The world's supply of fossil fuels is large but limited. Using reported (by British Petroleum) values for known reserves and consumption, the supply of the main fossil fuels is as follows:
In other words, the known coal reserves at current mining rate will be depleted in 132 years. If we convert all our energy use to coal, the supply will only last 38 years.
Fossil fuels are burned to generate heat, to propel vehicles, and to create electricity (from steam turbines) to light our cities. Because burning is never perfect, this burning creates carbon monoxide, nitrous oxides, and other pollutants. Because the input fuel may not be pure, this burning can create compounds of sulfur, mercury, and other toxins.
Fossil fuels have been known for decades to be major contributors of pollution. These fuels are more abundant (for now) and cheaper than alternatives, but are being gradually replaced where possible in the developed world. Replacement being not always possible or desirable, we will actually be using fossil fuels right up until they run out.
The Energy Information Agency of the Department of Energy projects that the world will need ever increasing amounts of energy sources:
Burning nighttime lights remind us of how much we rely on energy to live the way we do.