(a) The frequency distribution of total abatement cost for each of the 384 technology scenarios applied to each of three CO2 targets
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Figure 2. (a) The frequency distribution of total abatement cost for each of the 384 technology scenarios applied to each of three CO2 targets. The incremental abatement cost Cxz for target x is the additional cost relative to a target 50 ppm higher. (b) The broad and narrow distributions for climate sensitivity. (c) Expected temperature along GCAM's reference technology path. Expected temperature under the narrow distribution is nearly identical to that with climate sensitivity fixed at 3 °C. (d) Damages along GCAM's reference technology path using 3 °C climate sensitivity and functions calibrated to give a 1.7% loss from 2.5 °C of warming.
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.