evidence: We can thank water vapor for about half of the "greenhouse effect" keeping heat from the sun inside our atmosphere.1 "It’s the most important greenhouse gas in our climate system, because of its relatively high concentrations," says Kerry Emanuel, professor emeritus of atmospheric science at MIT. "It can vary from almost nothing to as much as 3% of a volume of air." Compare that to CO2, which today makes up about 420 parts per million of our atmosphere—0.04%—and you can see immediately why water vapor is such a linchpin of our climate system. [...] But water vapor differs in one crucial way from other greenhouse gases like CO2, methane, and nitrous oxide. Those greenhouse gases are always gases (at least when they’re in our atmosphere). Water isn’t. It can turn from a gas to a liquid at temperatures and pressures very common in our atmosphere, and so it frequently does. When it’s colder it falls from the air as rain or snow; when it’s hotter it evaporates and rises up as a gas again.