The greenhouse effect is the trapping of heat by a layer of gases surrounding the Earth. The heat is trapped by atmospheric gases such as carbon dioxide, methane, water vapor and nitrous oxide.
The heat coming from the sun that enters Earth's atmosphere is reflected back toward space. This reflected heat is redirected back into the atmosphere, where it is held by the layer of greenhouse gases. Because of the greenhouse effect, Earth is warm enough that water can exist as a liquid.
In theory, the amount of greenhouse gases should remain constant; however, human activity has contributed to an increased amount of greenhouse gases, such as carbon dioxide, in the atmosphere. This increase in greenhouse gases may lead to global warming, a rise in average temperatures around the world.