<p><strong>Figure 1.</strong> (a) Warming. (b) Damages 100 years after step forcing. (c) Abatement cost. Normally distributed uncertainty about equilibrium feedbacks and about an aggregate technology index translate into distributions for damages and abatement cost that are skewed towards undesirable outcomes. The feedback distribution follows [<a href="http://iopscience.iop.org/1748-9326/8/3/034019/article#erl475796bib13" target="_blank">13</a>] in using normally distributed equilibrium feedbacks with a mean of 0.65 and a standard deviation of 0.13. The economic loss coefficient <em>a</em> in the damage calculations has 2.5 °C of warming reducing output by 1.7% under the quadratic specification [<a href="http://iopscience.iop.org/1748-9326/8/3/034019/article#erl475796bib19" target="_blank">19</a>]. The abatement cost function assumes constant elasticity of −0.5 with respect to technology (i.e., achieving 1% more breakthroughs lowers abatement cost by 0.5%) and is normalized to the 650 ppm target's no-breakthrough scenario. The cost of each CO<sub>2</sub> target under the no-breakthrough scenario comes from the Global Change Assessment Model 3.0.</p> <p><strong>Abstract</strong></p> <p>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.</p
