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. Beginning with 1999 data, EPA reports GHG emissions data in teragrams (Tg), while EIA continues to use metric ton units. Total U.S. transportation carbon dioxide emissions in 1999 were 496 million metric tons of carbon equivalent (mmtce) according to EIA but 1,717 Tg of carbon dioxide equivalents (468 mmtce) according to EPA.
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.
EIA releases its data five to six months before EPA does, and its data could be considered preliminary estimates. EPA data undergo external as well as internal review before they are released in time to meet a United Nations deadline of April each year. While EPA data are not as timely as are EIA data, EPA provides more detail of interest to transportation. For instance, EPA breaks down GHG data by modes and by various GHGs, such as carbon dioxide, nitrous oxide, and methane.