Environmentalists believe that the 20th-century warming was caused by human activity, primarily the burning of fossil fuels. That produces carbon dioxide—one of several "greenhouse gases." The argument is that their release into the atmosphere wraps the Earth in an invisible shroud. This makes the escape of heat into outer space slightly more difficult than its initial absorption from sunlight. This is the Greenhouse Effect. So the Earth warms up.
Read more at http://spectator.org/articles/55208/false-alert-global-warming