The primary cause of global warming is the collection of carbon dioxide in the atmosphere. Carbon dioxide and other pollutants trap heat from the sun, making the planet warmer.
In the United States, the two biggest sources of carbon dioxide pollution are coal-burning power plants and automobiles. Coal-burning power plants produce 2.5 billion tons of carbon dioxide pollution each year, according to the Natural Resources Defense Council. NRDC experts say automobiles produce another 1.5 billion tons of carbon dioxide pollution each year.
Deforestation, which is the clearing of rainforest ecosystems, contributes to carbon dioxide pollution. Plants absorb carbon dioxide and use it to produce their own food. When plants are removed from an ecosystem, less carbon dioxide is absorbed.
Ozone depletion and the presence of aerosols in the atmosphere are also responsible for global warming. Stratospheric ozone protects living organisms from the sun's ultraviolet rays. Ozone depletion allows more UV radiation to reach the Earth, damaging the environment and putting people at risk for skin cancer and other health problems.
Atmospheric aerosols absorb infrared radiation, warming the planet. These aerosols also alter the physical and chemical properties of clouds. Industrial and agricultural processes produce aerosols in many ways. Automobile emissions also contain aerosols as well as chemicals that are converted into aerosols by chemical reactions.