Offsetting climate and land use effects on wetland methane emissions over 1901-2020

Atmospheric methane (CH₄) concentration has more than doubled since per-industrial time. The contribution of natural wetland ecosystems, currently the largest natural emitters of CH₄, to this increase is not well known. While temperature may have contributed to an growth in emissions, the extent wetlands has declined over this period due to drainage and land use conversion. In this study, we combine a new reconstruction of global wetland extent with simulated CH₄ flux from nine land surface model to estimate wetland CH4 emissions over 1901-2020. Our analysis evaluates: 1) the uncertainty of modelled emissions over the century timescale, 2) the separate and combined effects of climate and land use change on emissions, and 3) the global and regional trends of wetland CH4 emissions. We show that prognostic outputs from model of wetland extent are highly uncertain over the century timescale although most prognostic models suggest an emission increase. We find that inclusion of wetland drainage reducing wetland area primarily in temperate latitudes is sufficient to offset the increase in modeled emission in the absence of land use, with an ensemble mean displaying no significant trend between 1901-1920 and 2001-2020. We evaluate the contribution of individual land uses to the decline in global emissions, in particular the conversion to irrigated rice and wet cultivation which are also methane emitting areas. These results diverge from previous source attributions at the century timescale and may require upward revisions to other biogenic sources to balance the budget and remain within δ13C isotope constraints.  In the future, we will investigate the effect of wetland drainage on CO2, CH4 and aquatic carbon export for a more complete accounting of this land use change on global carbon fluxes.