Enhanced response of global wetland methane emissions to the 2015–2016 El Niño-Southern Oscillation event

Abstract

Wetlands are thought to be the major contributor to interannual variability in the growth rate of atmospheric methane (CH _4 ) with anomalies driven by the influence of the El Niño-Southern Oscillation (ENSO). Yet it remains unclear whether (i) the increase in total global CH _4 emissions during El Niño versus La Niña events is from wetlands and (ii) how large the contribution of wetland CH _4 emissions is to the interannual variability of atmospheric CH _4 . We used a terrestrial ecosystem model that includes permafrost and wetland dynamics to estimate CH _4 emissions, forced by three separate meteorological reanalyses and one gridded observational climate dataset, to simulate the spatio-temporal dynamics of wetland CH _4 emissions from 1980–2016. The simulations show that while wetland CH _4 responds with negative annual anomalies during the El Niño events, the instantaneous growth rate of wetland CH _4 emissions exhibits complex phase dynamics. We find that wetland CH _4 instantaneous growth rates were declined at the onset of the 2015–2016 El Niño event but then increased to a record-high at later stages of the El Niño event (January through May 2016). We also find evidence for a step increase of CH _4 emissions by 7.8±1.6 Tg CH _4 yr ^−1 during 2007–2014 compared to the average of 2000–2006 from simulations using meteorological reanalyses, which is equivalent to a ~3.5 ppb yr ^−1 rise in CH _4 concentrations. The step increase is mainly caused by the expansion of wetland area in the tropics (30°S–30°N) due to an enhancement of tropical precipitation as indicated by the suite of the meteorological reanalyses. Our study highlights the role of wetlands, and the complex temporal phasing with ENSO, in driving the variability and trends of atmospheric CH _4 concentrations. In addition, the need to account for uncertainty in meteorological forcings is highlighted in addressing the interannual variability and decadal-scale trends of wetland CH _4 fluxes.

Keywords

• DGVM
• global methane budget
• ENSO
• greenhouse gas