Energy > Climate Change
March 2022
The importance of water vapor
This article is part of our series focused on the causes and consequences of a warming planet
Water vapor is the most prevalent greenhouse gas. It causes roughly half of the warming of the planet. Like other greenhouse gases, it lets almost all of the sunlight reach the earth’s surface but absorbs heat radiated upward from the earth.
Most of us have experienced water vapor as a greenhouse gas. During periods of high humidity, the air cools slowly when the sun goes down because high levels of water vapor trapped heat are in the atmosphere. Conversely, temperatures in dry climates drop rapidly at night due to low levels of heat-trapping water vapor in the atmosphere. You will experience the same feeling when you move out of the sunshine into the shadows. You will feel little change in temperature if humidity is high but more change in temperature if humidity is low.
Although water vapor is a greenhouse gas, it is different than carbon dioxide. The concentration of water vapor in the atmosphere varies greatly by rapidly moving in and out of the atmosphere. By comparison, carbon dioxide is relatively uniformly distributed in the atmosphere and remains in the atmosphere for a hundred years or more.
Moreover, due to its short life in the atmosphere, water vapor concentration varies greatly among geographic regions of the earth. Conversely, due to the long life of carbon dioxide, the mixing caused by global winds results in carbon dioxide’s concentration to be evenly distributed among regions of the earth. This blending means that carbon dioxide emissions from a facility will eventually be distributed across all of the earth’s atmosphere.
The air’s water vapor holding capacity is highly sensitive to temperature. It ranges from a maximum holding capacity of only 0.01% in extremely cold temperatures up to 3% in saturated air at about 90 degrees. So, as the earth warms, the atmosphere can hold more water vapor. More specifically, the atmosphere can hold about 4% more water vapor for every one degree Fahrenheit increase in temperature.
Water vapor has an important role in the warming of the Earth because the atmosphere’s water vapor holding capacity increases as air temperature rises. When carbon dioxide warms the atmosphere, the atmosphere can hold more water vapor, and the additional water vapor, because of its greenhouse effect, warms the atmosphere even more. Water vapor significantly magnifies the warming caused by carbon dioxide.
See the Ag Decision Maker website for more from this series.
Don Hofstrand, retired extension agricultural business specialist, agdm@iastate.edu
Reviewed by Dr. Eugene Takle, retired professor emeritus Iowa State University
