Many governments and commentators in recent years have focused on the development of interim mid-century emissions reductions goals, contending that this would preserve long-term options in terms of avoiding temperature increases beyond 2C pre-industrial, provide more guidance to policymakers who engage in multidecadal planning, and provide more guidance on emissions paths for the next few decades than long-term goals. A new study in PNAS, O’Neill, et al., Mitigation implications of midcentury targets that preserve long-term climate options, 107 PNAS 1011-1016 (2010) (open access) seeks to assess the implications of potential interim objectives and long-term outcomes. As such, it could be an excellent reading for students in discussing the timing of emissions reductions and its policy implications. Also, this could be a good reading for students engaged in simulated negotiations for a successor to the Kyoto Protocol

Among the key takeaways from the study:

1. It may be possible that global emissions could be higher in 2050 than predicted by the IPCC and still achieve the goal of stabilizing atmospheric concentrations of carbon dioxide at 495-535ppm. Whereas the IPCC concluded that emissions would have to be 30-60% below 2000 levels to effectuate this objective, this study concludes that it might be possible to meet the target by only reducing emissions by 20% below 2000 levels, though the study also concludes that this would not be cost optimal;
2. To have a 50% chance of limiting warming to 2C would require limiting annual emissions to between 5.5 and 6.5 gigatons of carbon dioxide in 2050, under a least-cost scenario, using the IPCC’s B2 baseline scenario. If emissions were 7 gigatons, however, it would become infeasible to have a 50% chance of exceeding 2C by 2100. The researchers noted, however, that IPCC scenario assumptions are crucial however. For example, should the A2r scenario play out (high energy demand/low technology diffusion), avoiding temperature increases above 2C with a 50% probability becomes only virtually feasible, and would require reductions of emissions by more than 50% by 2050;
3. Energy systems costs for a wide range of emissons (between 7-10 gigatons of carbon dioxide annually) are roughly similar, suggesting that it’s possible to reduce emissions below some potentially critical thresholds for little or no cost. However, costs do rise substantially once emissions are reduced below 7 gigatons of carbon dioxide annually.

I think this article could stir some lively discussion on how climate policymaking should be conducted in the face of scientific and socio-economic uncertainty. The role of risk assessment and management is also germane here. Should we, for example, seek to reduce emissions to certain levels if this still poses a 50% threshold of exceeding 2C in 2100? Should we be seeking to reduce that risk to 25%, 5%? What are the implications of the principles of intergenerational equity or the precautionary principle? What the are the tradeoffs that this generation may have to take in seeking to effectuate deeper cuts in emissions?

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