Carbon is the fourth most abundant chemical element in the universe by mass after hydrogen, helium, and oxygen, but it constitutes only about 0.025 percent of the earth's crust, making it the 15th most abundant element in the crust.
Carbon, along with hydrogen, is a core element in all organic compounds, which are the basis for all known life. It is the second most abundant element in the human body by mass at about 18.5 percent, after oxygen. Plants consist of about 45 percent carbon, which they obtain from carbon dioxide that they absorb from the atmosphere.
The earth's carbon is stored in several forms in addition to atmospheric carbon dioxide and the bodies of living animals and plants, mostly in limestone, dolomite, methane hydrates, coal, petroleum, peat and natural gas.
Carbon has been increasingly in the news in recent years, especially with regard to carbon capture, carbon sequestration and carbon taxes, because of the rapid growth of carbon dioxide in the atmosphere and mounting awareness of the dramatic effects it will have on the world's climate.