Tens of billions of metric tons of carbon are exchanged every year among the Earth's plants, animals, land, oceans and air in complex cycles. In the most familiar cycle, people exhale carbon dioxide as a byproduct of turning food into energy for their bodies. Green plants take CO2 in from the air to build their cells. Most scientists are convinced that human activities are upsetting the balance of those cycles and increasing the amount of heat-trapping carbon dioxide in the atmosphere. This illustration shows why the planet's natural cleansing cycle may not be able to compensate for burning fossil fuels. A car that consumes 600 gallons of gasoline a year, by traveling 15,000 miles at an average of 25 mpg, puts about 12,000 pounds of carbon dioxide into the air. In contrast, a typical tree takes in an estimated 50 pounds a year for its own growth. (KYLE RAETZ)
