One of the main causes of uncertainty projecting the effects of climate change is how water all over the globe will react to increased anthropogenic carbon dioxide emissions. In other lessons, we have learned that without the radiative forcing due to greenhouse gases, the average surface temperature of earth would be approximately 33°C cooler than it is today. Atmospheric water, including clouds and vapour, accounts for 35-85% of that radiative forcing, depending on geographic location. As such, water is one of the most climatically important compounds on the planet.

Each phase of water plays a different role in regulating earth's climate. Depending on its physical state and where it is found, water can act as a greenhouse gas, or it can alter earth's albedo as reflective clouds or ice. Each of these states of water and their climate regulating effects are controlled through feedback loops.