Radiative forcing of methane emission completely offsets net carbon dioxide uptake in a temperate freshwater marsh from the present to future

Abstract

Wetlands serve a dual function as carbon dioxide (CO2) sinks and methane (CH4) sources. Emissions of CH4 from wetlands have long been considered to potentially offset net cooling effect of CO2 uptakes, yet there is a paucity of detailed knowledge on the tradeoff of wetland CO2 and CH4 fluxes under current and future climate conditions. This study used the eddy covariance technique to measure ecosystem-scale CO2 and CH4 fluxes in a Phragmites australis-dominated freshwater marsh from 2020 to 2023. Furthermore, we projected future CO2 and CH4 fluxes based on six Earth System Models under four Representative Concentration Pathways (RCP). Our results show that the P. australis marsh is a large CO2 sink and a hotspot for CH4 emissions, with a 3-year annual average net CO2 uptake of 926 g CO2 m‒2 and CH4 emission of 42 g CH4 m‒2. The negative radiative forcing caused by CO2 uptake was entirely counterbalanced by wetland CH4 emissions, resulting in a mean annual net CO2-equivalent (CO2-eq) emission of 970 g CO2-eq m‒2 over a 100-year time horizon. The interannual variability of net CO2-eq fluxes was driven by hydrological variations, with lower net CO2-eq fluxes occurring during dry years. In future scenarios, the radiative forcing of CH4 emissions could persistently and completely offset the net CO2 uptake in this freshwater marsh. Alarmingly, future net CO2-eq fluxes exhibit a significant increasing trend under the RCP8.5 scenario because CH4 emissions are more responsive than CO2 uptake to increasing temperatures and precipitation. Conversely, under the RCP2.6 scenario, future net CO2-eq fluxes show a significant downward trend, primarily due to the slowdown of positive warming-wetland CH4 feedback and the radiation-driven continuous rise in CO2 uptake. Overall, our findings affirm the role of wetlands in sequestrating atmospheric CO2 but highlight the current and growing importance of wetland CH4 emissions in regulating wetland-climate feedback.