Global warming and related shifts in climate are driven by increased levels of common gases in our air. These gases are already in the air (or, atmosphere) in certain quantities at levels that typically remain stable. These common gases are carbon dioxide (CO2, or the bubbles in soda pop and the air we exhale), methane (CH4, cow burps and cooking gas), nitrous oxide (N2O, laughing gas), and water vapor (H2O, humidity). When we increase the concentrations of any of these chemicals in the air, it causes the atmosphere to trap more heat, much like insulation in a house. The only problem is that we cannot turn down the source of the heat (in this case, the sun), like we can with a thermostat in a house.
Though water vapor, nitrous oxide, and methane have a much stronger insulating effect than does CO2, those chemicals will break down into other chemicals much more quickly than CO2. While the other greenhouse gases only take weeks or months to change forms, thus quickly reducing their warming effects, CO2 takes two to four centuries to break down if left alone in the air. CO2's sustained presence in the air is what allows for the atmosphere to warm up and stay warm, even after the other gases break down. This is why reducing atmospheric CO2 is such an urgent matter; even if we stop putting more CO2 into the atmosphere right now, it will still take at least 200 years for present levels to come down significantly. The more CO2 we put into the air, the longer the planet will be warmer.
For more information on greenhouse gases: