Greenhouse Gases: Carbon Dioxide

Carbon Dioxide

Carbon Cycle

Carbon, used by all living organisms, continuously circulates in Earth’s ecosystem. In the atmosphere it exists as carbon dioxide, a colorless, odorless gas. Plants absorb carbon dioxide in the process of photosynthesis. Animals acquire the carbon stored in plant tissue when they eat, and they exhale carbon dioxide as a by-product of metabolism. Surface bodies of water, especially oceans, absorb vast amounts of atmospheric carbon dioxide. Some carbon is removed from circulation as deposits of coal, oil, natural gas, and limestone. The burning of fossil fuels adds additional carbon dioxide to the atmosphere.

      Carbon dioxide is the second most abundant greenhouse gas, after water vapor. Carbon dioxide constantly circulates in the environment through a variety of natural processes known as the carbon cycle. It is released into the atmosphere from natural processes such as eruptions of volcanoes; the respiration of animals, which breathe in oxygen and exhale carbon dioxide; and the burning or decay of plants and other organic matter. Carbon dioxide leaves the atmosphere when it is absorbed into water, especially the oceans, and by plants, especially trees. Through a process called photosynthesis, plants use the energy of light to convert carbon dioxide and water into simple sugars, which they use as food. In the process, plants store carbon in new tissue and release oxygen as a byproduct.

 Humans are significantly increasing the amount of carbon dioxide released to the atmosphere through the burning of fossil fuels (such as coal, oil, and natural gas), solid wastes, and wood and wood products to heat buildings, drive vehicles, and generate electricity. At the same time, the number of trees available to absorb carbon dioxide through photosynthesis has been greatly reduced by deforestation, the widespread cutting of trees for lumber or to clear land for agriculture.

Coal-Burning Power Plant

This power plant burns coal to produce electricity. The burning of coal and other fossil fuels releases carbon dioxide into the atmosphere. Carbon dioxide is the second most abundant greenhouse gas, after water vapor. Human activities have significantly increased the amount of greenhouse gases in the atmosphere, and scientists have linked this increase to global warming.

       Human activities are causing carbon dioxide to be released to the atmosphere much faster than Earth’s natural processes can remove it. In addition, carbon dioxide can remain in the atmosphere a century or more before nature can dispose of it. Before the Industrial Revolution began in the mid-1700s, there were about 280 molecules of carbon dioxide per million molecules of air (abbreviated as parts per million, or ppm). Concentrations of carbon dioxide have risen since then as industrial production and fossil fuel-based transportation and electricity generation have spread around the world, accelerating in the last 50 years. In 2007 the Intergovernmental Panel on Climate Change (IPCC), a major scientific organization, reported that levels of carbon dioxide had risen to a record high of 379 ppm and are increasing an average of 1.9 ppm per year.

       To stabilize atmospheric concentrations of carbon dioxide, global emissions would need to be cut significantly—on the order of 70 to 80 percent. If efforts are not made to reduce greenhouse gas emissions, carbon dioxide is projected to reach concentrations more than double or even triple the level prior to the Industrial Revolution by 2100. In a higher-emissions scenario carbon dioxide is projected to reach 970 ppm by 2100, more than tripling preindustrial concentrations. In a lower-emissions scenario, carbon dioxide is projected to reach 540 ppm by 2100, still almost doubling preindustrial concentrations. (For a description of these two emissions scenarios, see the Introduction: Global Warming in the Future section of this article.)