Improved calculation of warming-equivalent emissions for short-lived climate pollutants

Michelle Cain,John Lynch,David J Frame,Adrian H Macey,Myles R Allen,Jan S Fuglestvedt

doi:10.1038/s41612-019-0086-4

Abstract

Anthropogenic global warming at a given time is largely determined by the cumulative total emissions (or stock) of long-lived climate pollutants (LLCPs), predominantly carbon dioxide (CO2), and the emission rates (or flow) of short-lived climate pollutants (SLCPs) immediately prior to that time. Under the United Nations Framework Convention on Climate Change (UNFCCC), reporting of greenhouse gas emissions has been standardised in terms of CO2-equivalent (CO2-e) emissions using Global Warming Potentials (GWP) over 100-years, but the conventional usage of GWP does not adequately capture the different behaviours of LLCPs and SLCPs, or their impact on global mean surface temperature. An alternative usage of GWP, denoted GWP*, overcomes this problem by equating an increase in the emission rate of an SLCP with a one-off “pulse” emission of CO2. We show that this approach, while an improvement on the conventional usage, slightly underestimates the impact of recent increases in SLCP emissions on current rates of warming because the climate does not respond instantaneously to radiative forcing. We resolve this with a modification of the GWP* definition, which incorporates a term for each of the short-timescale and long-timescale climate responses to changes in radiative forcing. The amended version allows “CO2-warming-equivalent” (CO2-we) emissions to be calculated directly from reported emissions. Thus SLCPs can be incorporated directly into carbon budgets consistent with long-term temperature goals, because every unit of CO2-we emitted generates approximately the same amount of warming, whether it is emitted as a SLCP or a LLCP. This is not the case for conventionally derived CO2-e.

Highlights

Comprehensive climate policies must appraise a range of greenhouse gases and aerosols, which can differ significantly in their radiative efficiencies and atmospheric lifespans, and the nature of their climate impacts.[1]
CO2-we pulse corresponding to a change in short-lived climate pollutants (SLCPs) emission rate over Δt
The rate term approximates the shortchange in the emission rate of an SLCP as equivalent to a single timescale climate response to a change in radiative forcing; the emissions pulse of a long-lived pollutant

Summary

Introduction

Comprehensive climate policies must appraise a range of greenhouse gases and aerosols, which can differ significantly in their radiative efficiencies and atmospheric lifespans, and the nature of their climate impacts.[1]. Despite its prevalence in the UNFCCC and national climate policies, GWP has received criticism,[2,3,4] not least that it cannot be used to appraise temperature-related goals,[5] and other equivalence metrics have been proposed.[6,7,8,9] Shine[3] notes that strong caveats were in place when GWP was introduced in the Intergovernmental Panel on Climate Change’s First Assessment Report10: “It must be stressed that there is no universally accepted methodology for combining all the relevant factors into a single [metric]. In AR4, the GWPs were the recommended metric to compare the effects of long-lived greenhouse gases,[11] and AR5 values of GWP100 have been adopted for emissions reporting

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Conclusion