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Why does warmer air usually lead to more water vapor in the lower atmosphere?

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World map shaded to show annual carbon dioxide emissions by country in 2022, with China, the United States, and India highlighted as top emitters in dark red. A bar chart below lists the top ten emitting countries by million metric tons. — Greenhouse gas Map of annual carbon dioxide emissions by country in 2014.

Warmer air leads to more water vapor in the lower atmosphere primarily because the saturation vapour pressure of air increases with temperature.1

At a molecular level, when liquid water and its surrounding vapor are warmed, a greater number of molecules escape from the liquid phase than return to it, causing the concentration of water vapor to rise. Consequently, warmer surface temperatures increase the rate of evaporation from the Earth's surface. Because warmer air has a higher capacity to hold water vapor, this increased evaporation results in a higher concentration of water vapor in the lower atmosphere.123

In the context of greenhouse gases, this process acts as a climate feedback. The additional water vapor is capable of absorbing infrared radiation and emitting it back toward the surface, which further influences the climate system's activity.2