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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 (CH4) 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 CH4 on ozone (O3). Here the differing effects of CH4 and CO2 on land carbon storage, including the effects of surface O3, lead to an additional increase in the allowable carbon emissions with CH4 mitigation. We find a simple robust relationship between the change in the 2100 CH4 concentration and the extra allowable cumulative carbon emissions between now and 2100 (0.27 ± 0.05 GtC per ppb CH4). This relationship is independent of modelled climate sensitivity and precise temperature target, although later mitigation of CH4 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 CH4 emissions would significantly increase the feasibility of stabilising global warming below 1.5 °C, alongside having co-benefits for human and ecosystem health.
Highlights
Meeting the Paris temperature targets by reducing CO2 emissions alone represents a huge challenge, even for the more optimistic assessments of the allowable carbon budgets (Millar et al 2017)
The uncertainty is due to the range of climate sensitivities of the CMIP5 models emulated by the IMOGEN framework. Rather than these absolute budgets we focus on the differences in the cumulative carbon emissions from the inversions for the different CH4 scenarios
The loss in benefit from delaying CH4 mitigation according to the Late CH4 scenario is 40 GtC. These values are similar to a study comparing no mitigation with stringent mitigation (Rogelj et al 2015b) which calculated an increase of 130 GtC in the carbon budget, with a 30 GtC penalty for late mitigation
Summary
Meeting the Paris temperature targets by reducing CO2 emissions alone represents a huge challenge, even for the more optimistic assessments of the allowable carbon budgets (Millar et al 2017). Most existing scenarios that avoid 2 ◦C of global warming, and almost all of those that avoid 1.5 ◦C, assume periods of negative global CO2 emissions in order to stay within the implied cumulative carbon budgets (Rogelj et al 2015a). Care needs to be taken as the scenarios used in the IPCC 5th Assessment Report (AR5) (Pachauri et al 2014) assumed specific changes in non-CO2 agents such as aerosols and CH4. These calculations did not include biogeochemical feedbacks that might affect the concentrations of the greenhouse gases such as changes in permafrost and wetlands (Comyn-Platt et al 2018). As a consequence cumulative carbon budgets are very sensitive to assumptions in scenarios for non-CO2 greenhouse gases (Rogelj et al 2015b)
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