By Alan Buis, NASA's Jet Propulsion Laboratory

Credit: John Fowler on Unsplash

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.

Since the late 1800s, global average surface temperatures have increased by about 2 degrees Fahrenheit (1.1 degrees Celsius). Data from satellites, weather balloons, and ground measurements confirm the amount of atmospheric water vapor is increasing as the climate warms. (The United Nations’ Intergovernmental Panel on Climate Change Sixth Assessment Report states total atmospheric water vapor is increasing 1 to 2% 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%, 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 global warming. Instead, it’s a consequence of it. Increased water vapor in the atmosphere amplifies the warming caused by other greenhouse gases.

It works like this: As greenhouse gases like carbon dioxide and methane increase, Earth’s temperature rises in response. This increases evaporation from both water and land areas. Because warmer air holds more moisture, its concentration of water vapor increases. Specifically, this happens because water vapor does not condense and precipitate out of the atmosphere as easily at higher temperatures. The water vapor then absorbs heat radiated from Earth and prevents it from escaping out to space. This further warms the atmosphere, resulting in even more water vapor 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.

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% 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 is driven directly by human activities.

All five of these greenhouse gases 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. As atmospheric temperatures change, the concentration of non-condensable gases remains stable.

But water vapor is a different animal. It’s condensable – it can be changed from a gas into a liquid. Its concentration depends on the temperature of the atmosphere. This makes water vapor the only greenhouse gas whose concentration increases because the atmosphere is warming, and causes it to warm even more.

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 responsible for a third of the total warming of Earth’s climate due to human-produced greenhouse gases. Small increases in its concentration have major effects. A key reason is the length of time carbon dioxide 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. For these reasons, they remain in the atmosphere for anywhere from years to centuries or even longer, depending on the gas.

In contrast, a molecule of water vapor stays in the atmosphere just nine days, on average. It then gets recycled as 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.”

Increases in atmospheric water vapor also amplify the global water cycle. They contribute to making wet regions wetter and dry regions drier. The more water vapor that air contains, the more energy it holds. This energy fuels intense storms, particularly over land. This results in more extreme weather events.

But more evaporation from the land also dries soils out. When water from intense storms falls on hard, dry ground, it runs off into rivers and streams instead of dampening soils. This increases the risk 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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