Water vapor
Water vapor is the gaseous state of water, formed through the evaporation of liquid water or the sublimation of ice. It consists of two hydrogen atoms bonded to one oxygen atom, and its molecular weight can vary between 18 to 22 atomic mass units. Water vapor plays a significant role in the Earth's climate system; it is the primary greenhouse gas responsible for warming the Earth's surface, being far more effective than both carbon dioxide and methane in absorbing infrared radiation. Concentrations of water vapor in the atmosphere can range from 0% to 4%, with the highest levels found in tropical regions near oceans and rainforests, and the lowest in deserts and polar areas.
As temperatures rise, increased evaporation leads to higher concentrations of water vapor, which can amplify warming through positive feedback mechanisms. However, water vapor also contributes to cloud formation, which can reflect sunlight and cool the planet. Human activities, particularly agriculture and industrial processes, have increased atmospheric water vapor levels, influencing climate dynamics. Intensive irrigation practices, such as those derived from aquifers and rivers, play a significant role in this increase, presenting complex challenges in understanding and modeling the effects on global climate change.
Subject Terms
Water vapor
Definition
Water vapor is water in its gaseous state. Water vapor may form by evaporation of liquid water or by sublimation of solid water (ice). Water vapor (H2O) is a chemical combination of two atoms of hydrogen and one of oxygen in which both the hydrogen and oxygen possess isotopes. Thus, the molecular weight of water vapor may vary from a low of 18 u to a high of (typically) 22 u. A u is equal to 1.66 10-27 kilogram. For a sample of water at a given temperature, the molecules of lower mass move with higher speeds than the more massive molecules and therefore more readily evaporate or sublimate. Thus, atmospheric water vapor is enriched in the lighter forms. In epochs of higher average planetary surface temperature, more low-mass water vapor forms, some of which is ultimately deposited as snow and ice in polar regions. The ratio of low-to-high mass water in ice cores yields a profile of how temperature has varied through time.
Significance for Climate Change
Water vapor is the principal atmospheric gas responsible for greenhouse warming of the Earth’s surface. In terms of its ability to absorb solar infrared emissions, wavelengths from approximately 0.7 micron to 100 microns, water vapor is about one hundred times more effective as a greenhouse gas (GHG) than carbon dioxide (CO2) and four times more effective than methane (CH4). The concentration of water vapor in Earth’s atmosphere varies with height and by region and season from 0 to 4 percent. Normally, it is the third most abundant gas in the atmosphere after nitrogen and oxygen.
The highest concentrations of water vapor in Earth’s atmosphere occur in the planet’s equatorial regions over the oceans and rainforests. In contrast, over continental deserts such as the Gobi and Sahara, the water vapor concentration approaches 0 percent, as it does in the frigid air of the polar regions. Nearly all of atmospheric water vapor is found within the troposphere, a region of Earth’s atmosphere that varies in thickness from about 20 kilometers over equatorial regions to about 5 kilometers over the poles.
Deducing the effects of atmospheric water vapor on global climate change is extremely complex. As the surface temperature rises, the rate of evaporation increases, leading to a higher average concentration of water vapor in the atmosphere. This result alone suggests that the higher concentration of water vapor would lead to a higher surface temperature, a positive feedback that would amplify the warming caused by some other factor, such as an increase in solar radiation incidents on Earth or an increased concentration of some other GHG. The increase in the amount of water vapor in the atmosphere, however, also contributes to an increase in cloudiness, making Earth more reflective and cooling the planet by lowering the amount of solar radiation reaching the surface.
The by-products of industrial activity release small particles into the atmosphere that serve as nucleation centers for the condensation of water vapor, stimulating the formation of clouds. In the condensation process, latent heat of vaporization is released. This deposition of energy within the atmosphere powers convection and the formation of storms that redistribute the energy to cooler regions of the atmosphere. These various effects are notoriously difficult to model.
Human activity contributes significantly to increasing levels of atmospheric water vapor. The combustion of fuels produces both water vapor and CO2. The most significant deposition of water vapor into the atmosphere, however, does not result from combustion but from the irrigation of land for agricultural purposes. Making marginal regions such as steppes and grasslands agriculturally productive requires intensive irrigation. River and lake water or water from underground aquifers is spread across the landscape where evaporation and by plants increase the water vapor concentration.
In the case of pumping sequestered underground water to the surface and spreading it over crop fields, water that was formerly removed from the atmosphere and not subject to the global water cycle now participates. For example, the Ogallala Aquifer, located beneath the Great Plains of the United States west of the Mississippi River, is decreasing by 12 billion cubic meters of water per year, a rate equivalent to eighteen times the flow of the Colorado River. Some 90 percent of the water removed from the Ogallala is used for agricultural irrigation. Most of this water is deposited in the atmosphere; very little percolates back into the ground to recharge the aquifer.
Similarly, the semiarid American Southwest becomes agriculturally productive when irrigated by water removed from the Colorado River and other water transported from the Northwest. As this water evaporates, it enters the atmosphere and contributes to further greenhouse warming. Extensive irrigation projects across the globe, such as China’s Three Gorges Project, Libya’s Great Man-Made River Project, and the draining of the Caspian Sea, exacerbate global warming.
Bibliography
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Hosanky, David. "Rising Temperatures Aren't Producing Expected Increase in Atmospheric Moisture Over Dry Regions." NIDIS, 6 Feb. 2024, www.drought.gov/news/climate-change-isnt-producing-expected-increase-atmospheric-moisture-over-dry-regions-2024-02. Accessed 13 Dec. 2024.
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University of Miami Rosenstiel School of Marine & Atmospheric Science. "Global Warming Amplifier: Rising Water Vapor in Upper Troposphere to Intensify Climate Change." ScienceDaily, 28 July 2014, www.sciencedaily.com/releases/2014/07/140728153933.htm. Accessed 13 Dec. 2024.