Synergistic Amplification of Methane Emissions During Compound Drought-Heatwave Events in a Temperate Reed-Cattail Wetland

Abstract. Compound drought-heatwave events are increasing in frequency and severity under global climate change, yet their joint impacts on wetland methane (CH₄) dynamics remain poorly constrained. Here, we analyzed continuous half-hourly eddy covariance flux measurements collected from a temperate reed-cattail wetland in northern China (2022–2025) to evaluate CH₄ emissions across four climate regimes: near-normal conditions, drought, heatwaves, and compound drought-heatwaves. Relative to the near-normal conditions, mean daily CH₄ emissions were suppressed by 69% during drought and remained unchanged during heatwaves. However, during drought-heatwave events, daily CH₄ emissions increased by 29%, despite lower water tables and elevated atmospheric vapor pressure deficits. Deseasonalized flux analyses revealed that CH₄ emissions during compound events were significantly greater than the additive sum of individual drought and heatwave effects, demonstrating non-additive synergistic interactions. High-frequency flux distribution analysis showed that this emission enhancement was driven by a systematic redistribution toward moderately high and high sub-daily fluxes rather than an intensification of extreme peak fluxes. These findings demonstrate that heatwave co-occurrence overrides drought-induced emission suppressions. Consequently, biogeochemical models and Earth system frameworks relying on isolated climate-stressor response functions risk systematically underestimating wetland CH₄ emissions as compound extreme events intensify.