Gasses that absorb in the infrared part of the spectrum have more potential for greenhouse effect. Although Nitrogen (N₂) and Oxygen (O₂) comprise roughly 99.96% of our (assumed dry) atmosphere by volume, they have almost no influence in the infrared range. Water vapor (H₂O), Methane (CH₄), Carbon Dioxide (CO₂) and some others have significant infrared impact. Their net effect on the atmosphere depends primarily on their concentration.

Water vapor can absorb a tremendous portion of infrared radiation with sufficient concentration. The proportion of water vapor in the atmosphere varies from 0% to 4%. Differences over time and location are significant. The absorptive characteristics and volume of the atmosphere make water vapor the most significant greenhouse gas by orders of magnitude.

Methane has significant infrared absorptive characteristics, similar to water vapor, and it covers the portion of the spectrum that water does not, which can contribute to warming even in areas saturated with humidity. However, the proportion of methane in the atmosphere is about 0.0017% which significantly limits its impact, especially compared to water's average of 2.0%. These are orders of magnitude differences.  

Carbon Dioxide has some ability to absorb infrared radiation, but much less than water vapor. Given that Carbon Dioxide comprises approximately 0.035% of the atmosphere; its relative impact compared to water vapor (average of 2.000%) would appear to be much less than that of water vapor by several orders of magnitude.

There are differences between the spectrums for water vapor and Carbon Dioxide. Here they are plotted together to show how how little that Carbon Dioxide adds to the spectral cover already in place by the more dominant water vapor.