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greenhouse gas effects:

Water Vapor

The largest feedback is water vapor. Water vapor is a strong greenhouse gas. In fact, because of its abundance in the atmosphere, water vapor causes about two-thirds of greenhouse warming, a key factor in keeping temperatures in the habitable range on Earth. But as temperatures warm, more water vapor evaporates from the surface into the atmosphere, where it can cause temperatures to climb further.

The question that scientists ask is, how much water vapor will be in the atmosphere in a warming world? The atmosphere currently has an average equilibrium or balance between water vapor concentration and temperature. As temperatures warm, the atmosphere becomes capable of containing more water vapor, and so water vapor concentrations go up to regain equilibrium. Will that trend hold as temperatures continue to warm?

The amount of water vapor that enters the atmosphere ultimately determines how much additional warming will occur due to the water vapor feedback. The atmosphere responds quickly to the water vapor feedback. So far, most of the atmosphere has maintained a near constant balance between temperature and water vapor concentration as temperatures have gone up in recent decades. If this trend continues, and many models say that it will, water vapor has the capacity to double the warming caused by carbon dioxide alone.


Closely related to the water vapor feedback is the cloud feedback. Clouds cause cooling by reflecting solar energy, but they also cause warming by absorbing infrared energy (like greenhouse gases) from the surface when they are over areas that are warmer than they are. In our current climate, clouds have a cooling effect overall, but that could change in a warmer environment.

Astronaut photograph of clouds over Florida.

Clouds can both cool the planet (by reflecting visible light from the sun) and warm the planet (by absorbing heat radiation emitted by the surface). On balance, clouds slightly cool the Earth. (NASA Astronaut Photograph STS31-E-9552courtesy Johnson space Center Earth Observations Lab.)

If clouds become brighter, or the geographical extent of bright clouds expands, they will tend to cool Earth’s surface. Clouds can become brighter if more moisture converges in a particular region or if more fine particles (aerosols) enter the air. If fewer bright clouds form, it will contribute to warming from the cloud feedback.

See Ship Tracks South of Alaska to learn how aerosols can make clouds brighter.

Clouds, like greenhouse gases, also absorb and re-emit infrared energy. Low, warm clouds emit more energy than high, cold clouds. However, in many parts of the world, energy emitted by low clouds can be absorbed by the abundant water vapor above them. Further, low clouds often have nearly the same temperatures as the Earth’s surface, and so emit similar amounts of infrared energy. In a world without low clouds, the amount of emitted infrared energy escaping to space would not be too different from a world with low clouds.

Thermal infrared image of the Western Hemisphere from GOES.

Clouds emit thermal infrared (heat) radiation in proportion to their temperature, which is related to altitude. This image shows the Western Hemisphere in the thermal infrared. Warm ocean and land surface areas are white and light gray; cool, low-level clouds are medium gray; and cold, high-altitude clouds are dark gray and black. (NASA image courtesy GOES Project Science.)