Earth's degassing: a missing ethane and propane source.

Current emission inventories require an additional "unknown" source to balance the global atmospheric budgets of ethane (C2H6). Here, we provide evidence that a substantial part of the missing source can be attributed to natural gas seepage from petroliferous, geothermal, and volcanic areas. Such geologic sources also inject propane (C3H8) into the atmosphere. The analysis of a large data set of methane (CH4), ethane, and propane concentrations in surface gas emissions of 238 sites from different geographic and geologic areas, coupled with published estimates of geomethane emissions, suggests that Earth's degassing accounts for at least 17% and 10% of total ethane and propane emissions, respectively.