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.