A hydrocarbon is a substance that is composed of hydrogen and carbon. A pure hydrocarbon consists of only carbon and hydrogen atoms, while an impure one also contains one or more other elements, such as nitrogen and sulfur. There are numerous forms of hydrocarbons, differing in the number and arrangement of the atoms in the molecules, and thus differing greatly in their chemical and physical properties. Some are merely simple, long chains of carbon and hydrogen atoms, while in others the atoms are arranged in elaborate rings and other shapes. Also, the types of bonds between individual atoms in organic molecules can differ, and often many of the atoms are joined together with multiple bonds.
Hydrocarbons formed over hundreds of millions of years from the decay, pressurization and heating of vast quantities of organic matter, and they are abundant in the earth's crust in the forms of petroleum, coal, and natural gas. Although they have been known and used to a small extent since ancient times, it has been only in the past two centuries or so that these fossil fuels came to be used on a large scale. In fact, the scale is so large and the number and range of applications so great that hydrocarbons are widely regarded as fundamental to modern civilization. Indeed, it can be difficult to imagine how a complex, highly interconnected global civilization could survive without them.
However, as attractive as their vast array of applications may seem, there are also extremely severe disadvantages to the large scale extraction and use of hydrocarbons. That is, they are the dominant source of greenhouse gases and the consequent climate change, they are the main cause of ocean acidification, and they are largely responsible for the increasingly widespread poisoning of the land. Thus, as difficult as it may seem, it will be necessary to greatly reduce the use of hydrocarbons in order to solve these problems. This decarbonization can be accomplished in part by both reducing energy consumption and developing alternative energy sources. Also necessary will be a reduction in the consumption of raw materials together with developing an extensive range of materials that are biodegradable.