Water Vapor

Water vapor is the most effective, fastest changing, and least understood of the greenhouse gases.

Water vapor is a powerful greenhouse gas; as a matter of fact, it is the dominant greenhouse gas. But scientists don't know with certainty how much water vapor affects global climate because it is neither well-mixed nor long-lived. Scientists can more accurately determine the radiative forcing of other gases because the gases are so much more stable. Water vapor is not!! Its atmospheric concentration changes from day to day and from place to place. To confuse matters more, it also changes physical state! It goes from gas to liquid to solid in the atmosphere, and, as it does, its radiative forcing also changes. Much more research is needed to better understand the impact water vapor has on global climate. This is one major focus of the Atmospheric Radiation Measurement (ARM) Program.

Impact of Water Vapor on Global Warming

Water vapor has a definite positive radiative forcing in the lower troposphere, which results in warming of the atmosphere. If the atmosphere is warmed, its holding capacity for water vapor increases, which would further enhance warming. But as water vapor moves up in the atmosphere it can condense and change to water or ice. When this happens, clouds may form.

Impact of Clouds on Global Warming

The impact of clouds on global climate change is as difficult to determine as is water vapor because not only can clouds absorb radiation (positive radiative forcing) but they can also give off radiation (negative radiative forcing). Whether clouds absorb more or give off more radiation depends on their height, thickness, and makeup. Latitude can also be a factor that will determine their radiative forcing. All these factors are hard to sort out, but considerable progress has been made in describing and modeling the cloud processes that will determine their radiative forcing.

Uncertainties About Water

Yet uncertainty still exists, and there is a definite need to more fully understand the effects of water vapor and clouds on global climate change. With the ARM Program working on both of these parameters, we are getting a better idea of what happens. ARM currently has three test sites equipped with special instruments that provide us with data. We are using the data to test models about the impact that clouds and water vapor have on the global climate.

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