The National Wildlife Federation explains that global warming is thought to be caused by the greenhouse effect, which is the result of certain gases in the atmosphere trapping the heat of the sun and preventing it from radiating back into space. There are many gases responsible for this effect, including water vapor, carbon dioxide and methane. Artificially-produced chlorofluorocarbons, known as CFCs, are extremely effective greenhouse agents, molecule for molecule, but are present only in small concentrations.
Much of the sun's light passes through the earth's atmosphere and strikes the ground. The light that's reflected back up is often of a different wavelength than the incoming light. Climate Data Information explains that these wavelengths can be absorbed by molecules of greenhouse gases, which raises the temperature of the air. Water vapor, caused by evaporation, acts over a broad range of wavelengths. Carbon dioxide and methane are just as effective at absorbing energy, but over a shorter part of the spectrum.
Climate Data Information shows how some of these gases occur naturally. Methane is a natural breakdown product of certain bacteria, and water vapor enters the air naturally as part of the water cycle. Recently, however, human agriculture and industry have dramatically increased the concentrations of these gases in the air. It is estimated that since the beginning of the Industrial Revolution, humans have emitted roughly 500 billion tons of carbon dioxide, approximately half of which remains in the atmosphere. These gases are the major cause of warming over the last century, according to the National Wildlife Federation.