Future scenarios for emissions need continual adjustment

Sir

In their Commentary ‘Dangerous assumptions’ (Nature 452, 531–531; 2008), Pielke et al. show that the 2000 Special Report on Emissions Scenarios (SRES) reflects unrealistic progress on both the supply and demand sides of the energy sector. These unduly optimistic baselines cause serious underestimation of the costs of policy-induced mitigation required to achieve a given stabilization level.

This is well known among experts but perhaps not to the public, which may explain why some politicians overstate the impact of their (plans for) climate policy, and why others argue incorrectly that 'available' off-the-shelf technologies can reduce emissions at very little or no cost.

The numbers presented by Pielke et al. are revealing, but they divert attention from a more serious problem underlying the SRES approach to calculating mitigation costs: a failure to incorporate the dynamic nature of the decision problem into climate-policy analysis. Until we can keep adjusting the analysis by continually incorporating uncertainty, correction and learning, we shall continue to offer policy-makers an incomplete guide to decision-making.

The focus of policy analysis should not be on what to do over the next 100 years, but on what to do today in the face of many important long-term uncertainties. The minute details of any particular scenario for 2100 are then not that important. This can be achieved through an iterative risk-management approach in which uncertain long-term goals are used to develop short-term emission targets. As new information arises, emission scenarios, long-term goals and short-term targets are adjusted as necessary. Analyses would be conducted periodically (every 5–10 years), making it easier to distinguish autonomous trends from policy-induced developments — a major concern of Pielke and colleagues. If actual emissions are carefully monitored and analysed, the true efficacy and costs of past policies would be revealed and estimates of the impact of future policy interventions would be less uncertain.

Such an approach would incorporate recent actions by developed and developing countries. In an 'act then learn' framework, climate policy is altered in response to how businesses change their behaviour in reaction to existing climate policies and in anticipation of future ones. This differs from SRES-like analyses, which ignore the dynamic nature of the decision process and opportunities for mid-course corrections as they compare scenarios without policy with global, century-long plans.

See also:

Long-range energy forecasts are no more than fairy tales

Energy assumptions were reasonable at the time, but not now

Climate policies will stimulate technology development

IPCC’s climate-policy assumptions were justified

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Richels, R., Tol, R. & Yohe, G. Future scenarios for emissions need continual adjustment. Nature 453, 155 (2008) doi:10.1038/453155a

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