Significant climate benefits from near-term climate forcer mitigation in spite of aerosol reductions

Robert J Allen,Philippe Le Sager,Shinichiro Fujimori,Vaishali Naik,Larry W Horowitz,Fiona M O'Connor,Twan Van Noije,William J Collins,Steven Turnock,Sungbo Shim,Lori T Sentman,Conor Broderick,Makoto Deushi,Gerd A Folberth,Susanne E Bauer,Kostas Tsigaridis,Naga Oshima,Jasmin G John

doi:10.1088/1748-9326/abe06b

Abstract

Near-term climate forcers (NTCFs), including aerosols and chemically reactive gases such as tropospheric ozone and methane, offer a potential way to mitigate climate change and improve air quality—so called ‘win-win’ mitigation policies. Prior studies support improved air quality under NTCF mitigation, but with conflicting climate impacts that range from a significant reduction in the rate of global warming to only a modest impact. Here, we use state-of-the-art chemistry-climate model simulations conducted as part of the Aerosol and Chemistry Model Intercomparison Project (AerChemMIP) to quantify the 21st-century impact of NTCF reductions, using a realistic future emission scenario with a consistent air quality policy. Non-methane NTCF (NMNTCF; aerosols and ozone precursors) mitigation improves air quality, but leads to significant increases in global mean precipitation of 1.3% by mid-century and 1.4% by end-of-the-century, and corresponding surface warming of 0.23 and 0.21 K. NTCF (all-NTCF; including methane) mitigation further improves air quality, with larger reductions of up to 45% for ozone pollution, while offsetting half of the wetting by mid-century (0.7% increase) and all the wetting by end-of-the-century (non-significant 0.1% increase) and leading to surface cooling of −0.15 K by mid-century and −0.50 K by end-of-the-century. This suggests that methane mitigation offsets warming induced from reductions in NMNTCFs, while also leading to net improvements in air quality.

Highlights

Near-term climate forcers (NTCFs), known as short-lived climate forcers (SLCFs), include aerosols such as sulfate, nitrate, organic carbon (OC) and black carbon (BC) and chemically reactive gases including ozone (O3), sulfur dioxide and methane (CH4)
Methane reductions offset Non-methane NTCF (NMNTCF) wetting by end-of-the-century, only about half of the wetting is offset by mid-century
The large Arctic warming under NMNTCF mitigation is countered under all-NTCF mitigation— by end-of-the-century

Summary

Introduction

Near-term climate forcers (NTCFs), known as short-lived climate forcers (SLCFs), include aerosols such as sulfate, nitrate, organic carbon (OC) and black carbon (BC) and chemically reactive gases including ozone (O3), sulfur dioxide and methane (CH4). NTCFs affect the radiative balance of the Earth, leading to a climate forcing nearly equal in magnitude to that of CO2 (Myhre et al 2013, Shindell et al 2013) This forcing drives climate perturbations, including surface warming (CH4, BC, some OC components, O3) or cooling (sulfate, nitrate, OC), as well as altered precipitation patterns (Allen 2015, Allen et al 2015, Rotstayn et al 2015b, Liu et al 2018). NTCF mitigation is of particular importance to both the United Nation’s Sustainable Development Goals (Haines et al 2017, Lelieveld 2017, Shindell et al 2017), as well as the Paris Agreement, which strives to keep global mean surface temperature to well below 2 ◦C above preindustrial values (IPCC 2018) Policies that combine both climate and air pollution mitigation (‘win-win’ policies) have clear societal and economic benefits compared to separate mitigation (Clarke et al 2014).

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Results

Conclusion