Chapter 8: Some Dynamic Economic Consequences of the Climate-Sensitivity Inference Dilemma
Martin L. Weitzman 1 INTRODUCTION Equilibrium climate sensitivity is a key parameter that serves as a very useful macro-indicator of the eventual aggregate response of temperature change to the aggregate level of greenhouse gases (GHGs). Decades of scientific research have failed to constrain the upper range of climate sensitivity, which means that alarmingly high distant-future temperature responses are not excluded. This chapter highlights a generic statisticalinference mechanism that makes it difficult to thin down to zero inherently fat-tailed probability estimates of rare extreme outcomes. Let D ln CO2 be sustained relative change in concentrations of atmospheric carbon dioxide while DT is equilibrium mean global surface temperature response. Equilibrium climate sensitivity (here denoted l) is a benchmark amplifying or scaling multiplier for converting D ln CO2 into DT by the (reasonably accurate) linear approximation DT < (l / ln 2) 3 D ln CO2. As the Intergovernmental Panel on Climate Change in its IPCC-AR4 (2007) Executive Summary phrases it: The equilibrium climate sensitivity is a measure of the climate system response to sustained radiative forcing. It is not a projection but is defined as the global average surface warming following a doubling of carbon dioxide concentrations. It is likely to be in the range 2 to 4.5° C with a best estimate of 3° C, and is very unlikely to be less than 1.5° C. Values substantially higher than 4.5° C cannot be excluded, but agreement of models with observations is not as good for those values (italics in original). In this...
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