Water vapor is the gaseous phase of water. It is abundant in the earth's atmosphere and is also present on every other planet in the solar system as well as in some moons, comets and even large asteroids.
Water vapor is a key part of the earth's water cycle. Its atmospheric concentration varies greatly according to the location and time, from close to zero in the coldest and driest air to about five percent in humid tropical regions. The total amount of water vapor in the atmosphere is sufficient to cover the entire surface of the earth with a layer of liquid water several centimeters deep.
The water vapor content of the atmosphere is being continuously depleted by precipitation, but it is also simultaneously being replenished, mostly by evaporation from oceans, lakes, rivers, other water bodies and moist earth, but also from combustion, volcanic eruptions, respiration, transpiration and other geological and biological processes.
While in the atmosphere, water vapor acts as a greenhouse gas and contributes more to the greenhouse effect than the total of other gases such as carbon dioxide and methane. Greenhouse gases keep the earth livable, and without them its surface temperature would be far colder than at present.
However, water vapor is not the main driver of the earth's current rapid warming; rather, it is a consequence of it, and it amplifies the warming caused by other greenhouse gases in what is termed a "positive feedback loop." As other greenhouse gases increase, the earth's temperature rises in response, thus increasing evaporation. Because warmer air holds more moisture, its concentration of water vapor increases. This increased water vapor then absorbs more heat radiated from the earth and prevents it from escaping out into space, thereby further warming the atmosphere and resulting in even more water vapor accumulating in it.