Water's Role in Earth Systems
The most water is stored in the deep Earth, but is essentially locked up and unavailable to life, the oceans, or the atmosphere. Almost no water is stored in the atmosphe, yet the tiny amount in the air makes clouds, rain, and the basis for our weather.
Water Vapor
Atmospheric water (vapor) is a strong greenhouse gas, yet people can neither add nor diminish water vapor by any purposeful means.
Water Storage
The vast majority of surface water is stored in the oceans. A smaller percentage is in glacial ice and groundwater. A very miniscule amount of surface water is in lakes and streams.
Water Movement
Water is stored in each of the 'spheres' and is moved from one to another in the hydrologic cycle.
What's water's role in the greenhouse effect?
Burning fossil fuels adds carbon dioxide to the atmosphere which triggers global warming. As the atmosphere warms, the air can hold more water vapor. More water vapor in the air means there are more water molecules that will trap heat and add to the greenhouse effect. So carbon dioxide's effect on global warming is greatly amplified by water vapor, leading to a potentially dangerous positive feedback.