Carbon dioxide in the atmosphere: 1  allows sunlight and heat into the atmosphere, but doesn't let it get out, causing the Earth to heat up. 2  along with water vapor, methane, and a few other gases keep the Earth from becoming a deep freeze on the side away from the Sun. Venus and Mars have about 95% carbon dioxide in their atmospheres, but the atmosphere of Venus is very much thicker. Earth has only a small amount of carbon dioxide, but it's enough to have a greenhouse effect.