People produce a great deal of carbon dioxide (CO2) from many different sources, and most of this CO2 ends up in the atmosphere. As of 2002, for example, there were 531 million cars worldwide [source: World Watch]. Each of those cars, on average, produces 20 pounds of carbon dioxide for every gallon of gas it consumes [source: United States Dept. of Energy]. A great deal of our electricity is generated by coal-fired power plants as well, and they are are some of the worst polluters. When you factor in other sources of CO2, such as cow manure and cleared forests, the U.S. Department of Energy estimates that humans are responsible each year for emitting nearly 6 billion metric tons of carbon dioxide.
It's probably worth noting that CO2 is released in natural ways too. In addition to our cars and coal plants, there is a natural "carbon cycle" found in the animal and plant kingdoms as well as in the oceans. These processes create natural greenhouse gases [source: EPA].
CO2, of course, is at the heart of heating up the Earth, so emissions of it matter, especially to the animal kingdom, which takes a hit when this happens. Warming trends have been a major cause of destabilization of animal life for hundreds of years. When global temperatures rise, the natural habitats of many animals are thrown into chaos. Food can become scarce, new predators can enter the fray and sometimes the mere temperature itself rises high enough to kill an animal. Some animals have even been forced to migrate to new habitats entirely when their original homes became inhospitable, either due to rising temperatures or human interference. Some scientists believe that if global temperatures rise, by 2050 almost a third of the plant and animal species alive on Earth today run a significant risk of extinction [sources: EPA, Scientific American].
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