Methane emission from high latitude lakes: methane-centric lake classification and satellite-driven annual cycle of...

Methane (CH₄) is emitted from lakes by several processes: bubbles released from bottom sediments that reach the atmosphere (ebullition); spring release of CH₄ trapped in bubbles in and under the ice during fall freeze (bubble release), and diffusion of CH₄ from sediments to the surface. Each of these emission routes is highly variable over space and time, and episodic in the extreme, making reliable measurements difficult to carry out. However, lakes are receiving increasing interest for their important contribution to global CH₄ emissions. Their area, distribution and emissions respond to interannual and longer-term climate fluctuations and close to half the world's lake area is in high northern latitudes that are experiencing rapidly-warming temperatures and lengthening thaw periods. We report on a new spatially-explicit data set of lakes > 50°N, classified with methane-relevant criteria. The seasonality of daily CH₄ fluxes is driven with satellite observations of thaw timing and duration. We found that observed thaw seasons are 10-30% shorter than those assumed in previous studies. The area of lakes is 1,095 × 10³ km² and total CH₄ emission is 13.8-17.7 Tg CH₄ year^-1: 11.2-14.4 Tg via diffusion and ebullition and 2.6-3.3 Tg from spring release of CH₄ stored in bubbles in winter lake ice. This novel suite of data and methodologies provides a unique framework to model CH₄ emission from lakes under current, past and future climates.