Global warming is a result of heat trapping by atmospheric greenhouse gases such as carbon dioxide and methane. Cars, trucks and planes powered by fossil fuels emit carbon dioxide and contribute to the enhanced greenhouse effect seen in recent decades. Almost a third of the greenhouse gas emissions due to human activity come from transportation. We contribute greenhouse gases not only by transporting ourselves, but also by buying food and other products that have to be shipped by trucks or ships that burn fossil fuels.
Electric power plants that use fossil fuels contribute about a third of all human-produced greenhouse gases to the atmosphere. Plants that use coal emit more greenhouse gases per unit of electricity than those that use natural gas. Greenhouse gases are also a byproduct of industrial processes such as those used in the production of cement, steel and aluminum. Most of these emissions are in the form of carbon dioxide. Other emissions include methane, nitrous oxide and chlorofluorocarbons (CFCs).
Global agriculture contributes about 20 percent of human-generated greenhouse gases to the atmosphere. Though other sources primarily emit carbon dioxide, agricultural sources also emit a lot of methane and nitrous oxide, both of which are far more potent greenhouse gases than carbon dioxide. Direct agricultural sources of greenhouse gas emissions include emissions from field machinery, manure management and production of methane by cows. Indirect sources are production processes for nitrogen fertilizers as well as transportation related to farming.
People contribute to global warming not only by emitting greenhouse gases but also by cutting down trees that absorb carbon dioxide. Widespread deforestation worldwide for purposes such as building, logging, farming and grazing is responsible for 25 percent of the carbon emissions generated by human activity. Individual choices, such as the choice to eat beef or to live in the suburbs, indirectly affect the level of deforestation.