Simultaneous Hot and Dry Extreme‐Events Increase Wetland Methane Emissions: An Assessment of Compound Extreme‐Event Impacts Using Ameriflux and FLUXNET‐CH4 Site Data Sets

Wetlands are the largest natural source of global atmospheric methane (CH4). Despite advances to our understanding of changes in temperature and precipitation extremes, their impacts on carbon‐rich ecosystems such as wetlands, remain significantly understudied. Here, we quantify the impacts of extreme temperature, precipitation, and dry events on wetland CH4 dynamics by investigating the effects of both compound and discrete extreme‐events. We use long‐term climate data to identify extreme‐events and 45 eddy covariance sites data sets sourced from the FLUXNET‐CH4 database and Ameriflux project to assess impacts on wetland CH4 emissions. These findings reveal that compound hot + dry extreme‐events lead to large increases in daily CH4 emissions. However, per event, discrete dry‐only extreme‐events cause the largest total decrease in CH4 emissions, due to their long duration. Despite dry‐only extreme‐events leading to an overall reduction in CH4 emissions, enhanced fluxes are often observed for the first days of dry‐only extreme‐events. These effects differ depending on wetland type, where marsh sites tend to be sensitive to most types of extreme‐events. Lagged impacts are significant for at least the 12 months following several types of extreme‐events. These findings have implications for understanding how extreme‐event impacts may evolve in the context of climate change, where changes in the frequency and intensity of temperature and precipitation extreme‐events are already observed. With increasing occurrences of enhanced CH4 fluxes in response to hot‐only extreme‐events and hot + wet extreme‐events and fewer occurrences of reduced CH4 fluxes during cold‐only extreme‐events, the impact of wetland CH4 emissions on climate warming may be increasing. Key Points Compound extreme‐events (e.g., hot + dry extreme‐events) cause large impacts on daily CH4 emissions relative to discrete extreme‐events Dry‐only extreme‐events show large total decreases in CH4 emissions due to the long duration of events, despite initial flux increases Lagged impacts are significant for at least the 12 months following most types of extreme‐events