Greenhouse gases include carbon dioxide, methane, and water vapor. These gases are naturally found in small amounts in our atmosphere. However, over the last century, atmospheric concentrations of these gases have been increasing, reaching levels that exceed historic concentrations.

Climate experts see a link between greenhouse gas concentrations and global temperatures because these gases are responsible for keeping Earth warm in the first place. Carbon dioxide, water vapor, methane, nitrous oxide, and a few other compounds readily absorb infrared radiation (heat) coming from Earth's surface, essentially trapping the heat at the Earth's surface. In the proper balance, greenhouse gases make life on Earth possible. However, any increase in the concentration of these gases poses the risk of altering the natural balance and dramatically changing global temperatures.