Geology and geography: Subterranean forests

Graphic "Bursting at the seams"

Coal is formed from vegetation at high temperatures and pressures, cut off from the air. The older the coal, the more carbon and energy it contains. Deposits are located in all continents. A chapter from the Coal Atlas.

Coal is a brownish to black sedimentary rock made up of organic material. It was formed in the Carboniferous, a period that lasted 60 million years and spans from about 359 million to 299 million years ago. The name "Carboniferous" comes from "carbo", the Latin word for coal, because so much of this type of rock dates from this period. The Latin in turn comes from the presumed Indo-European word *ker, meaning "burn".

The climate was generally warm in the Carboniferous, and the atmosphere was richer in oxygen – 35 percent, compared to just 21 percent today. That stimulated the growth of plants. Vast forests spread over the land surface. A now-extinct tree known as lepidodendrales (from the Greek for "scale tree" after the appearance of their trunks) grew up to 40 metres tall.

Relatives of horsetails, now inconspicuous plants that grow on the edges of fields, reached 20 metres in height. Giant ferns formed massive swamp forests. All these plants accumulated large amounts of biomass. They used chlorophyll, the substance that makes leaves green, to use the energy from sunlight to convert carbon dioxide and hydrogen into organic material. They absorbed enormous quantities of greenhouse gases and turned them into lignin, resins and proteins.

When the vegetation died, the process of coal formation began. Many dead plants sank beneath the water, where they did not rot because of the lack of oxygen, but formed peat. Sediments such as clay or sand were deposited on top, raising the pressure and heat and squeezing out the water.

As the carbon content of the organic layers increased, the peat turned into denser, firmer lignite, or brown coal. Most deposits of this type date from 40 to 50 million years ago, from the Palaeogene period, formerly known as the Tertiary. Lignite has a moisture content of 45 to 60 percent. The remains of vegetation, such as roots, can still be seen in some pieces of lignite. Hard coal is much older – around 250 to 350 million years old. Lumps of this coal still bear the imprints of past vegetation. Most hard coal has a moisture content of 15 to 20 percent.

The more carbon coal contains, the more energy and the higher its calorific value – its value as fuel. So hard coal is preferable to brown coal. The best type is known as anthracite, which contains very little water or other ingredients. The only minerals that have more carbon are graphite and diamond, which are both usually of volcanic origin.

Heavy industry loves anthracite. It can contain more than 90 percent carbon

Ultimately, coal is energy from the sun, preserved in the form of plant remains. The historian Rolf Peter Sieferle refers to coal as a “subterranean forest”. Along with oil and natural gas, lignite and hard coal are fossil fuels. The term “fossil” indicates that they were formed from organic materials in the geological past. Coal and lignite come from vegetation; oil and natural gas are the remains of tiny organisms that were deposited on the sea floor. They were formed between 400 and 100 million years ago – at around the same time as hard coal. More recent deposits, such as those in the North Sea, were, like lignite, formed in the Palaeogene.

The German Federal Institute for Geosciences and Natural Resources estimates the world’s coal reserves at 968 gigatonnes (968 billion tonnes). It classifies reserves as deposits that can be exploited economically and profitably using current technology. In 2013 alone, humankind mined and burned 8 gigatonnes, or 253 tonnes every second. In addition to the reserves, the Earth has vast deposits of coal that have been proven but are currently uneconomic to exploit. Altogether, it is estimated that global deposits of lignite and hard coal may amount to 22,000 gigatonnes.

Hard coal takes a mere 60 million years to form - a brief period on a geological timescale

The largest deposits of the economically more important hard coal are found in Asia, Australia, North America and the Commonwealth of Independent States, an organisation of former Soviet Republics. The United States has the biggest reserves of hard coal and anthracite, with 223 gigatonnes. China comes next, with 121 gigatonnes, followed by India, with 82. In 2013, China dug up 3.7 gigatonnes of hard coal, more than half the world’s total output. The United States followed with 12 percent, and then India, with 8 percent. About 20 percent of the world’s hard coal output is traded internationally.

Once upon a time, a map of coal deposits reflected natural wealth. Now i shows where problems may lie

Lignite, on the other hand, is difficult to transport and contains less energy, so it is used as fuel only in the immediate vicinity of the open-cast mines where it is extracted. Some 37 countries around the world exploit lignite, but only eleven account for 82 percent of worldwide production. The biggest producer in 2013 was Germany, with 183 million tonnes, followed by China and Russia. Germany’s lignite production has risen sharply after the country’s move away from nuclear power. This has significantly worsened its carbon footprint. In 2014, renewables overtook lignite as Germany’s most important source of energy, but only by a small margin.

Unlike oil, there is no official shortage of coal. In the long term, output will decline because the atmosphere can absorb only so much carbon dioxide. However, the Energy Watch Group, an international network of specialists, thinks that official estimates of coal reserves are too high. The global estimates are continually being revised downwards – between 1980 and 2005 by about half, despite higher figures for India and Australia. The group expects we will reach peak global coal production as soon as 2020.