CARBON IN THE AIR: Carbon, in the form of carbon dioxide, is a greenhouse gas continually released into the atmosphere as a direct result of human activities. The concentration of atmospheric carbon dioxide has already increased by about 30% since the dawn of the industrial revolution in the 19th century. Most of this increase comes from the use of fossil fuel -- coal, oil and natural gas -- for energy, but approximately one-quarter of it can be attributed to changes in land use, such as the clearing of forests and the cultivation of soils for food production. Natural sources of atmospheric carbon include gases emitted by volcanoes, and respiration of living things, including people. We breathe in oxygen, and breathe out carbon dioxide.
ABOUT GLOBAL WARMING: Global warming refers to an increase in the earth's average temperature -- which has risen about 1 degree F over the past 100 years. A warmer earth may lead to changes in rainfall patterns, and a rise in sea level, for example, as polar glaciers melt. Some of this rise is due to the greenhouse effect: certain gases in the atmosphere trap energy from the sun so that heat can't escape back into space. Without the greenhouse effect, the earth would be too cold for humans to survive, but if it becomes too strong, the earth could become much warmer than usual, causing problems for humans, plants and animals.