PNNL

Abstract

Freshwater ecosystem contributions to the global methane budget remains the most uncertain among natural sources. With warming and accompanying carbon release from thawed permafrost and thermokarst lake expansion, the increase of methane emissions could be large. However, the impact and relative importance of various factors related to warming remain uncertain. Based on diverse lake characteristics incorporated in modeling and observational data, we calibrate and verify a lake biogeochemistry model. The model is then applied to estimate global lake methane emissions and examine the impacts of temperature increase, permafrost thaw, and thermokarst lake area changes for the first and the last decades of the 21st century under different climate scenarios. We find that current emissions are 24.0±8.4 Tg CH4 yr-1 from lakes larger than 0.1 km2, accounting for 11% of the global total natural source as estimated based on atmospheric inversion. Future projections under the RCP8.5 scenario suggest a 57-86% growth in emissions from lakes. While more active methanogenesis and additional carbon substrates from thawing permafrost will enhance methane production under warming, methane oxidation in lake water can be an effective sink to reduce the release from lakes.