Power plants, like this one in Serbia and Monetenegro, contribute to the 6 billion metric tons of carbon dioxide we humans emit each year.

Ermal Meta/AFP/Getty Images

With every gallon of gas it burns, your car exhausts roughly 20 pounds of carbon dioxide (CO2) [source: United States Dept. of Energy]. With 531 million cars worldwide in 2002, that can add up to a lot of CO2 released into the atmosphere pretty quickly [source: World Watch]. This is not to mention the carbon resulting from all of the coal-fire power plants, cleared forests, cow manure from farms and other sources. In total, we humans emit somewhere around 6 billion metric tons of CO2 each year [source: U.S. Dept. of Energy].

It's not that the Earth can't handle a little carbon dioxide. Just because people can't breathe pure CO2 doesn't mean it's bad. Plants love the stuff, using carbon dioxide as fuel for photosynthesis and emitting precious oxygen as waste. Photosynthesis is one part of the carbon cycle, one of Earth's biogeochemical processes.

Through this process, the extant carbon on the planet is shuffled from one place to another. Soil, oceans and the atmosphere all store carbon temporarily. Along the way, living organisms ingest CO2, effectively making them storehouses as well.

But what happens when the surplus gets to be too much? We don't really know what will happen if carbon stores eventually become flooded, as it appears they will with the accelerated rate at which we're releasing carbon dioxide.

Keep in mind, your car isn't generating the carbon dioxide released into the atmosphere. It was stored in the oil drilled out of the ground, and in the gasoline refined from it. But by burning it for energy, humans release it.

So if we're releasing too much CO2 into the atmosphere, can't we just capture it and stash it somewhere? Yes. Read about some plans to do just that on the next page.