The greenhouse effect is a natural process that warms the Earth's surface. When the sun's energy reaches the Earth's atmosphere, some of it is reflected back to space and the rest is absorbed. The absorbed energy warms both the atmosphere and the earth's surface, both of which then emit heat back toward space as longwave radiation. This outgoing longwave radiation is partially trapped in the atmosphere by greenhouse gases such as carbon dioxide, methane and water vapour which then radiate the energy in all directions, further warming the Earth's surface and the lower levels of the atmosphere. Without these greenhouse gases the Earth's average surface temperature would be about 35 degrees Celsius cooler.

The enhanced greenhouse effect refers to the addition of more heat trapping gases to the atmosphere. The extra greenhouse gases are not 'natural' in origin, and are primarily due to the burning of fossil fuels (coal, oil and natural gas), with smaller contributions from land use changes, such as land clearing. The enhanced greenhouse effect is contributing to a warming of the Earth's surface.