Since 2008, atmospheric methane concentrations have surged. This change has previously been attributed to an increase in biogenic methane sources, particularly agriculture. But new research suggests this may not be the case (Biogeosci. 2019, DOI: 10.5194/bg-16-3033-2019). Biogenic sources of methane have a characteristic isotopic signature that contains less carbon-13 than fossil fuels. As atmospheric methane’s signature has been becoming more depleted in 13C since 2008, researchers have attributed the rise in emissions to biogenic sources. However, previous researchers assumed that all fossil fuels have the same carbon isotope ratios. Robert Howarth, a Earth systems scientist at Cornell University, wondered if different fossil fuel sources might also have different isotopic signatures. He found that shale gas is depleted in 13C compared with traditional natural gas sources, which suggests that fracking, not agriculture, could be responsible for methane’s surge in the past decade. His analysis suggests that one-third of the increase can be attributed to shale gas production. Critics of Howarth’s work say his selection of shale wells is not representative—notably, he did not include the most productive natural gas source in the US, the Marcellus Shale. Howarth says shale gas–specific isotopic data do not exist for the Marcellus, but such data would be helpful in future analyses. The shales he analyzed are also major US sources.