Earth's climate is the result of a balance between the amount of incoming energy from the sun, and energy being radiated out into space.
Incoming solar radiation strikes Earth's atmosphere in the form of visible light, plus ultraviolet and infrared radiation (which are invisible to the human eye), according to the Earth Observatory of the National Aeronautics and Space Administration (NASA).
Ultraviolet (UV) radiation has a higher energy level than visible light, and infrared (IR) radiation has a weaker energy level. Some of the sun's incoming radiation is absorbed by the atmosphere, the oceans and the surface of the Earth.
Much of it, however, is reflected back out to space as low-energy IR radiation. For Earth's temperature to remain stable, the amount of incoming solar radiation should be roughly equal to the amount of IR radiation leaving the atmosphere.
As Earth's atmosphere changes, however, the amount of IR radiation leaving the atmosphere also changes. And since the Industrial Revolution, the burning of fossil fuels like coal, oil and gasoline have greatly increased the amount of carbon dioxide (CO2) in the atmosphere, according to NASA's Earth Observatory.
Along with other gases like methane and nitrous oxide, CO2 acts like a blanket, absorbing IR radiation and preventing it from leaving the atmosphere. The net effect causes the gradual heating of Earth's atmosphere and surface. [Related: Effects of Global Warming]
This is called the "greenhouse effect" because a similar process occurs in a greenhouse: Relatively high-energy UV and visible radiation penetrate the glass walls and roof of a greenhouse, but weaker IR radiation isn't able to pass out through the glass. The trapped IR radiation keeps the greenhouse warm, even in the coldest winter weather.
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