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How does water vapor act as a feedback to global warming?

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Grinnell Glacier shrinkage — Global warming A series of photographs of the Grinnell Glacier taken from the summit of Mount Gould in Glacier National Park, Montana, in (from left) 1938, 1981, 1998, and 2006. In 1938 the Grinnell Glacier filled the entire area at the bottom of the image. By 2006 it had largely disappeared from this view.

Water vapor acts as a positive feedback mechanism in the context of global warming, significantly amplifying the warming effect initiated by other greenhouse gases like carbon dioxide.12

This process functions through the following steps:

Temperature-Dependent Capacity: According to the Clausius-Clapeyron equation, the capacity of the atmosphere to hold water vapor increases as air temperature rises.1
Increased Evaporation: As the Earth's surface warms, the rate of evaporation from oceans and other water sources increases, leading to a higher concentration of water vapor in the lower atmosphere.2
Enhanced Greenhouse Effect: Because water vapor is a potent greenhouse gas, this increased concentration traps more infrared radiation and emits it back toward the surface, causing further warming.12

Unlike other greenhouse gases, water vapor is not a direct agent of radiative forcing driven by human behavior; rather, its atmospheric concentration is determined by temperature. This "water vapor feedback" is the primary reason that climate sensitivity is substantially higher than it would be if only non-condensing greenhouse gases were considered.21