evidence: What about water vapor? Water vapor is the most abundant heat-trapping gas, but rarely discussed when considering human-induced climate change. The principal reason is that water vapor has a short cycle in the atmosphere (10 days on average) before it is incorporated into weather events and falls to Earth, so it cannot build up in the atmosphere in the same way as carbon dioxide does. However, a vicious cycle exists with water vapor, in which as more CO2 is emitted into the atmosphere and the Earth’s temperature rises, more water evaporates into the Earth’s atmosphere, which increases the temperature of the planet. The higher temperature atmosphere can then hold more water vapor than before. [...] Long-wave radiation absorbed by these gases in turn is re-emitted in all directions, including back toward Earth, and some of this re-emitted energy is absorbed again by these gases and re-emitted in all directions.