Substantial hysteresis in emergent temperature sensitivity of global wetland CH4 emissions

Wetland methane (CH₄) emissions (FCH₄) are important in global carbon budgets and climate change assessments. Currently, FCH₄ projections rely on prescribed static temperature sensitivity that varies among biogeochemical models. Meta-analyses have proposed a consistent FCH₄ temperature dependence across spatial scales for use in models; however, site-level studies demonstrate that FCH₄ are often controlled by factors beyond temperature. Here, we evaluate the relationship between FCH₄ and temperature using observations from the FLUXNET-CH₄ database. Measurements collected across the globe show substantial seasonal hysteresis between FCH₄ and temperature, suggesting larger FCH₄ sensitivity to temperature later in the frost-free season (about 77% of site-years). Results derived from a machine-learning model and several regression models highlight the importance of representing the large spatial and temporal variability within site-years and ecosystem types. Mechanistic advancements in biogeochemical model parameterization and detailed measurements in factors modulating CH₄ production are thus needed to improve global CH₄ budget assessments.