Ebullition dominates high methane emissions globally across all lake sizes

Methane is emitted from lakes by diffusion and ebullition. Methane diffusion is constrained by diffusion from sediments to water and water to the atmosphere, as well as oxidation. Methane ebullition from shallow water sediments bypasses these constraints but requires high methane production to form bubbles. We tested if ebullition dominates at high emissions with a Danish dataset and a global dataset comprising 973 measurements. Upper limits of methane diffusion were more constrained than ebullition. During periods of low total emissions, diffusive methane emissions predominated, whereas ebullition prevailed during periods of high emissions. The relative contribution of ebullition changed predictably, being 50% at 1.5–1.6 mmol m−2 d−1 and 75% at 5.1–6.4 mmol m−2 d−1 total methane emission. The probability of ebullitive flux was highly affected by the magnitude of the diffusive flux, and water temperature. Thus, when data was divided into the water temperature intervals ≤10, 10–20, and >20 °C, ebullition occurred in 69, 69 and 95% of the observations, respectively, and emission increased from 0.29, 0.71 to 3.6 mmol m−2 d−1 between the three temperature intervals. Summed across all measurements, ebullition accounted for the majority (75–83%) of total methane emissions. Thus, to attain reliable whole-lake emission and global estimates, many ebullition measurements are required to cover their extensive spatial and temporal variability.