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." [...] And of course, temperatures today are rising, thanks to humans’ emissions of longer-lasting greenhouse gases like CO2. Water vapor amplifies that effect. "If the temperature rises, the amount of water vapor rises with it," says Emanuel. "But since water vapor is itself a greenhouse gas, rising water vapor causes yet higher temperatures. We refer to this process as a positive feedback, and it is thought to be the most important positive feedback in the climate system." 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.