Coal is the largest source of human energy worldwide. It is a fossilized carbon, that forms from plant matter, undergoing many geological changes over a period of millions of years. The carboniferous period ≈ 300 million years ago is when the vast majority of coal was formed, though it has formed in most other time periods. There are various forms of coal (8 rough classifications) with four primarily used as fuel: Sub-bituminous coal, Bituminous coal, "Steam coal", and Anthracite. Each is classified by the amount of volatiles contained within the coal itself, with the "best" quality coal containing the fewest amount of volatiles.
(CC Image (right) from USGS [Public domain], via Wikimedia Commons)
Coal is also the largest contributor to worldwide carbon dioxide (CO2) emissions, which contributes enormously to climate change. Furthermore, burning coal releases toxic and dangerous elements into the atmosphere. Coal acts as a filter in the Earth and binds to heavy metals such as mercury, as well as, selenium and arsenic and the process of burning coal releases these back to the environment and the air we breathe. Burning coal also creates fly ash byproducts that contain throrium, uranium, and other toxic elements. Newer regulations and "clean coal" technologies typically filter these elements to varying degrees before being released into the air. High sulfur coal is also a leading cause of acid rain.
Mining and burning of coal has been definitively linked to an increase in lung cancer and breathing disorders around the world. It's health effects on the greater environment are extensive as well. The Appalachian mountain range is rich with coal seams, and with new technology it has become much "safer" and "cheaper" to use explosives to blow-off the surface obstructions (trees, animals, soil, sedimentary rocks, etc.) to access the coal this way.
(CC Image (left) of mountaintop removal in West Virginia taking place by Roston (Own work) [Public domain], via Wikimedia Commons)
Known as mountaintop removal (MTR), this has led to the creation of what some call "moonscapes". This manner of coal removal obliterates local ecosystems and water sources often become contaminated or lost in the process. Headwaters often become buried in "valley-fills" where the removed land and materials are dumped into adjacent valley's for disposal. This process can lead to land slides, contaminated water, loss of water, and loss of habitat, as well as a ruined aesthetic aspect to the land.
While the negative externalities and direct effects of mining and burning coal on humans health, biodiversity loss, deforestation, pollution, and climate change are well documented and clear, there is a long history of using coal, with little evidence showing a slow-down or stopping anytime soon. This is because coal is an abundant and cheap form of energy. Coal has been used commonly for hundreds of years for heating, steam generation, industrial applications, and of course electricity generation.
(CC Image (right) of valley fill, by Flashdark (Own work) [Public domain], via Wikimedia Commons)
This fact has not changed today. While our understanding of burning coal and its effects have advanced in recent decades, the political and social motivation has wavered only slightly to change. Our dependence on coal is reflective of our dependence on energy. A dependence that is driven by our capitalistic consumer economy that seeks competitive advantage in order to align the markets supply and demand. Until regulations restrict coals use, or perverse subsidies are changed to drive innovation into renewable energies, conservation, and limits to growth are realized, it is not likely we will see coal use decline, until coal reserves (like all fossil fuels) run out. This has been estimated to be in ≈ 140 years.
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(CC Image of major countries coal consumption by Plazak (Own work) [CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons)