Inconsistencies when applying novel metrics for emissions accounting to the Paris agreement

Alexander Nauels,William Hare,Joeri Rogelj,Michiel Schaeffer,Carl-Friedrich Schleussner

doi:10.1088/1748-9326/ab56e7

Abstract

Addressing emissions of non-CO2 greenhouse gases (GHGs) is an integral part of efficient climate change mitigation and therefore an essential part of climate policy. Metrics are used to aggregate and compare emissions of short- and long-lived GHGs and need to account for the difference in both magnitude and persistence of their climatic effects. Different metrics describe different approaches and perspectives, and hence yield different numerical estimates for aggregated GHG emissions. When interpreting GHG emission reduction targets, being mindful of the underlying metrical choices thus proves to be essential. Here we present the impact a recently proposed GHG metric related to the concept of CO2 forcing-equivalent emissions (called GWP*) would have on the internal consistency and environmental integrity of the Paris Agreement. We show that interpreting the Paris Agreement goals in a metric like GWP* that is significantly different from the standard metric used in the IPCC Fifth Assessment Report can lead to profound inconsistencies in the mitigation architecture of the Agreement. It could even undermine the integrity of the Agreement’s mitigation target altogether by failing to deliver net-zero CO2 emissions and therewith failing to ensure warming is halted. Our results indicate that great care needs to be taken when applying new concepts that appear scientifically favourable to a pre-existing climate policy context.

Highlights

To achieve the climate targets expressed in the 2015 Paris Agreement (UNFCCC 2015a), stringent reductions in all greenhouse gas (GHGs) emissions are required (IPCC 2018)
We show that interpreting the Paris Agreement goals in a metric like global warming potentials’ (GWPs)* that is significantly different from the standard metric used in the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report can lead to profound inconsistencies in the mitigation architecture of the Agreement
The use of GWP* instead of GWP with a 100 year time horizon (GWP100) leads to substantial differences in the accounting of greenhouse gases (GHGs) emissions, the timing of net-zero GHG emissions, and the global mean temperature (GMT) trajectories resulting from achieving Article 4 of the Paris Agreement

Summary

Introduction

To achieve the climate targets expressed in the 2015 Paris Agreement (UNFCCC 2015a), stringent reductions in all greenhouse gas (GHGs) emissions are required (IPCC 2018). Different metrics are used to account for and compare the contributions of different GHGs. Different metrics are used to account for and compare the contributions of different GHGs They typically provide conversion factors between the emissions of non-CO2 GHGs and equivalent emissions of CO2, often over a chosen set time horizon. The most common approaches to account for different GHGs are using forcing centred metrics known as ‘global warming potentials’ (GWPs). GWPs express the ratio of the time-integrated radiative forcing effect of a pulse emission of a certain GHG relative to the effect of a pulse emission of an equal mass of CO2 (Myhre et al 2013). Metrics focussing directly on the temperature effects like ‘global temperature potentials’ (GTPs) have been proposed (Shine et al 2005). There are significant uncertainties related to both GWPs and GTPs, the relative uncertainties are larger for GTPs (Myhre et al 2013)

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Results

Conclusion