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As we discuss in the section, “Focusing On Carbon Dioxide: Leader of the Pack,” earlier in this chapter, water vapor is a huge player in the greenhouse effect. As shocking as it may seem, good ol’ H₂O (two parts hydrogen, one part water) causes the majority — 60 percent — of the planet’s greenhouse effect. But the ramped-up threat of climate change isn’t tied to water vapor. Water vapor remains an essential reason the planet is warm enough to sustain current life forms.

Unlike the production of the other GHGs, humans don’t directly cause the increase of water vapor. But the other gases that are produced heat up the atmosphere. When plants, soil, and water warm up, more water evaporates from their surfaces and ends up in the atmosphere as water vapor. A warmer atmosphere can absorb more moisture. The atmosphere will continue to absorb more moisture while temperatures continue to rise. See Figure 2-6.

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FIGURE 2-6: Water evaporates and lingers in the atmosphere.

Water vapor also differs greatly from other GHGs because the atmosphere can hold only so much of it. When you watch a weather forecast, you hear the term relative humidity, which refers to the amount of water vapor currently in the atmosphere compared to how much the atmosphere can hold. On a really hot and sticky day, the relative humidity may be 90 percent — the atmosphere has just about taken in all the water vapor it can. When the relative humidity reaches 100 percent, clouds form, and then precipitation falls, releasing the water from the air.