Abstract
Quantifying natural geological sources of methane (CH(4)) allows to improve the assessment of anthropogenic emissions to the atmosphere from fossil fuel industries. The global CH(4) flux of geological gas is, however, an object of debate. Recent fossil ((14)C-free) CH(4) measurements in preindustrial-era ice cores suggest very low global geological emissions (~ 1.6 Tg year(-1)), implying a larger fossil fuel industry source. This is however in contrast with previously published bottom-up and top-down geo-emission estimates (~ 45 Tg year(-1)) and even regional-scale emissions of ~ 1-2 Tg year(-1). Here we report on significant geological CH(4) emissions from the Lusi hydrothermal system (Indonesia), measured by ground-based and satellite (TROPOMI) techniques. Both techniques indicate a total CH(4) output of ~ 0.1 Tg year(-1), equivalent to the minimum value of global geo-emission derived by ice core (14)CH(4) estimates. Our results are consistent with the order of magnitude of the emission factors of large seeps used in global bottom-up estimates, and endorse a substantial contribution from natural Earth's CH(4) degassing. The preindustrial ice core assessments of geological CH(4) release may be underestimated and require further study. Satellite measurements can help to test geological CH(4) emission factors and explain the gap between the contrasting estimates.