Two groups offer different explanations—both stemming from human activity—for a recent mysterious decline in the increase of atmospheric methane (Nature, DOI: 10.1038/nature10352, 10.1038/nature10259). Methane, a greenhouse gas that’s produced by microbial emissions and by burning fossil fuels, had been increasing in the atmosphere over the 20th century. But in the past three decades, the rate of increase leveled off. Michael J. Prather at the University of California, Irvine, and colleagues measured ethane levels trapped in air bubbles in Greenland and Antarctica. (Ethane and methane are both released by burning fossil fuels.) From those measurements and an atmospheric model, the authors conclude that the methane emission rate slowdown was caused by decreases in fossil-fuel burning beginning in the 1980s. Separately, UC Irvine’s James T. Randerson and colleagues compared isotopic signatures of methane from fossil fuel and microbial sources. They find the decline is due partly to increased fertilizer and decreased water use in Asian rice agriculture. These conflicting studies illustrate the importance of long-term observations of methane concentration and isotope composition, and of trace gases such as ethane, Martin Heimann of the Max Planck Institute for Biogeochemistry writes in perspective accompanying the two groups’ reports.