The Dominant Role of Tropical Wetlands in Dedacal-Scale Changes in the Global Methane Budget

Atmospheric methane has an important role in changing climate, a role that may be gaining increased importance. It is emitted by a varied set of processes and sources. Emission rates are often small and variable, both temporally and spatially, making quantification of emissions difficult, except at global scales. NOAA observations of globally averaged atmospheric CH4 began in 1983, and the data are rich in features, capturing small changes in its budget of emissions and sinks. From the start of measurements through 2006, the rate of increase of atmospheric CH4 was decreasing and averaged less than 1 ppb yr-1 from 1999 to 2006. Assuming no trend in CH4 lifetime from 1983-2006, this implies that total global CH4 emissions were constant and atmospheric CH4 nearly achieved steady state. Since 2007, the growth rate has averaged ~6 ppb yr-1, implying an increase in emissions of ~16 Tg CH4 yr-1. Many potential explanations exist for this increase, but the most likely involves a significant contribution from increased emissions from tropical wetlands. One thing is clear: the increase that started in 2007 can no longer be considered a short-term anomaly in growth rate. It has persisted for 8 years, and analysis of preliminary data suggests the growth rate increased further in 2014.