Warming Amplifies the Variability of Methane Emissions From a Coastal Wetland

Abstract

Climate warming is expected to increase methane (CH 4 ) emissions from coastal wetlands, but its effects on the temporal dynamics of emissions remain poorly resolved. Because short‐lived high‐emission events are poorly captured by low‐frequency monitoring and by existing models, warming‐driven increases in variability could worsen a systematic bias in CH 4 budget estimates. Here, we analyzed the effects of soil warming on flux variability using an in situ warming experiment in a brackish coastal wetland. We found that warming‐driven increases in CH 4 emissions disproportionately resulted from an increase in emissions during infrequent extreme events (top 0.2%–5% of fluxes) that are unlikely to be captured in low‐frequency monitoring. These high flux events include ebullitive (bubbling) events, which were more common and influential at higher soil temperatures. Consequently, simulated monthly sampling underestimated total emissions by a greater percentage under elevated soil temperatures (+5.7°C; 29% underestimate) compared with ambient temperatures (3.4% underestimate). While warming was associated with increases in the variability of CH 4 fluxes across all temporal scales (hourly, daily, weekly, and seasonal), the change in short‐term (hourly) variability was proportionally greater than the change in seasonal variability, highlighting the increased importance of rapid‐scale emission dynamics under warming. Hourly and daily variability in emissions were predominantly associated with variability in water level, while the seasonal pattern of emissions was related to seasonal changes in soil temperature. Combined, our results indicate that understanding and modeling future CH 4 dynamics may require accounting for an increasing magnitude of CH 4 emitted during short‐lived periods of high emission in a warming climate.