The main greenhouse gases are carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and water vapor. In addition to these natural compounds, synthetic fluorinated gases also function as greenhouse gases. Different greenhouse gases have different chemical properties and are removed from the atmosphere over time by various processes. Carbon dioxide is the primary greenhouse gas emitted through human activities and accounts for 79% of all U.S. greenhouse gas emissions from human activities. Methane is the second most abundant greenhouse gas and is emitted from natural and human sources, including livestock, landfills, and oil and gas production. Nitrous oxide is emitted from natural and human sources, including agriculture and fossil fuel combustion. Water vapor is the most abundant greenhouse gas, but it is not included in U.S. or international greenhouse gas inventories because it is not directly emitted by human activity.