Before the Industrial Revolution, the amount of CO₂ released to the atmosphere was about equal to the amount that the atmosphere could absorb. However, due to the increased burning of fossil fuels, the CO₂ levels have risen drastically, and the atmosphere is getting an excess CO₂.  Half of the excess is being absorbed by oceans and plants, the other half is just accumulating in the atmosphere.

This creates what is known as "The Greenhouse Effect". The Greenhouse Effect is a natural occurrence whereby the greenhouse gases allow sunlight to enter the Earth's atmosphere, but trap some of the outgoing infrared radiation, warming the Earth's surface more than what would occur otherwise.

Since 1978, the energy reaching Earth has been measured by satellites. These measurements have not increased significantly since 1978, meaning the warming is not coming from an increase in sunlight. Also, there have not been any significant increases or decreases in the numbers of volcanic eruptions. (Volcanic eruptions reflect sunlight back to space, creating cooling). Therefore there is no known naturally occurring reason why the Earth should be warming.