There are lots of different greenhouse gases – including water vapor, methane, nitrous oxide, and, of course, carbon dioxide. So why is everyone so concerned about just carbon dioxide – why aren’t we focusing on the other gases? Part of it has to do with global warming potential, which is a measure of how much heat energy is absorbed by each gas and how long those gases stay in the atmosphere.
The above is a graph of the different greenhouse gases and how concentrated they are in the atmosphere over time – found at http://commons.wikimedia.org/wiki/File:Annual_greenhouse_gas_index,_1979-2008_%28EPA,_2010%29._Indicator_of_radiative_forcing.png.
Greenhouse gases become a problem when they absorb a lot of energy and stay in the atmosphere for a very long time, allowing a single molecule of the gas to absorb far more heat as time goes on.
For example, water vapor is one of the most potent greenhouse gases because it absorbs a lot of heat (far more so than carbon dioxide), but its concentration in the atmosphere changes almost daily as atmospheric temperatures change and precipitation happens. Therefore, it does not have time to sit in the atmosphere and collect heat, making the atmosphere warmer. (However, there is some evidence that if temperatures keep increasing due to other gases, like carbon dioxide, more water vapor will collect in the atmosphere, further exacerbating global warming – this is known as a positive feedback).
On the other hand, nitrous oxide stays in the atmosphere for a long time (about 114 years) and can absorb a lot of heat in that time, making it even more potent a greenhouse gas than carbon dioxide. To see a full list of global warming potentials from the UN, go here: http://unfccc.int/ghg_data/items/3825.php.
So, if there are much more potent greenhouse gases, why focus on carbon dioxide? Because carbon dioxide is a potent greenhouse gas and it is increasing in concentration far faster than the other gases, making it a big problem. There have been other efforts to reduce some of the other gases (the Montreal Protocol banned in some countries CFC’s and other gases with high global warming potential), but at the moment it seems that carbon dioxide is the biggest concern.