The relationship between short-term emissions and long-term concentration targets

The relationship between long-term climate goals and short/medium-term emission targets forms crucial information for the design of international climate policy. Since IPCC's 4th Assessment Report (AR4), a large number of new scenario studies have been published. This paper reviews this new literature and finds that there is more flexibility in the timing of short-term emission reductions compared to the earlier scenarios assessed by the AR4. For instance, the current literature suggests that a peak of emissions in 2020 and even 2030 would be consistent with limiting temperature change to about 2A degrees C in the long term. The timing when emissions peak depends on whether negative emissions in the long-term can be achieved. The recent scenarios further indicate that global emissions by 2050 should be 40-80% below 2000 levels. Above all, the paper argues that there is no clear, single law that would directly determine the required emissions levels in 2020, but that instead policy-makers need to consider trade-offs between the likelihood of achieving long-term targets, the short-term costs, and their expectation with respect to future technologies (and their possible failure). The higher flexibility might be important in finding acceptable agreements on international climate policy.