Coal is a black or brownish, readily combustible, rock-like substance that consists mostly of carbon, with varying amounts of other elements, mainly hydrogen, sulfur, oxygen and nitrogen. It was formed from plant remains that were buried, compressed, hardened, chemically altered, and metamorphosed by heat and pressure within the earth's crust over a span of millions of years. It is found in numerous locations throughout the world and has been utilized in small quantities since ancient times.
Accompanying the Industrial Revolution and the invention of the steam engine, the demand for coal grew rapidly. Today its main use is to generate electricity, and it accounts for about a third of the world's total electricity production. It is also used in some industrial processes, particularly iron and steel making.
The large-scale mining and burning of coal has been extremely damaging to the natural environment and human health. Foremost is its release of massive quantities of carbon dioxide into the atmosphere, thus contributing greatly to the greenhouse effect and climate change. Coal-fired electric power plants account for toughly 40 percent of the total fossil fuel emissions. Coal burning also results in large emissions of other harmful substances, including sulfur dioxide and mercury. Coal mining can emit methane, a far more potent greenhouse gas than carbon dioxide, and a gas which is also highly hazardous to the miners.
Moreover, the burning of coal creates vast amounts of coal ash and other toxic wastes, which can contain small but dangerous amounts of mercury, thorium, uranium, arsenic and other heavy metals. The ever-growing accumulations of these wastes are usually stored locally but can escape into the air, water and soil and harm nearby populations.
Although the harmful effects of coal have been known for decades, coal not only is still being burned on a massive scale, but also its total consumption for the world as a whole is continuing to increase. The coal industry in major countries has vigorously fought attempts to reduce or eliminate its use and continues to try to sway public opinion, including through the use of the deceptive phrase "clean coal."