Greenhouse gases absorb solar heat as it is bounced from the Earth toward space, subsequently re-radiating the heat in all directions. The constant absorbing and radiating of heat serves to retain the heat in the atmosphere for longer periods of time. Greenhouse gases produce the greenhouse effect, which keeps the atmosphere warm and stable enough to support terrestrial life. The increase in greenhouse gases due to human-caused emissions causes an enhanced greenhouse effect, gradually making the planet warmer. Greenhouse gases include carbon dioxide, water vapor, methane, nitrous oxide and chlorofluorocarbons.
During the past 150 years, the carbon dioxide concentration in the atmosphere has increased from 280 to 390 parts per million. Most human carbon dioxide emissions result from the burning of fossil fuels, including oil, coal and natural gas. According to the U.S. Energy Information Administration, combustion of fossil fuels accounts for over 80 percent of annual greenhouse emissions from human sources within the United States. The United States produces around 25 percent of global carbon dioxide emissions from fossil fuels.
Livestock reared for human consumption contribute significantly to the greenhouse gas emissions, representing a greater percentage of annual emissions than the transportation sector. Livestock accounts for approximately 9 percent of total carbon dioxide emissions. Animal manure produces over half of human-caused nitrous oxide, which is a much more harmful greenhouse gas than carbon dioxide. Ruminants also produce large quantities of methane during the digestive process, representing nearly 40 percent of human-caused emissions.
Deforestation also contributes to global warming, representing 25 percent of greenhouse gas emissions worldwide. The destruction of forests releases the carbon that is stored within trees and forest plants. Logging, agriculture and ranching are contributing to heavy deforestation in tropical regions, particularly in Indonesia, Brazil and Africa. Deforestation, in addition to releasing huge quantities of atmospheric carbon, also reduces the planet's capacity to absorb future carbon emissions.