The temperatures of the Earth and any other planet depends mainly on (1) the amount of sunlight received, (2) the amount of sunlight reflected into space, and (3) the extent to which the atmosphere retains heat. Over the last two million years, changes in the timing and amount of sunlight striking the earth has been responsible for inducing ice ages (see glossary), during which temperatures were about 5°C (9°F) colder than today, and interglacial warm periods during which temperatures have been approximately the temperature of today. During the 20th and 21st century, however, the other two factors may be more important.
The water vapor and carbon dioxide found naturally in the atmosphere keep the earth warmer than it would otherwise be. Our clear atmosphere allows sunlight to penetrate to the earth's surface and warm it. The surface releases this energy as infrared radiation, which is absorbed by water vapor and CO2 in the atmosphere. This mechanism is commonly known as the "greenhouse effect." Without the greenhouse effect, the earth would be about 33°C (60°F) colder than it is currently.
Humanity is altering the energy balance of our planet by adding gases that absorb infrared radiation to the atmosphere, and thereby strengthening the greenhouse effect. The chief "greenhouse gases" are CO2, methane, and nitrous oxide. Whenever oil, coal, gas, or wood are burned, carbon dioxide is released into the atmosphere. Approximately half of the CO2 that is released is soon absorbed by the oceans or by increased plant photosynthesis. The other half remains in the atmosphere for many decades. As a result, the atmospheric concentration of CO2 is increasing. The average concentration of carbon dioxide has increased from around 275 parts per million before the industrial revolution, to 315 ppm when precise monitoring stations were set up in 1958, to 368 ppm in 1999. This change has increased the amount of energy striking the earth's surface by about 1.5 watts for every square meter of the earth's surface. This increased energy is equal to about 1 percent of the energy in the sunlight that reaches the earth's surface.
About two thirds of the current emissions of methane into the atmosphere result from cattle farming, rice paddies, landfills, coal mining, oil and gas production, and several other human activities. The other third comes from natural sources, particularly wetlands and termites. The total greenhouse effect from methane has increased by about 0.5 watts (0.3%) the energy striking each square meter of the earth's surface.
Several other gases collectively may have as much of a greenhouse effect as methane. Nitrous oxide (also known as "laughing gas") is released by the use of nitrogen fertilizers, the burning of wood, and some industrial processes. Higher levels of ozone, an urban pollutant regulated by EPA NAAQS, also add to the greenhouse effect. (The loss of ozone in the upper atmosphere tends to reduce the greenhouse effect.) Other gases with a greenhouse effect include chlorofluorocarbons (CFCs), hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), hydrochlorofluorocarbons (HCFCs), and sulfur hexafluoride (SF6)
Unlike CO2, methane, and other greenhouse gases, which remain in the atmosphere for decades or longer, most of the sulfates are removed by precipitation within a few weeks of being emitted. As a result, sulfates tend to be concentrated in the areas immediately downwind of major industrial areas. (There is more on sulfates in the Climate – Trends section.) The ability of sulfates to scatter light also causes visibility problems in the Grand Canyon and other scenic vistas. Sulfates also cause acid rain.