What Causes Global Warming?

There are several different existence theories pertaining to the causes of global warming. Some say it is due to the natural course of the world, why others say the fault lies with us, the inhabitants of the earth.
Scientists have spent decades trying to find out what causes global warming. They have looked at the natural cycles and events that have influenced nature. The US has created a group (IPCC) that learned of the various greenhouse gases, emitted by humans, which causing global warming. 
It has been found that the gas responsible for the most warming is carbon dioxide. Carbon dioxide is emitted by both humans and animals.