Coal the heat and pressure of deep burial causes the loss of water, methane and carbon dioxide and an increase in the proportion of carbon. Thus first lignite (also called "brown coal"), then sub-bituminous coal, bituminous coal, and lastly anthracite (also called "hard coal" or "black coal") may be formed. The wide, shallow seas of the Carboniferous Period provided ideal conditions for coal formation, although coal is known from most geological periods. The exception is the coal gap in the Permian–Triassic extinction event, where coal is rare. Coal is known from Precambrian strata, which predate land plants—this coal is presumed to
Energy policy of the United States US burns 1 billion tons of coal every year. Concern for climate change has led to a call for a moratorium on all coal consumption, unless carbon capture is utilized. Coal is the largest potential source of CO emissions. Integrated Gasification Combined Cycle (IGCC) is the cleanest currently operational coal-fired electricity generation technology. FutureGen is an experimental U.S. research project to investigate the possibility of sequestering IGCC CO emissions underground. Natural gas production and consumption quadrupled between 1950 and 1970 to , but declined steadily to stabilize in 1986. Since then, the United States imports a rising share of its