There are a number of theories about this which include CO2 emissions (that are in higher concentration now than when the world population and industry was much smaller) from businesses, automobiles, planes, and cows. It is a topic of wide debate because there are also a number of theories that suggest that CO2 levels have simply spiked regardless of human activity in our Earth's past. You may find this article interesting if you would like to know more about global warming and the human element: www.guardian.co.uk It is based on Mark Linas' book entitled: Six Degrees: Our Future on a Hotter Planet
