Impact of interannual and multidecadal trends on methane-climate feedbacks and sensitivity
Chin-Hsien Cheng, Simon A. T. Redfern
Abstract
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 δ 13 C H 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.