Coal-burning power plants, car exhausts, factory smokestacks, and other man-made waste gas vents give off about 23 billion tons of carbon dioxide and other greenhouse gases into the Earth's atmosphere each year. The amount of CO in the air is about 31% more than it was around 1750. About three-quarters of the CO that people have put in the air during the past 20 years are due to burning fossil fuel like coal or oil. The rest mostly comes from changes in how land is used, like cutting down trees. 