Harmonization vs. fragmentation: overview of climate policy scenarios in EMF27

This paper synthesizes results of the multi-model Energy Modeling Forum 27 (EMF27) with a focus on climate policy scenarios. The study included two harmonized long-term climate targets of 450 ppm CO 2 -e (enforced in 2100) and 550 pm CO 2 -e (not-to-exceed) as well as two more fragmented policies based on national and regional emissions targets. Stabilizing atmospheric GHG concentrations at 450 and 550 ppm CO 2 -e requires a dramatic reduction of carbon emissions compared to baseline levels. Mitigation pathways for the 450 CO 2 -e target are largely overlapping with the 550 CO 2 -e pathways in the first half of the century, and the lower level is achieved through rapid reductions in atmospheric concentrations in the second half of the century aided by negative anthropogenic carbon flows. A fragmented scenario designed to extrapolate current levels of ambition into the future falls short of the emissions reductions required under the harmonized targets. In a more aggressive scenario intended to capture a break from observed levels of stringency, emissions are still somewhat higher in the second half due to unabated emissions from non-participating countries, emphasizing that a phase-out of global emissions in the long term can only be reached with full global participation. A key finding is that a large range of energy-related CO 2 emissions can be compatible with a given long-term target, depending on assumptions about carbon cycle response, non-CO 2 and land use CO 2 emissions abatement, partly explaining the spread in mitigation costs. Copyright Springer Science+Business Media Dordrecht 2014