Moving targets—cost-effective climate policy under scientific uncertainty
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The IPCC’s fifth assessment report of Working Group III has just come out. It pays special attention to the 2 °C temperature target and tells us that the window of opportunity to prevent such climate change is rapidly closing. Yet, the report also presents a portfolio of stabilization targets, reflecting a fundamental ambiguity: there is no unique “dangerous” climate threshold. Here, we describe a framework for the evaluation of optimal climate policy given an uncertain formal climate threshold. We find that uncertainty leads to moving targets: even when the available information does not change, future regulators will tend to relax current climate plans.
We develop a reduced form integrated assessment model to assess the quantitative significance of our findings. We calibrate preferences such that in 2000 a stabilization target of 450 ppmv maintains the optimal balance between climate risks and abatement costs. The naïve equilibrium ultimately reaches a peak of 570 ppmv, missing the 2000 stabilizations targets by a wide margin. Our results offer an explanation for the inertia in mitigation efforts over the past decades: policies often delay the majority of abatement efforts. Yet, believing that subsequent regulators will uphold the planned future efforts is self-defeating.
KeywordsClimate Policy Carbon Price Integrate Assessment Model Cumulative Emission Stabilization Target
RG wrote the code for the GAMS model and STATA figures. RG and TM both contributed to the final text.
Conflict of interest
The authors declare that they have no competing interests.
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