evidence: With all the attention given to humans’ climate-warming carbon dioxide (CO2) emissions, you might be surprised to learn that CO2 is not the most important greenhouse gas affecting the Earth’s temperature. That distinction belongs to water. [...] 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. [...] In short, it’s true that water vapor is in some sense the "biggest" greenhouse gas involved in climate change, but it’s not in the driver’s seat. CO2 is still the main culprit of the global warming we’re experiencing today. Water vapor is just one of the features of our climate that our CO2 emissions are pushing out of balance—well beyond the stable levels humanity has enjoyed for thousands of years.