Increased importance of methane reduction for a 1.5 degree target

Abstract

To understand the importance of methane on the levels of carbon emission reductions required to achieve temperature goals, a processed-based approach is necessary rather than reliance on the transient climate response to emissions. We show that plausible levels of methane (CH 4 ) mitigation can make a substantial difference to the feasibility of achieving the Paris climate targets through increasing the allowable carbon emissions. This benefit is enhanced by the indirect effects of CH 4 on ozone (O 3 ). Here the differing effects of CH 4 and CO 2 on land carbon storage, including the effects of surface O 3 , lead to an additional increase in the allowable carbon emissions with CH 4 mitigation. We find a simple robust relationship between the change in the 2100 CH 4 concentration and the extra allowable cumulative carbon emissions between now and 2100 (0.27 ± 0.05 GtC per ppb CH 4 ). This relationship is independent of modelled climate sensitivity and precise temperature target, although later mitigation of CH 4 reduces its value and thus methane reduction effectiveness. Up to 12% of this increase in allowable emissions is due to the effect of surface ozone. We conclude early mitigation of CH 4 emissions would significantly increase the feasibility of stabilising global warming below 1.5 • C, alongside having co-benefits for human and ecosystem health. ORCID iDsWilliam J Collins https://orcid.org/0000-0002-7419-0850

Demonstrating GWP*: a means of reporting warming-equivalent emissions that captures the contrasting impacts of short- and long-lived climate pollutants

“…decline noted above suggests cooling relative to the current temperature ( figure 5(b), see [16] for discussion of the physical origin and uncertainty in this rate of decline). This is broadly the role of methane in ambitious mitigation pathways, where significant, permanent, reductions in methane emission rates can permit the emissions of a fixed amount of extra CO 2 , and hence the additional long-term warming it will cause, under a given temperature ceiling [35,36]. This concept is explored further below when demonstrating the use of GWP * to assess alternative mitigations.…”

“…The 'stock' component, respresenting the longterm temperature adjustment to methane emissions, is still important, and the scale of contemporary methane emissions is such that this element could equate to a significant increase in total allowable CO 2 emissions for warming of 1.5°C if action is taken on methane earlier rather than later [35], but by a much smaller amount than the direct GWP 100 equivalence between methane and CO 2 would suggest. Figure 8.…”

Stable climate metrics for emissions of short and long-lived species—combining steps and pulses

“…This concept was introduced into metrics in Allen et al (2016) where they used a single number (GWP X (100)×100) to equate permanent step changes in SLCF emissions to a one-off pulse emission of CO 2 . Collins et al (2018) calculated the impact of methane mitigation on allowable carbon budgets. Using a simple climate model coupled to a carbon cycle model, they showed that for a fixed temperature target, a step reduction in methane emissions of 1 Gt(CH 4 ) yr −1 was approximately equivalent to an increase in allowed cumulative CO 2 emissions of 2900-3300 Gt(CO 2 ).…”

UKESM1: Description and Evaluation of the U.K. Earth System Model

“…Primary among these are estimates of available carbon emission budgets commensurate with limiting global warming below specific targets (Matthews et al, 2017). Introducing prognostic atmospheric chemistry into GC3.1 allows an assessment of the mitigation potential from reducing short lived climate forcers such as methane, black carbon, and tropospheric ozone (Collins et al, 2018;Stohl et al, 2015), while coupling atmospheric chemistry to terrestrial biogeochemistry opens up the potential to study the risk of permafrost thaw, methane release, and their impacts on Arctic warming. A full atmosphere treatment of ozone chemistry supports a joined-up study of stratospheric ozone recovery and climate change as well as analysis of interactions between climate warming, tropospheric ozone, and vegetation (Sadiq et al, 2017).…”