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Water vapor is Earth’s most abundant greenhouse gas. It’s responsible for about half of Earth’s greenhouse effect — the process that occurs when gases in Earth’s atmosphere trap the Sun’s heat. Greenhouse gases keep our planet livable. Without them, Earth’s surface temperature would be about 59 degrees Fahrenheit (33 degrees Celsius) colder. Water vapor is also a key part of Earth’s water cycle: the path that all water follows as it moves around Earth’s atmosphere, land, and ocean as liquid water, solid ice, and gaseous water vapor.
Global average surface temperatures have risen by approximately 2 degrees Fahrenheit (1.1° Celsius) since the late 1800s. Satellite data, weather balloons and ground measurements confirm that atmospheric water vapor is increasing with climate warming. (The United Nations’ Intergovernmental Panel on Climate Change Sixth Assessment Report states total atmospheric water vapor is increasing one to two percent per decade). For every degree Celsius that Earth’s atmospheric temperature rises, the amount of water vapor in the atmosphere can increase by about 7 percent, according to the laws of thermodynamics.
Some people mistakenly believe water vapor is the main driver of Earth’s current warming. But increased water vapor doesn’t cause Human-caused global warming. Instead, it’s a consequence of it. The warming caused by other greenhouse gases is supercharged by increased water vapor in our atmosphere.
It works like this: as greenhouse gases like carbon dioxide and methane increase, Earth’s temperature rises in response. This causes more water to evaporate from land and water. Warmer air has more moisture so its concentration of water vapour increases. This is because water vapor doesn’t condense or precipitate as easily at higher temperatures. The water vapor absorbs heat from Earth and stops it from escaping to space. This further warms up the atmosphere, resulting is even more water vapour in the atmosphere. This is what scientists call a ‘positive feedback loop.’ Scientists estimate this effect more than doubles the warming that would happen due to increasing carbon dioxide alone.
A Different Kind Of Greenhouse Gas
The greenhouse gases in the dry air in Earth’s atmosphere include carbon dioxide, methane, nitrous oxide, ozone, and chlorofluorocarbons. While making up around 0.05 percent of Earth’s total atmosphere, they play major roles in trapping Earth’s radiant heat from the Sun and keeping it from escaping into space. Each one is directly driven by human activities.
All five of these greenhouse gasses are non-condensable. Non-condensable gases can’t be changed into liquid at the very cold temperatures present at the top of Earth’s troposphere, where it meets the stratosphere. The concentration of non-condensable gases remains stable despite changes in atmospheric temperatures.
But water vapor, however, is a completely different animal. It’s condensable – it can be changed from a gas into a liquid. Its concentration is dependent on the temperature in the atmosphere. This makes water vapor the only greenhouse gases whose concentrations increase. because The atmosphere is warming, which causes it to get warmer.
If non-condensable gases weren’t increasing, the amount of atmospheric water vapor would be unchanged from its pre-industrial revolution levels.
Carbon Dioxide Is Still King
Carbon dioxide is responsible for a third of the total warming of Earth’s climate due to human-produced greenhouse gases. Its concentration can have dramatic effects, even if it is only slightly increased. One reason is the amount of carbon dioxide that remains in the atmosphere.
Methane, carbon dioxide, and chlorofluorocarbons don’t condense, and they aren’t particularly chemically reactive or easily broken down by light in the troposphere. They remain in the atmosphere for years to decades or longer depending on the gas.
* Carbon dioxide’s lifetime cannot be represented with a single value because the gas is not destroyed over time, but instead moves among different parts of the ocean–atmosphere–land system. Some carbon dioxide is absorbed quickly, such as by the ocean surface, but others will remain in the atmosphere for thousands upon thousands of years. This is due to the slow process of carbon being transferred to ocean sediments.
** The lifetimes shown for methane and nitrous oxide are perturbation lifetimes, which have been used to calculate the global warming potentials shown here. Credit: EPA
A molecule of water vapour, however, stays in the atmosphere for an average of nine days. It is then used to make rain or snow. Its amounts don’t accumulate, despite its much larger relative quantities.
“Carbon dioxide and other non-condensable greenhouse gases act as control knobs for the climate,” said Andrew Dessler, a professor of Atmospheric Sciences at Texas A&M University in College Station. “As humans add carbon dioxide to the atmosphere, small changes in climate are amplified by changes in water vapor. This makes carbon dioxide a much more potent greenhouse gas than it would be on a planet without water vapor.”
This map shows the areas where the water cycle has been increasing or decreasing across the U.S. continental from 1945-1974 to 1986-2014. Areas in blue show where the water cycle has been speeding up—moving through the various stages faster or with more volume. Red areas experienced a decrease in precipitation and evapotranspiration, and experienced slower or more intense cycles. A region with a higher intensity value indicates that there was more water cycling, mainly due to increased precipitation. Credit: NASA Earth Observatory image taken by Lauren Dauphin using data from Huntington Thomas and others. (2018).
The Global Water Cycle is in a state of chaos
Global water cycles are also amplified by an increase in atmospheric water vapour. They make dry areas drier and wetter. The more water vapor in air, the greater its energy. This energy fuels severe storms, particularly on land. This causes more extreme weather events.
More evaporation from land also dry out soils. When water from severe storms falls on dry ground, it evaporates into rivers and streams, rather than dampening the soils. This increases the chance of drought.
In short, when atmospheric water vapor meets increased levels of other greenhouse gases, its impacts on Earth’s climate are substantial.
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