Quantifying global biogenic methane emissions from land and freshwater ecosystems across the landscape 

Land and freshwater ecosystems play a significant role in affecting the global methane budget. With future warming, the increase of methane emissions could create large positive feedbacks to the global climate system.  We have used observation data of methane fluxes from diverse land and freshwater ecosystems to calibrate and evaluate extant land and freshwater biogeochemistry models of the Terrestrial Ecosystem Model (TEM) and the Arctic Lake Biogeochemistry Model (ALBM) to quantify the global methane emissions for the past few decades and the 21st century in a temporally and spatially explicit manner. Land ecosystems could emit methane from wetlands while uplands could uptake atmospheric methane. TEM simulates that global wetlands emissions are 212 ± 62 and 212 ± 32 Tg CH4 yr−1 due to uncertain parameters and wetland type distribution, respectively, during 2000–2012. After combining the global upland methane consumption of −34 to −46 Tg CH4 yr−1, we estimate that the global net land methane emissions are 149–176 Tg CH4 yr−1 due to uncertain wetland distribution and meteorological input. During 1950–2016, both wetland emissions and upland consumption increased during El Niño events and decreased during La Niña events. For freshwater ecosystems, we find that current emissions are 24.0 ± 8.4 Tg CH4 yr−1 from lakes larger than 0.1 km2. Future projections under the RCP8.5 scenario suggest a 58–86% growth in emissions from lakes.  Warming enhanced methane oxidation in lake water can be an effective sink to reduce the net release from global lakes. Additionally, these studies identify the key biogeochemical and physical processes of controlling methane production, consumption, and transport in various hotspot emission regions.  We also highlight the need for more in situ methane flux data, more accurate wetland and lake type and their area distribution dynamics information to better constrain the quantification uncertainty of global biogenic methane emissions across the landscape.