Per kilo, atmospheric methane is about 34 times as potent a greenhouse gas as carbon dioxide is. Whether natural or anthropogenic, sources of CH4 can therefore have a big impact on radiative forcing. Mary Kang of Princeton University and her colleagues have quantified one previously unaccounted source of atmospheric CH4: abandoned oil and gas wells. Estimates put the number of such wells in the US at 3 million. In her study, Kang focused on 19 wells in the region where oil and gas were first extracted in the US: Western Pennsylvania. She and her colleagues placed enclosing chambers over the wellheads, like the one shown here, and measured the methane flux. They repeated the procedure at control sites near the wells. On average, the methane flux per well was 11 000 mg/h, whereas the mean flux per control site was 0.19 mg/h. Oil and gas extraction in Pennsylvania has taken place—and continues to take place—in more than the two counties included in the Princeton study. Extrapolating across the entire state, the researchers calculate that the CH4 emission from abandoned wells constitutes 4–7% of the state's total anthropogenic emissions. Extrapolating across the entire US is difficult, given differences in terrain, history of oil and gas development, and other local conditions, but it's clear that CH4 emissions from abandoned wells may significantly augment those from active systems, which are estimated to contribute 28% of the nation's total. (M. Kang et al., Proc. Natl. Acad. Sci. USA, published online on 8 December 2014.)
