Petroleum

petroleum, complex mixture of hydrocarbons that occur in Earth in liquid, gaseous, or solid form. The term is often restricted to the liquid form, commonly called crude oil, but, as a technical term, petroleum also refers to natural gas and the viscous or solid form known as bitumen, which is found in tar sands. The liquid and gaseous phases of petroleum constitute the most important of the primary fossil fuels.

Liquid and gaseous hydrocarbons are so intimately associated in nature that it has become customary to shorten the expression “petroleum and natural gas” to “petroleum” when referring to both. The first use of the word petroleum (literally “rock oil” from the Latin petra, “rock” or “stone,” and oleum, “oil”) is often attributed to a treatise published in 1556 by the German mineralogist Georg Bauer, known as Georgius Agricola. However, there is evidence that it may have originated with Persian philosopher-scientist Avicenna some five centuries earlier.

The burning of all fossil fuels (coal and biomass included) releases large quantities of carbon dioxide (CO₂) into the atmosphere. The CO₂ molecules do not allow much of the long-wave solar radiation absorbed by Earth’s surface to reradiate from the surface and escape into space. The CO₂ absorbs upward-propagating infrared radiation and reemits a portion of it downward, causing the lower atmosphere to remain warmer than it would otherwise be. This phenomenon has the effect of enhancing Earth’s natural greenhouse effect, producing what scientists refer to as anthropogenic (human-generated) global warming. There is substantial evidence that higher concentrations of CO₂ and other greenhouse gases have contributed greatly to the increase of Earth’s near-surface mean temperature since 1950.