This is for two primary reasons:

• There is a greater amount of water vapor in the atmosphere than any other greenhouse gas. Water vapor can be up to 2% of the atmosphere (the amount varies place to place and over time), whereas carbon dioxide only comprises 0.37% of the atmosphere.
• Water vapor traps heat very effectively. Because it is a clear gas (like the other greenhouse gases), radiation passes through it relatively undiminished on its way to the earth. Once the Earth’s surface re-radiates that energy in the form of heat (infrared radiation), it is absorbed very efficiently by water vapor, which in turn causes a warming effect.

It is estimated that water vapor is responsible for about two-thirds of the natural greenhouse effect!

One important feature of water vapor is that, unlike other greenhouse gases, human activities do not seem to directly change the concentration of water vapor in the atmosphere. Moreover, water vapor does not accumulate in the atmosphere over many years as other greenhouse gases do. Natural process such as rain remove water vapor from the atmosphere when it exceeds certain limits.

However, human activities can indirectly increase the amount of water vapor in the atmosphere. Because the amount of water vapor in the air tends to rise as air temperatures increase, rising temperatures may result in a “positive feedback loop” (i.e. a cycle that reinforces itself): rising temperatures lead to increases in water vapor concentrations, which in turn lead to further increases in temperature, and so forth.

Water formula

In the atmosphere, water vapor condenses into clouds and rain when combined with lower temperatures and pressures. Clouds play a key role in regulating the climate. On the one hand, clouds can reflect some radiation from the sun, cooling the atmosphere and the Earth below. On the other hand, clouds can also absorb heat radiating up from the Earth, warming the lower atmosphere. Currently it is believed that clouds have, on balance, a small cooling effect, but the effect they will have in the future is unknown.