Water vapor added to the atmosphere by human activities has little effect on the greenhouse effect.
The concentration of carbon dioxide is much smaller than that of water vapor. As a result, the fairly large input of carbon dioxide from human activities can significantly affect the amount of heat trapped in the atmosphere . Atmospheric levels of carbon dioxide, CFCs, methane, and nitrous oxides have risen significantly in recent decades . The accumulation of CO 2 over the last century has added as much heat to the Earth's climate system as would a one half percent increase in the sun's energy output. The increase in these greenhouse gases is projected to increase the earth s natural greenhouse effect and lead to global warming .