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
Brown carbon to reduce their emissions and their effects. Brown carbon has attracted interest as a possible cause of climate change. This class of organic carbon, known for its brownish color, absorbs strongly in the ultraviolet wavelengths and less significantly going into the visible wavelengths. Types of brown carbon include tar materials from smoldering fires or coal combustion, breakdown products from biomass burning, a mixture of organic compounds emitted from soil, and volatile organic compounds given off by vegetation. Simulation models suggest that brown carbon contributes about 19% of the total atmospheric absorption by aerosols, while 72% is attributed to black carbon