Although concurring with some of the broad conclusions of the challenge chapter, there is no scientific consensus that a limit of 450 ppm of CO2 does not pass a cost-benefit test; in fact, broad political consensus implies that limiting temperature rise to 2°C is cost effective. Neither are annual emissions caps inherently costly, depending on the policy measures used to achieve them. Overall, bringing fossil fuel prices more in line with their social costs necessarily produces greater benefits, which suggests that negotiating higher and converging carbon prices may hold more promise than trying to agree on quantity targets.

Any policy portfolio that focuses primarily on R&D and/or renewable energy deployment is inherently more costly than one that incorporates a fundamental role for carbon pricing, which encourages the use of all abatement options. To avoid costly delay in taking action on mitigation while waiting for international agreement on emissions targets, we should immediately work toward aligning the prices of emitting energy sources with their social costs – removing fossil fuel subsidies, using market-based mechanisms to regulate conventional air pollutants, and pricing carbon.

Galiana cites a few studies, which find that even less-ambitious stabilization targets than 2are unlikely to be cost effective in a conventional analysis. However, models are limited in their ability to estimate the full range of potential costs and do not necessarily take account of possible catastrophic damage caused by uncertain events. Neither can modeling address the ethical and distributional effects of climate change. Environmental justice argues for an even lower target, for example, to protect vulnerable small island states. In contrast to the challenge chapter, the IEA emphasizes the feasibility of meeting climate goals cost effectively with existing technologies.

Bringing down the cost of low-carbon technologies – or even finding breakthrough technologies – has many advantages, particularly in a worldreluctant to take costly measures to reduce emissions. If low-carbon energy sources can become cost-competitive on their own, they will naturally displace fossil-based sources. This is particularly relevant fo r developing countries, which could then decarbonize without sacrificing growth.