The gaseous phase of water. It is the greenhouse gas that is the largest contributor to the greenhouse effect.
1. "The global warming problem remains enormously complex because the atmospheric concentration of the most important greenhouse gas, water vapour, can't be specified."
(Philander, Is the Temperature Rising?, 197)
2. It is added to the atmosphere by the evaporation of water or by the sublimation of ice; it is removed by condensation.
3. For every 1°C rise in sea surface temperature, air can hold another 7% of evaporated moisture.
4. Click here to read Real Climate's article Water vapour: feedback or forcing?.
Extract: "Long-wave (or thermal) radiation is emitted from the surface of the planet and is largely absorbed in the atmosphere. Water vapour is the principle absorber of this radiation (and acknowledged as such by everybody). But exactly how important is it? In terms of mass, water vapour is much more prevalent (about 0.3% of atmospheric mass, compared to about 0.06% for CO2), and so is ~80% of all greenhouse gases by mass (~90% by volume). However, the radiative importance is less (since all molecules are not created equal)."
5. Click here to read the IPCC TAR report on water vapour feedback.