Impact of interannual and multidecadal trends on methane-climate feedbacks and sensitivity.

Abstract

We estimate the causal contributions of spatiotemporal changes in temperature (T) and precipitation (Pr) to changes in Earth's atmospheric methane concentration (C_CH4) and its isotope ratio δ¹³CH₄ over the last four decades. We identify oscillations between positive and negative feedbacks, showing that both contribute to increasing C_CH4. Interannually, increased emissions via positive feedbacks (e.g. wetland emissions and wildfires) with higher land surface air temperature (LSAT) are often followed by increasing C_CH4 due to weakened methane sink via atmospheric ^•OH, via negative feedbacks with lowered sea surface temperatures (SST), especially in the tropics. Over decadal time scales, we find alternating rate-limiting factors for methane oxidation: when C_CH4 is limiting, positive methane-climate feedback via direct oceanic emissions dominates; when ^•OH is limiting, negative feedback is favoured. Incorporating the interannually increasing C_CH4 via negative feedbacks gives historical methane-climate feedback sensitivity ≈ 0.08 W m^-2 °C^-1, much higher than the IPCC AR6 estimate.