Methane accounts for an important share of greenhouse gas emissions in the United States, but estimates of North American emissions vary by more than 50 percent. This range is too wide to inform policy decisions on future greenhouse gas reductions. Traditional emissions inventories use accounting methods to tabulate methane sources. In this study, by contrast, we utilize observations of methane in the atmosphere and HPC tools to estimate emissions across the United States and Canada. Our results show that the current methane inventory from the U.S. Environmental Protection Agency (EPA) underestimates emissions nationally by a factor of about 1.5. This discrepancy is particularly pronounced in the south-central United States, where we find total emissions are about 2.7 times greater than in most inventories and account for (24 plus or minus 3) percent of national emissions. These existing inventories likely underestimate emissions from two industries: agriculture (i.e., ruminants and manure) and the natural gas and oil industries. In fact, we find that methane emissions from the oil and gas industries in the south-central United States could be (4.9 plus or minus 2.6) times larger than inventory estimates. These results cast doubt on the U.S. EPA’s recent decision to downscale its estimate of national natural gas emissions by 25 to 30 percent. Additional Abstract Authors: Steven C. Wofsy, Anna M. Michalak, Eric A. Kort, Arlyn E. Andrews, Sebastien C. Biraud, Edward J. Dlugokencky, Janusz Eluszkiewicz, Marc L. Fischer, Greet Janssens-Maenhout, Ben R. Miller, John B. Miller, Stephen A. Montzka, Thomas Nehrkorn, Colm Sweeney, Doug E.J. Worthy