Tuesday, December 11, 2007

Of fat-tailed catastrophes

Estimating the long-term costs of climate change has become something of a stumbling block in determining what we should be doing now to mitigate the worse effects. There's continuing debate about whether the short-term costs of mitigation outweigh the uncertain long-term costs of doing nowt - something complicated by questions about what the appropriate discount rate should be, as noted below. In the US, industry lobby groups have deployed cost-benefit analysis (CBA) to argue that it's just not cost-effective to do anything to reduce emissions now - given the long time horizons, it'll be better to deal with any problems if and when they happen, even if the cost is many times that of acting now. Basically, prevention isn't better than treatment.

Martin Weitzman, of Harvard's economics department, has now countered that device with a paper considering the possibility (however low) of a genuinely catastrophic event resulting from a failure to act now. Such events are usually ignored in standard CBA, mainly because they're a bit difficult to work with. Weitzman's model does include the possibility of extreme events (which he terms 'fat-tailed catastrophes'), and the results significantly shift the balance of costs. When applied to the current knowledge relating to climate change and emissions, Weitzman's analysis shows that mitigation investment makes a hell of a lot of sense in minimising expected future costs.

As Weitzman concludes:
Even just acknowledging more openly the incredible magnitude of the uncertainties that are involved in climate-change analysis - and explaining better to policy makers that the artificial crispness conveyed by conventional IAM-based CBAs here is especially and unusually misleading compared with more-ordinary non-climate-change CBA situations - would in my opinion go a long way towards elevating the level of a reality-based public discourse concerning what to do about global warming.

The fairly technical paper is available in draft as a PDF here. For the less technically inclined, New Scientist gives a good summary.

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