Both the Energy Information Administration (EIA) and the U.S. Environmental Protection Agency (EPA) estimate annual U.S. greenhouse gas (GHG) emissions. EPA's data are the official inventory for the United States for reporting required under the United Nations Framework Convention on Climate Change (UNFCCC). Although EPA uses EIA fuel consumption data as a basis for some of its estimates, there are differences in the two agencies' methodologies that result in different datasets. For instance, EPA estimated total 1998 U.S. transportation sector carbon dioxide emissions at 450.3 million metric tons of carbon (mmtc), while EIA reported 482 mmtc.
The Intergovernmental Panel on Climate Change (IPCC) was set up as the scientific body under UNFCCC, and EPA largely adheres to IPCC methodology guidelines designed to assure data harmonization among all reporting countries. EIA has more discretion in deciding which IPCC guidelines to follow. For instance, EIA's data cover 50 states and the District of Columbia, while EPA must include all U.S. territories, as well. Some numbers EPA gets from EIA are revised. EIA fuel consumption data are gathered in physical units and EPA converts them to energy equivalents. In some cases, EPA emission estimates (e.g., for industrial coal) are lower than EIA's to avoid double counting problems.