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How does water vapor act as a greenhouse gas in the atmosphere?
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Greenhouse gas
Map of annual carbon dioxide emissions by country in 2014.
Water vapor is the most potent greenhouse gas in Earth's atmosphere, but it functions differently than other greenhouse gases because its concentration is primarily determined by air temperature rather than direct human activity.12
The Water Vapor Feedback
Rather than acting as a direct agent of radiative forcing, water vapor serves as a critical climate feedback. This process, known as the "water vapor feedback," works as follows:1
- Temperature Dependence: The capacity of air to hold water vapor is governed by the Clausius-Clapeyron equation, which dictates that warmer air can hold significantly more moisture.23
- Amplification: As the surface and lower atmosphere warm, the rate of evaporation increases, leading to a higher concentration of water vapor.12
- Enhanced Warming: Because water vapor is a powerful greenhouse gas, this increased concentration absorbs more infrared radiation and emits it back toward the surface, further strengthening the greenhouse effect and leading to additional warming.12
This positive feedback loop is a primary reason why the climate system is highly sensitive to changes in temperature.2