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

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 δ(13)CH(4) 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.