorporating that shadow price for emissions.