Unveiling Cascading Lag Effects of Wetland Methane Emissions: Evidence from Lake Chad in Africa

Wetland methane (CH₄) emission seasonality is a major yet uncertain component of the global CH₄ seasonal cycle, limited by challenges in accurately characterizing wetland dynamics and CH₄ emissions. Advances in satellite observations, methodology and computing capabilities enable the production of tropical basin-scale wetland-CH₄ seasonality at a finer scale. Here, we quantify monthly CH₄ emissions in the Lake Chad Basin, Africa—a region with pronounced hydroclimatic variability—using an advanced divergence method and TROPOspheric Monitoring Instrument (TROPOMI) satellite observations. We distinguish open water and inundated vegetation and quantify their monthly area dynamics using high-resolution Sentinel-2 data (10 m). Our results reveal a strong seasonal amplitude (4.75 Tg a⁻¹) and significant seasonal hysteresis between CH₄ emissions and wetland inundation (i.e., greater CH₄ emissions during the reduction period of inundation, +1.68 Tg a⁻¹). The cascading link of precipitation–wetland inundation–vegetation succession–CH₄ emissions is proven to drive emission seasonality, comprising a three-month lag for wetland inundation and a subsequent four-month lag for CH₄ emissions. These findings provide new constraints for tropical CH₄ flux estimates, which are the dominant sources of uncertainty between the bottom-up models (± 44—65%), and differences from satellite observations. It is also crucial to improve our understanding of the driver(s) of tropical wetland CH₄ emissions to better assess the impact of future climate changes on these wetland emissions and potential positive feedback.