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
Environmental impact of electricity generation home. Heat pumps are very efficient and burn no natural gas, shifting the environmental impacts from homeowners to electric utilities. Switching from natural gas to electricity in Alberta Canada burns natural gas and coal at about a 40% efficiency to supply the heat pump. In Quebec Canada where electric resistance heating is common, the heat pump will use 70% less hydroelectricity. Heat pumps may be beneficial for the environment or not, it depends how your region generates electricity. Nuclear power plants do not burn fossil fuels and so do not directly emit carbon dioxide; because of the high energy yield