Impact of methane and black carbon mitigation on forcing and temperature: a multi-model scenario analysis

Abstract

The relatively short atmospheric lifetimes of methane (CH4) and black carbon (BC) have focused attention on the potential for reducing anthropogenic climate change by reducing Short-Lived Climate Forcer (SLCF) emissions. This paper examines radiative forcing and global mean temperature results from the Energy Modeling Forum (EMF)-30 multi-model suite of scenarios addressing CH4 and BC mitigation, the two major short-lived climate forcers. Central estimates of temperature reductions in 2040 from an idealized scenario focused on reductions in methane and black carbon emissions ranged from 0.18–0.26 °C across the nine participating models. Reductions in methane emissions drive 60% or more of these temperature reductions by 2040, although the methane impact also depends on auxiliary reductions that depend on the economic structure of the model. Climate model parameter uncertainty has a large impact on results, with SLCF reductions resulting in as much as 0.3–0.7 °C by 2040. We find that the substantial overlap between a SLCF-focused policy and a stringent and comprehensive climate policy that reduces greenhouse gas emissions means that additional SLCF emission reductions result in, at most, a small additional benefit of ~ 0.1 °C in the 2030–2040 time frame.

The Energy Modeling Forum (EMF)-30 study on short-lived climate forcers: introduction and overview

“…Global mean temperature change is often used as a metric to examine the effectiveness of climate mitigation policies, and we also do so here, recognizing that other metrics such as changes in regional climate, precipitation, and extremes can be important for specific impacts. Global temperature is reduced by 0.18-0.26°C in 2050 (0.11-0.23°C in 2040) by the SLCF (CH4 plus BC)-focused reductions examined in the EMF-30 scenarios (Smith et al 2020), with similar results in Harmsen et al (2019b). Harmsen et al (2019b) also find that SLCF reductions can have a somewhat larger relative reduction (up to 15%) on the rate of temperature change.…”

“…These can either enhance (additional forcing reduction) or offset the primary effect. In general, auxiliary changes were offsetting for BCOC-EndU (reduced cooling), enhancing for CH4-Only (additional forcing reduction), and offsetting for ClimPolicy (see Smith et al 2020 Figure S17). These results emphasize the importance of considering all emitted species in a consistent manner when analyzing emission reduction scenarios.…”

“…The EMF-30 scenarios also revealed that model structure plays a role in determining the magnitude of auxiliary reductions from a methane reduction policy. For the set of models that pass a methane price to methane-producing technologies, additional auxiliary reductions occur as economic activities shift to less methane-intensive activities (Smith et al 2020).…”

“…There were nine models participating in the EMF-30 exercise: AIM/CGE, DNE21+v.14, ENV-Linkages, GCAM4, IMAGE, MESSAGE-GLOBIOM, POLES, REMIND, and WITCH (see also Table S1.1 in Harmsen et al 2019a and supplemental section B in Smith et al 2020). These models span a range of computational methodologies and model structures.…”

“…Because, by definition, SLCFs have a relatively short atmospheric lifetimes, there has been a growing interest in developing strategies to reduce their emissions and consequently mitigate near-term climate change. See Harmsen et al (2019bHarmsen et al ( , 2020 and Smith et al (2020) in this issue for literature review and discussion of previous work. Many of the SLCFs are also air pollutants or air pollution precursors so reducing SLCF emissions will generally lead to reductions in air pollutant-related mortality and economic impacts, as also examined in this special issue.…”