“Some people mistakenly believe water vapor is the main driver of Earth’s current warming. But increased water vapor doesn’t cause global warming. Instead, it’s a consequence of it. Increased water vapor in the atmosphere amplifies the warming caused by other greenhouse gases.
It works like this: As greenhouse gases like carbon dioxide and methane increase, Earth’s temperature rises in response. This increases evaporation from both water and land areas. Because warmer air holds more moisture, its concentration of water vapor increases. Specifically, this happens because water vapor does not condense and precipitate out of the atmosphere as easily at higher temperatures. The water vapor then absorbs heat radiated from Earth and prevents it from escaping out to space. This further warms the atmosphere, resulting in even more water vapor in the atmosphere. This is what scientists call a “positive feedback loop.” Scientists estimate this effect more than doubles the warming that would happen due to increasing carbon dioxide alone.”
Source: Steamy Relationships: How Atmospheric Water Vapor Amplifies Earth’s Greenhouse Effect – Climate Change: Vital Signs of the Planet