How much methane removal is required to avoid overshooting 1.5 ∘C?

Abstract

Methane is the second most important anthropogenic greenhouse gas after carbon dioxide. With an atmospheric lifetime of around a decade, methane mitigation starting immediately has the potential to avoid substantial levels of additional warming by mid-century. In addition to the methane emissions reductions that are necessary to limit warming, we address the question of whether technological methane removal can provide additional benefits by avoiding global mean surface temperatures exceeding 1.5 ∘C above pre-industrial—the high-ambition Paris Agreement climate goal. Using an adaptive emissions methane removal routine in a simple climate model, we successfully limit peak warming to 1.5 ∘C for overshoots of up to around 0.3 ∘C. For substantially higher overshoots, methane removal alone is unable to limit warming to 1.5 ∘C, but in an extreme scenario could limit peak warming by an ensemble median 0.7 ∘C if all atmospheric methane was removed, requiring huge levels of net removal on the order of tens of petagrams cumulatively. The efficacy of methane removal depends on many emergent properties of the climate system, including climate sensitivity, aerosol forcing, and the committed warming after net zero CO2 (zero emissions commitment). To avoid overshooting 1.5 ∘C in the low-overshoot, strong-mitigation SSP1-1.9 scenario, a median cumulative methane removal of 1.2 PgCH4 is required, though this may be much higher if climate sensitivity is high or the zero emissions commitment is positive, and in these cases may require ongoing methane removal long after peak warming in order to stabilise warming below 1.5 ∘C.