The damages from 2.5 °C of warming (as a percentage of global output) that would equalize the additional abatement cost Cxz and expected climate benefits E[Bxzd|a] from adopting CO2 target x instead of one 50 ppm higher, all calculated with a 5% consumption discount rate
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Figure 3. The damages from 2.5 °C of warming (as a percentage of global output) that would equalize the additional abatement cost Cxz and expected climate benefits E[Bxzd|a] from adopting CO2 target x instead of one 50 ppm higher, all calculated with a 5% consumption discount rate. The range of technology scenarios (n = 384) is represented by each box (median and interquartile range) and its whiskers (minimum and maximum). Comparing across columns within a group reveals the effect of changing the distribution for climate sensitivity, comparing across groups reveals the effect of changing the damage function, and comparing across plots reveals the effect of changing the CO2 target. The darker shaded region indicates output losses within one standard deviation (σ) of the average best estimate (μ) summarized in , and the lighter shaded region indicates those losses within two standard deviations. (a) 450 ppm CO2 target (versus 500 ppm). (b) 500 ppm CO2 target (versus 550 ppm).
Climate change policies must trade off uncertainties about future warming, about the social and ecological impacts of warming, and about the cost of reducing greenhouse gas emissions. We show that laxer carbon targets produce broader distributions for climate damages, skewed towards severe outcomes. However, if potential low-carbon technologies fill overlapping niches, then more stringent carbon targets produce broader distributions for the cost of reducing emissions, skewed towards high-cost outcomes. We use the technology-rich GCAM integrated assessment model to assess the robustness of 450 and 500 ppm carbon targets to each uncertain factor. The 500 ppm target provides net benefits across a broad range of futures. The 450 ppm target provides net benefits only when impacts are greater than conventionally assumed, when multiple technological breakthroughs lower the cost of abatement, or when evaluated with a low discount rate. Policy evaluations are more sensitive to uncertainty about abatement technology and impacts than to uncertainty about warming.