What is the most abundant greenhouse gas in our atmosphere?
Water vapor is around 0.4% of the atmosphere which is the highest of any greenhouse gas. The reason we never talk about water vapor is for a couple of reasons. One, the actual amount of water vapor can vary greatly depending on where you are. And two (and most important), the amount of water vapor in the air is not increasing at a dramatic rate.
The second most abundant greenhouse gas is carbon dioxide, at around 0.04%, and it is increasing at a dramatic rate due to human action.
Water vapor absorbs electromagnetic radiation in various bands in the infra-red region. Water vapor represents about 2% of the mass of the atmosphere, compared to about 0.04% for carbon dioxide. Greenhouse gases have no detectable effect on the Earth's temperature either at the surface or in the depths of the oceans.