Instituting policies to manage or reduce GHGs would likely impact different states differently. Understanding these differences may provide for a more informed debate regarding potential policy approaches. However, multiple factors play a role in determining impacts, including alternative design elements of a GHG emissions reduction program, the availability and relative cost of mitigation options, and the regulated entities' abilities to pass compliance costs on to consumers. Three primary variables drive a state's human-related greenhouse gas (GHG) emission levels: population, per capita income, and the GHG emissions intensity. GHG emissions intensity is a performance measure. In this book, GHG intensity is a measure of GHG emissions from sources within a state compared with a state's economic output (gross state product, GSP). The GHG emissions intensity driver stands apart as the main target for climate change mitigation policy, because public policy generally considers population and income growth to be socially positive. The intensity of carbon dioxide (CO2) emissions largely determines overall GHG intensity, because CO2 emissions account for 85% of the GHG emissions in the United States. As 98% of U.S. CO2 emissions are energy-related, the primary factors that shape CO2 emissions intensity are a state's energy intensity and the carbon content of its energy use. Energy intensity measures the amount of energy a state uses to generate its overall economic output (measured by its GSP). Several underlying factors may impact a state's energy intensity: a state's economic structure, personal transportation use in a state (measured in vehicle miles travelled per person), and public policies regarding energy efficiency. The carbon content of energy use in a state is determined by a state's portfolio of energy sources. States that utilise a high percentage of coal, for example, will have a relatively high carbon content of energy use, compared to states with a lower dependence on coal. An additional factor is whether a state is a net exporter or importer of electricity, because CO2 emissions are attributed to electricity-producing states, but the electricity is used (and counted) in the consuming state. Between 1990 and 2000, the United States reduced its GHG intensity by 1.6% annually. Assuming that population and per capita income continue to grow as expected, the United States would need to reduce its GHG intensity at the rate of 3% per year in order to halt the annual growth in GHG emissions. Therefore, achieving reductions (or negative growth) in GHG emissions would necessitate further declines in GHG intensity.