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Equivalence of greenhouse-gas emissions for peak temperature limits


Climate policies address emissions of many greenhouse gases including carbon dioxide, methane, nitrous oxide and various halogen-containing compounds. These are aggregated and traded on a CO2-equivalent basis using the 100-year global warming potential (GWP100); however, the GWP100 has received scientific and economic criticism as a tool for policy1,2,3,4. In particular, given international agreement to limit global average warming to 2 °C, the GWP100 does not measure temperature and does not clearly signal the need to limit cumulative CO2 emissions5,6,7. Here, we show that future peak temperature is constrained by cumulative emissions of several long-lived gases (including CO2 and N2O) and emission rates of a separate basket of shorter-lived species (including CH4). For each basket we develop an emissions-equivalence metric allowing peak temperature to be estimated directly for any emissions scenario. Today’s emissions of shorter-lived species have a lesser impact on ultimate peak temperature than those nearer the time of peaking. The 2 °C limit could therefore be met by setting a limit to cumulative long-lived CO2-equivalent emissions while setting a maximum future rate for shorter-lived emissions.

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Figure 1: Time evolution of global average temperature rise for several GHGs under constant emission rates.
Figure 2: Correlation between peak temperature rise and cumulative emissions (r2Tmax, ΣE)) versus correlation between peak temperature rise and maximum emission rate (r2Tmax, Emax)) for key GHGs.
Figure 3: PCT- and SET-estimated temperature change for GHGs in the RCP2.6 scenario (ΔTM) compared with realized GHG warming simulated by MAGICC (black lines).
Figure 4: MAGICC-modelled contributions to ΔTmax in the RCP2.6 scenario from emissions of GHGs, black carbon and tropospheric ozone (trop. O3) precursors in each decade.


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We thank K. Shine and D. Shindell for discussions during the progress of this research. We are grateful to T. Wigley and the other model developers for making MAGICC freely available. S.M.S. was supported by the UK Committee on Climate Change (the views expressed herein represent those of the authors and do not necessarily represent the views of the committee). J.A.L. and L.K.G. were supported by the joint Department of Energy and Climate Change/Department for Environment, Food and Rural Affairs Met Office Hadley Centre Climate Programme (GA01101). N.H.A.B. was supported by a Natural Environment Research Council CASE studentship with the Met Office. M.R.A. received additional support from the US National Oceanic and Atmospheric Administration and the US Department of Energy through the International Detection and Attribution Group. C.H. was supported by the Centre for Ecology & Hydrology science budget fund.

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S.M.S. designed the research with input from J.A.L. Decadal emissions plots were devised by N.A.H.B., C.H. and M.R.A. S.M.S. carried out all simulations with advice on model setup from L.K.G. All authors contributed to writing the paper.

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Correspondence to Stephen M. Smith.

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Smith, S., Lowe, J., Bowerman, N. et al. Equivalence of greenhouse-gas emissions for peak temperature limits. Nature Clim Change 2, 535–538 (2012).

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