Estimating the Importance of Geologic Sources of Methane

The contribution of geologic sources to the annual global budget of atmospheric methane is thought to range anywhere from a few Tg (teragrams) of CH4 to tens of Tg of CH4. Indeed, this term is the most uncertain element of the CH4 budget as we currently understand it, and is generally neglected in studies of the atmospheric methane budget. Geologic methane sources are likely to be highly variable with the potential for sporadic catastrophic releases related to the instability of some reservoirs. The processes understood to lead to methane release from geologic reservoirs include seeps of oil and gas, mud volcanoes and gas hydrates. Although rough estimates of methane emissions from geologic sources have revealed the potential magnitude of the emissions, inventories for use in atmospheric models have not generally been available. We have developed a detailed inventory of likely geologic sources of methane, and we present here a study using an atmospheric transport model and CH4 flux estimates for non-geologic sources optimzed using observations. We consider the extent to which global ground-based and satellite observations are in agreement with spatial distribution of geologic methane emissions given the uncertainties in the non-geologic sources.