evidence: Water vapor is Earth’s most abundant greenhouse gas. It’s responsible for about half of Earth’s greenhouse effect — the process that occurs when gases in Earth’s atmosphere trap the Sun’s heat. Greenhouse gases keep our planet livable. Without them, Earth’s surface temperature would be about 59 degrees Fahrenheit (33 degrees Celsius) colder. Water vapor is also a key part of Earth’s water cycle: the path that all water follows as it moves around Earth’s atmosphere, land, and ocean as liquid water, solid ice, and gaseous water vapor. [...] 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. [...] But water vapor is a different animal. It’s condensable – it can be changed from a gas into a liquid. Its concentration depends on the temperature of the atmosphere. This makes water vapor the only greenhouse gas whose concentration increases because the atmosphere is warming, and causes it to warm even more.