Water vapor is the most present greenhouse gas on Earth. It’s responsible for about half of the Earth’s greenhouse effect, the process that occurs when gases in Earth’s atmosphere trap the Sun’s heat. Water vapor is also a key part of Earth’s water cycle: the path that all water follows as it moves around Earth’s atmosphere, land, and  ocean as liquid water, solid ice, and gaseous water vapor. For every degree Celsius that Earth’s atmospheric temperature rises, the amount of water vapor in the atmosphere can increase according to the laws of thermodynamics. 

It works like this: As greenhouse gases  increase, Earth’s temperature rises in response. This increases evaporation from both the Earth’s areas. Because warmer air holds more moisture, its concentration of water vapor increases.The water vapor then absorbs heat radiated from Earth and prevents it from escaping out to space. This further warms the atmosphere, resulting in even more water vapor in the atmosphere. This is what scientists call a "positive feedback loop."

The greenhouse gases in the dry air in Earth’s atmosphere include carbon dioxide , methane , nitrous oxide, ozone, and chlorofluorocarbons. All five of these greenhouse gases are non-condensable meaning that the atmospheric temperatures change, the concentration of non-condensable gases  remains stable. The water vapor is  condensable gas that can be changed from a gas into a liquid. This make it the only  greenhouse gas whose concentration increases because the atmosphere is warming, and causes it to  warm even more.