Greenhouse gasses have different average lifetimes and different capacity to trap infrared radiation (heat).
To compare the different warming effects of the various gases they are converted into a unit called carbon dioxide equivalents (CO2-e). This unit represents the amount of CO2 that would generate the equivalent warming effect, usually rated over a 100 year span.
For example, over 100 years, per mass, methane is listed as 25 times stronger as a greenhouse gas, nitrous oxide is 298 times stronger, and typical halocarbons are many thousands of times stronger. This is why comparatively small releases of non CO2 gases become significant in warming terms.
Therefore methane would have a GWP of 25 and nitrous oxide would be 298.
This is known as its Global Warming Potential (GWP) or sometimes referred to as a GHG emissions factor.
These numbers have been reviewed by the United Nations Intergovernmental Panel on Climte Change (IPCC).
The current GWP rankings are listed in Section 2.10 of the IPCC Fourth Assessment Report: Climate Change 2007 at: http://www.ipcc.ch/publications_and_data/ar4/wg1/en/ch2s2-10-2.html