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2020 emissions levels required to limit warming to below 2 °C


This paper presents a systematic scenario analysis of how different levels of short-term 2020 emissions would impact the technological and economic feasibility of achieving the 2 °C target in the long term. We find that although a relatively wide range of emissions in 2020—from 41 to 55 billion tons of carbon dioxide equivalent (Gt CO2e yr−1)—may preserve the option of meeting a 2 °C target, the size of this ‘feasibility window’ strongly depends on the prospects of key energy technologies, and in particular on the effectiveness of efficiency measures to limit the growth of energy demand. A shortfall of critical technologies—either for technological or socio-political reasons—would narrow the feasibility window, if not close it entirely. Targeting lower 2020 emissions levels of 41–47 Gt CO2e yr−1 would allow the 2 °C target to be achieved under a wide range of assumptions, and thus help to hedge against the risks of long-term uncertainties.

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Figure 1: Schematic representation of the two-stage model set-up to quantify the feasible 2020 emission windows to stay below 2 °C.
Figure 2: Feasibility windows for global GHG emissions in 2020 required to limit global temperature increase to below 2 °C relative to pre-industrial levels.
Figure 3: Characteristics of all feasible 2 °C scenarios with intermediate energy demand.


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We thank V. Krey, P. Kolp, M. Strubegger and A. Reisinger for their support in developing the model set-up and extracting the results, and A. Grubler and V. Krey for their constructive feedback on the analysis. J.R. was supported by the Swiss National Science Foundation (project 200021-135067) and the IIASA Young Scientists Summer Program 2011. K.R. and D.L.M. greatly acknowledge financial support from the EU-FP7 project AMPERE (FP7-265139).

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All authors were involved in designing the research; J.R. performed the research in close collaboration with D.L.M.; all authors contributed to writing the paper.

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Correspondence to Joeri Rogelj.

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The authors declare no competing financial interests.

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Rogelj, J., McCollum, D., O’Neill, B. et al. 2020 emissions levels required to limit warming to below 2 °C. Nature Clim Change 3, 405–412 (2013).

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