Water vapor (H₂O) is the most abundant of all these gases, so it's considered most important. The amount of water vapor in the air is limited by the air temperature (for a particular air temperature, there is a maximum amount of vapor that can be in the air; any in excess of that amount immediately condenses back out as liquid water or ice). Carbon dioxide is the second most abundant of these greenhouse gases, and has numerous, large anthropogenic sources. The other greenhouse gases have smaller source rates and atmospheric concentrations, but are still important because they can potentially produce more greenhouse warming on a per-molecule basis.