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.