Human beings add CO 2 to the atmosphere mainly by burning fossil fuels like coal and oil. Deforestation is the second major way we increase atmospheric CO 2 . Felled timber releases CO 2 as it burns or decays, and disturbed soils produce CO 2 from burned organic matter. Forests give way largely to annual crops that store CO 2 for only a season, or to cities with little vegetation at all. Besides the atmosphere, Earth has several other reservoirs of CO 2 including vegetation and the oceans.
Oceans constitute by far the largest reservoir, however.
Atmospheric CO 2 dissolves into surface waters, and over time, ocean circulation carries it to great depths and distant locations. Eventually, biochemical processes may turn it into shells and sediments that drift toward the bottom of the oceans. Because high levels of greenhouse gases may have a significant effect on climate, vegetation, and other aspects of our lives, it is important to determine when major increases will occur. Predictions become more complicated when they take other greenhouse gases into account. While increasing CO 2 is expected to be the main factor in enhancing Earth's atmospheric greenhouse, gases like methane, nitrous oxide and the CFCs almost certainly will contribute to changing climate as well.