Abstract
Wetlands are thought to be the major contributor to interannual variability in the growth rate of atmospheric methane (CH4) with anomalies driven by the influence of the El Niño-Southern Oscillation (ENSO). Yet it remains unclear whether (i) the increase in total global CH4 emissions during El Niño versus La Niña events is from wetlands and (ii) how large the contribution of wetland CH4 emissions is to the interannual variability of atmospheric CH4. We used a terrestrial ecosystem model that includes permafrost and wetland dynamics to estimate CH4 emissions, forced by three separate meteorological reanalyses and one gridded observational climate dataset, to simulate the spatio-temporal dynamics of wetland CH4 emissions from 1980–2016. The simulations show that while wetland CH4 responds with negative annual anomalies during the El Niño events, the instantaneous growth rate of wetland CH4 emissions exhibits complex phase dynamics. We find that wetland CH4 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 CH4 emissions by 7.8±1.6 Tg CH4 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 CH4 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 CH4 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 CH4 fluxes.
Highlights
Methane (CH4) is a potent greenhouse gas and has contributed to ∼20% of observed warming since pre-industrial times (IPCC 2013)
Wetlands are thought to be the major contributor to interannual variability in the growth rate of atmospheric methane (CH4) with anomalies driven by the influence of the El Niñ o-Southern Oscillation (ENSO)
We find significant negative correlations between the ENSO Multivariate ENSO index (MEI) index and monthly wetland CH4 anomalies, regardless of the climate data used in the simulations
Summary
Methane (CH4) is a potent greenhouse gas and has contributed to ∼20% of observed warming since pre-industrial times (IPCC 2013). El Niñ o, the positive phase of ENSO, influences water- and carbon- fluxes of tropical terrestrial ecosystems through a change in patterns of atmospheric pressure and sea surface temperature (Philander 1990). These changes induce strong warming and reduced precipitation patterns by shifting the Intertropical Convergence Zone southward, causing amplified wildfires (Worden et al 2013) and reduced wetland areal extent and CH4 emissions (Hodson et al 2011). Atmospheric measurements of CH4 provide evidence that the growth rate of global CH4 concentrations can rise during strong El Niño years (Nisbet et al 2016, Bousquet et al 2006), but terrestrial biogeochemical models suggest that tropical and global wetland CH4 emissions are usually found to decrease during El Niño (Hodson et al 2011, Zhu et al 2017, Ringeval et al 2014)
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