Abstract

Many nationally determined contributions (NDCs) under the Paris Agreement follow the established practice of specifying emissions levels in tonnes of CO2 equivalent emissions. The Global Warming Potential (GWP) is the emissions metric used most often to aggregate contributions from different greenhouse gases (GHGs). However, the climate impact of pathways expressed in this way is known to be ambiguous. For this reason, alternatives have been proposed but the ambiguity has not been quantified in the context of the Paris Agreement. Here we assess the variation in temperature using pathways consistent with the ambition of limiting temperature increases to well below 2 °C. These are taken from the IPCC Special Report on Global Warming of 1.5 °C (SR15). The CO2 emission levels are adjusted so that the pathways all have the same total CO2 equivalent emissions for a given emissions metric but have different proportions of short-lived and long-lived pollutants. We show that this difference affects projections by up to 0.17 °C when GWP100 is used. Options of reducing this ambiguity include using a different emissions metric or adding supplementary information in NDCs about the emissions levels of individual GHGs. We suggest the latter on the grounds of simplicity and because it does not require agreement on the use of a different emissions metric.

Highlights

  • Country-level contributions to the Paris Agreement are in the form of nationally determined contributions (NDCs) that typically contain quantified emission reduction commitments

  • The CO2 emission levels are adjusted so that the pathways all have the same total CO2 equivalent emissions for a given emissions metric but have different proportions of short-lived and long-lived pollutants. We show that this difference affects projections by up to 0.17 °C when Global Warming Potential measured over 100 years (GWP100) is used

  • Participants’ obligations to limit aggregate emissions of six key pollutants are quantified using a specified metric, Global Warming Potential measured over 100 years (GWP100)

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Summary

Introduction

Country-level contributions to the Paris Agreement are in the form of nationally determined contributions (NDCs) that typically contain quantified emission reduction commitments. There is extensive literature covering the limitations of metrics (Fuglestvedt et al 2003, Forster et al 2007, Shine et al 2007, Jenkins et al 2018) and GWP in particular (Shine et al 2005, Ocko et al 2017, Allen et al 2018). This motivates the question as to whether the use of metrics, especially GWP100, serves the purpose of the Country

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