Atmospheric concentration of methane (CH4), a potent greenhouse gas, has more than doubled since pre-industrial time. Natural wetland ecosystems are currently the largest natural emitters of CH4, yet their contribution to this increase is not well understood. While rising temperatures may have contributed to growth in emissions, the extent wetlands has declined over this period due to drainage and land use conversion.
Here, I will discuss how a new map reconstructing historical wetland extent combined with simulated wetland CH4 emissions by land surface models can help disentangle effects of humans and climate. The resulting wetland CH4 emissions over 1901-2020 shows that wetland drainage concentrated in temperate latitudes is sufficient to offset increase in emission rates, producing in a broadly stable flux since 1901. These findings diverge from previous source attributions at the century timescale using larger estimates of drained wetland area.
Our conclusions imply that an upward revision of other biogenic sources may be needed to balance the historical budget and isotopic constraints. To fully understand the contribution of wetland drainage to global radiative forcing, more work is needed to quantify the cumulative impact on CO2, CH4, N2O atmospheric fluxes and aquatic carbon export, as well as to provide a baseline for planning of conservation and restoration efforts.
