Estimating Global Natural Wetland Methane Emissions Using Process Modeling: The Spatiotemporal Patterns and the Contributions to Atmospheric Methane Fluctuations

Global wetland methane (CH4) emissions and its spatiotemporal patterns were evaluated using a new process-based model called TRIPLEX-GHG. The model was developed based on the Integrated Biosphere Simulator (IBIS), coupled with a new CH4 biogeochemistry module and a water table module to simulate CH4 emission processes in natural wetlands. The model has been validated using a number of field observations successfully. This study reported the initial results of global scale application, including total CH4 emissions, spatiotemporal patterns, and contributions to the atmospheric CH4 fluctuations. Global annual wetland CH4 emission ranged from 150 TgC yr-1 to 174 TgC yr-1 between 1901 and 2012 with the peak occurring in 1991 and 2012. Between 1990 and 2010, global CH4 emission decreased with a rate of approximately 0.38 TgC yr-1. It was largely due to the decrease of CH4 emissions from topical wetlands, which had a decreasing trend with a rate of 0.33 TgC yr-1 since the 1970s. CH4 emissions from tropical, temperate, and high latitude wetlands comprised 63%, 22% and 15% of global CH4 emission, respectively. Tropical wetlands are the primary contributor of the inter-annual variability of global wetland CH4 emissions as well as atmospheric CH4. The stable-to-decreasing wetland CH4 emissions owing to the balance of emissions between tropical and extratropical wetlands was a contributing factor to the slow-down atmospheric CH4 growth rate during 1990s. The rapid drop in tropical wetland CH4 emissions from 2000 onwards is supposed to offset the increases of the anthropogenic CH4 emissions, hence leading to a relatively stable level of atmospheric CH4 during 2000-2006. Increases of wetland CH4 emissions particularly after 2010 can be considered as an important contributor to the resumed growth of atmospheric CH4 since 2007 and for further increasing in the near future.