How the World Really Works: The Science Behind How We Got Here and Where We're Going

by Vaclav Smil

Water vapor is the principal generator of the natural greenhouse effect—but water vapor is not the cause of atmospheric warming because it does not control atmospheric temperature. In fact, it is the other way around: the changing temperature determines how much water can be present as a gas (the humidity of air increases with rising temperatures) and how much it condenses to liquid (condensation increases as the temperature drops). The Earth’s natural warming is controlled by trace gases whose concentration is not affected by the ambient temperature—that is, they do not condense and precipitate as temperatures decline. But the relatively small warming they cause increases evaporation and raises atmospheric water concentrations, and this feedback results in additional warming.