@article {609026,
	title = {Potentially Large Equilibrium Climate Sensitivity Tail Uncertainty},
	journal = {Economics Letters},
	volume = {168},
	year = {2018},
	month = {May 5, 2018},
	pages = {144-146},
	abstract = {Equilibrium\&nbsp; climate\&nbsp; sensitivity\&nbsp; (ECS),\&nbsp; the\&nbsp; link\&nbsp; between\&nbsp; concentrations\&nbsp; of greenhouse\&nbsp; gases in\&nbsp; the\&nbsp; atmosphere\&nbsp; and\&nbsp; eventual\&nbsp; global\&nbsp; average\&nbsp; tempera- tures, has been persistently and perhaps deeply uncertain.\&nbsp; Its {\textquoteleft}likely{\textquoteright} range has been approximately between 1.5 and 4.5 degrees Centigrade for almost 40 years (Wagner and Weitzman, 2015).\&nbsp; Moreover, Roe and Baker (2007), Weitzman\&nbsp; (2009),\&nbsp; and\&nbsp; others\&nbsp; have\&nbsp; argued\&nbsp; that\&nbsp; its\&nbsp; right-hand\&nbsp; tail\&nbsp; may\&nbsp; be long, {\textquoteleft}fat{\textquoteright} even.\&nbsp; Enter Cox et al. (2018), who use an {\textquoteleft}emergent constraint{\textquoteright} approach to characterize the probability distribution of ECS as having a cen- tral or best estimate of 2.8{\textopenbullet}C with a 66\% confidence interval of 2.2-3.4 {\textopenbullet}C. This\&nbsp; implies,\&nbsp; by\&nbsp; their\&nbsp; calculations,\&nbsp; that\&nbsp; the\&nbsp; probability of\&nbsp; ECS\&nbsp; exceeding 4.5{\textopenbullet}C is less than 1\%.\&nbsp; They characterize such kind of result as {\textquotedblleft}renewing hope that we may yet be able to avoid global warming exceeding 2[{\textopenbullet}C]{\textquotedblright}. We share the desire for less uncertainty around ECS (Weitzman, 2011; Wagner and Weitzman, 2015).\&nbsp; However, we are afraid that the upper-tail emergent constraint on ECS islargely a function of the assumed normal error terms in the regression analysis.\&nbsp; We do not attemptto evaluate Cox et al. (2018){\textquoteright}s physical modeling (aside from the normality assumption), leaving that task to physical scientists.\&nbsp; We take Cox et al. (2018){\textquoteright}s 66\% confidence interval as given and explore the implications of applying alternative probability distri- butions.\&nbsp; We find, for example, that moving from a normal to a log-normal distribution,\&nbsp; while\&nbsp; giving\&nbsp; identical probabilities\&nbsp; for\&nbsp; being\&nbsp; in\&nbsp; the\&nbsp; 2.2-3.4{\textopenbullet}C range, increases the probability of exceeding 4.5{\textopenbullet}C by over five times.\&nbsp; Using instead a fat-tailed Pareto distribution, an admittedly extreme case, increases the probability by over forty times.Keywords: climate change, climate sensitivity, fat tails},
	url = {https://doi.org/10.1016/j.econlet.2018.04.036},
	author = {Gernot Wagner and Martin L. Weitzman}
}
