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 CO2-e (enforced in 2100) and 550 pm CO2-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 CO2-e requires a dramatic reduction of carbon emissions compared to baseline levels. Mitigation pathways for the 450 CO2-e target are largely overlapping with the 550 CO2-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 CO2 emissions can be compatible with a given long-term target, depending on assumptions about carbon cycle response, non-CO2 and land use CO2 emissions abatement, partly explaining the spread in mitigation costs.
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A target for temperature instead may prove to be a more policy-relevant formulation, but it will require further methodological work on uncertainty and the representation of climate dynamics in integrated assessment models.
Following the approach in van Vuuren et al. (2011b), we exclude three agents whose forcing is more speculative and often treated exogenously in the models: nitrate aerosols, mineral dust aerosols, and land use albedo changes (Abbreviated 3A). The median estimate for 3A forcing in IPCC (2007) is −0.3 W/m2. We refer to the total controlled anthropogenic forcing in this study as AN(ot)3A forcing. See Rose et al. (this issue) for more details.
This formulation is based on the suggestion in Frankel (2007), in which negotiated reductions relative to a base year were translated into reductions from an expected baseline.
In both cases a discount rate of 5 % was used ex post, though it should be noted that individual models may use different rates and treat time preferences differently.
Note that in all cases the price rises significantly over time, so that high future levels are suppressed by reporting only the time-average price.
Models generally assume that the land use and non-CO2 sources are characterized by a limited amount of low cost mitigation options followed by steeply rising marginal costs of further emissions reductions (Figure S6). Once these options are exhausted, additional reductions must occur in the energy sector, whose marginal abatement cost curve is less convex. Further research on non-energy abatement options could change this characterization.
See Krey et al. (this issue) for further discussion on the impact of technology availability on costs.
One consequence of the bottom-up formulation is that model differences in relative growth rates across regions are emphasized in these scenarios, revealing an important dimension of variation and uncertainty that tends not to emerge as clearly with a harmonized target.
Only a few models represent energy-intensive industries and trade in non-energy goods with sufficient disaggregation to directly assess the potential extent of “off-shoring”.
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The authors are grateful to three anonymous reviewers for their insightful comments. The contributions of Elmar Kriegler and Massimo Tavoni to this research was supported by funding from the European Union’s Seventh Framework Programme FP7/2011 under grant agreement no. 282846 (LIMITS).
This article is part of the Special Issue on “The EMF27 Study on Global Technology and Climate Policy Strategies” edited by John Weyant, Elmar Kriegler, Geoffrey Blanford, Volker Krey, Jae Edmonds, Keywan Riahi, Richard Richels, and Massimo Tavoni.
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Blanford, G.J., Kriegler, E. & Tavoni, M. Harmonization vs. fragmentation: overview of climate policy scenarios in EMF27. Climatic Change 123, 383–396 (2014). https://doi.org/10.1007/s10584-013-0951-9
- Emission Reduction
- Climate Policy
- Carbon Price
- Policy Scenario
- Negative Emission