A solution to the misrepresentations of CO2-equivalent emissions of short-lived climate pollutants under ambitious mitigation

Myles R Allen,Adrian H Macey,David J Frame,Richard J Millar,Michelle Cain,Keith P Shine,Jan S Fuglestvedt

doi:10.1038/s41612-018-0026-8

Abstract

While cumulative carbon dioxide (CO2) emissions dominate anthropogenic warming over centuries, temperatures over the coming decades are also strongly affected by short-lived climate pollutants (SLCPs), complicating the estimation of cumulative emission budgets for ambitious mitigation goals. Using conventional Global Warming Potentials (GWPs) to convert SLCPs to “CO2-equivalent” emissions misrepresents their impact on global temperature. Here we show that peak warming under a range of mitigation scenarios is determined by a linear combination of cumulative CO2 emissions to the time of peak warming and non-CO2 radiative forcing immediately prior to that time. This may be understood by expressing aggregate non-CO2 forcing as cumulative CO2 forcing-equivalent (CO2-fe) emissions. We show further that contributions to CO2-fe emissions are well approximated by a new usage of GWP, denoted GWP*, which relates cumulative CO2 emissions to date with the current rate of emission of SLCPs. GWP* accurately indicates the impact of emissions of both long-lived and short-lived pollutants on radiative forcing and temperatures over a wide range of timescales, including under ambitious mitigation when conventional GWPs fail. Measured by GWP*, implementing the Paris Agreement would reduce the expected rate of warming in 2030 by 28% relative to a No Policy scenario. Expressing mitigation efforts in terms of their impact on future cumulative emissions aggregated using GWP* would relate them directly to contributions to future warming, better informing both burden-sharing discussions and long-term policies and measures in pursuit of ambitious global temperature goals.

Highlights

The Paris Agreement introduced a regular (5-yearly) “stocktake” of collective progress towards achieving its long-term temperature goals, but the metrics of progress to be used in these stocktakes remain under discussion.[1,2,3,4,5,6,7,8,9] Relating emissions to future temperatures remains ambiguous as long as contributions are expressed, as in the majority of “Nationally Determined Contributions” (NDCs), in terms of CO2-equivalent (CO2-e) emission rates in a specific year defined using a metric such as the 100-year Global Warming Potential (GWP100)
Remaining discrepancies likely result from different temperature responses to different forcings in the MAGICC model and the transition from concentrations-driven to emissions-driven integration after 2005.23 Figure 1d shows that, in multi-gas scenarios, temperatures stabilise when and only when the annual net rate of total anthropogenic CO2 forcing-equivalent (CO2-fe) emissions reaches zero, which is not the case for either CO2 emissions alone or CO2-e emissions computed using Global Warming Potentials (GWPs)
CO2-fe emissions depend on knowledge of the full scenario history and must be computed using a carbon cycle model, and so would be difficult to use directly as an emission metric

Summary

Introduction

The Paris Agreement introduced a regular (5-yearly) “stocktake” of collective progress towards achieving its long-term temperature goals, but the metrics of progress to be used in these stocktakes remain under discussion.[1,2,3,4,5,6,7,8,9] Relating emissions to future temperatures remains ambiguous as long as contributions are expressed, as in the majority of “Nationally Determined Contributions” (NDCs), in terms of CO2-equivalent (CO2-e) emission rates in a specific year defined using a metric such as the 100-year Global Warming Potential (GWP100). The two variables are positively correlated, but peak warming depends on scenario-dependent assumptions about emissions after 2030. Most of these scenarios extrapolate the implications of short-term commitments using some notion of “sustained ambition”, interpreted in a number of ways, including[13] an effective global carbon price increasing at a constant exponential rate over the 21st century.

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Conclusion