Abstract
AbstractAtmospheric methane (CH4) accounts for ~20% of the total direct anthropogenic radiative forcing by long‐lived greenhouse gases. Surface observations show a pause (1999–2006) followed by a resumption in CH4 growth, which remain largely unexplained. Using a land surface model, we estimate wetland CH4 emissions from 1993 to 2014 and study the regional contributions to changes in atmospheric CH4. Atmospheric model simulations using these emissions, together with other sources, compare well with surface and satellite CH4 data. Modeled global wetland emissions vary by ±3%/yr (σ = 4.8 Tg), mainly due to precipitation‐induced changes in wetland area, but the integrated effect makes only a small contribution to the pause in CH4 growth from 1999 to 2006. Increasing temperature, which increases wetland area, drives a long‐term trend in wetland CH4 emissions of +0.2%/yr (1999 to 2014). The increased growth post‐2006 was partly caused by increased wetland emissions (+3%), mainly from Tropical Asia, Southern Africa, and Australia.
Highlights
Two outstanding features of the recent global atmospheric CH4 record are as follows: (i) prior to 2007 atmospheric CH4 seemed to be approaching a stationary state with sources and sinks in balance and (ii) since 2007 growth has resumed [Rigby et al, 2008]
The isotopic signature of atmospheric CH4 has become more depleted in 13CH4 since 2007, which may be explained by a shift toward a larger fraction of biogenic sources, i.e., wetlands [Nisbet et al, 2014], or agricultural sources [Schaefer et al, 2016]
The modeled emissions, used within the TOMCAT chemical transport model (CTM), gave a good representation (R = 0.88) of the spatial and temporal variability in atmospheric CH4 when compared with satellite and surface data
Summary
Two outstanding features of the recent global atmospheric CH4 record are as follows: (i) prior to 2007 atmospheric CH4 seemed to be approaching a stationary state with sources and sinks in balance and (ii) since 2007 growth has resumed [Rigby et al, 2008] The reasons for these two features are not fully understood, but several indicators suggest that changes in wetlands contributed to both. The isotopic signature of atmospheric CH4 has become more depleted in 13CH4 since 2007, which may be explained by a shift toward a larger fraction of biogenic sources, i.e., wetlands [Nisbet et al, 2014], or agricultural sources [Schaefer et al, 2016] Both of these studies focused on the global isotopic signature of CH4 and so were unable to spatially resolve the regions of main CH4 emission changes
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