How Does Water Vapor Affect Climate?

What is water vapor? What is its role in the hydrologic cycle? How does it affect global warming? How does the water vapor feedback work? Do clouds warm or cool the planet? How does water vapor create hurricanes? How does it help in the formation of the the Hydroxyl Radical (OH)? We answer all these questions and more.
Thunderstorm cumulonimbus cloud
Thunderstorm as observed from C130 research aircraft. Photo: © NOAA

What is Water Vapor?

Water vapor is the gaseous form of water. It enters the atmosphere by evaporation, or plant transpiration, or (more rarely) due to sublimation of ice.

It accounts for roughly 0.01 percent of the water in Earth’s hydrosphere and on average remains in the atmosphere for no more than nine days. 1

The presence of water vapor in the atmosphere has a major influence on the climate system, due to its heat-trapping ability. In fact, it is the most abundant of all greenhouse gases on the planet.

Yet, as we shall see, climate science does not regard water vapor as a direct cause of global warming, but rather as an internal accelerant or feedback. This is because, unlike other greenhouse gases, we cannot control the amount of vapor in the atmosphere.

What determines the water-vapor content of the atmosphere (humidity) is temperature. This is the Clausius‐Clapeyron relation which states that a one degree increase in temperature increases the amount of vapor that the atmosphere can hold (before it falls as rain or snow) by 6‐7 percent.

For example, at 30°C (86°F), air can contain up to 4 percent water vapor. Whereas at minus 40°C (minus 40°F), it can contain no more than 0.2 percent. 2

Part of the Water Cycle

The movement of water from the Earth’s surface into the troposphere and back again, is part of the global water cycle, and is regulated by temperature. As air warms, liquid water evaporates from the surface of the Earth (land or sea) and rises upwards. The rising vapor is invisible to the naked eye.

As the rising air cools, the water vapor condenses back into liquid form, typically with the help of tiny particles of dust or other debris in the air. These microscopic specks of material provide the surfaces upon which the vapor can condense into liquid droplets. An accumulation of droplets is a cloud. Because evaporation is tied largely to temperature, more water evaporates in the tropics than in the higher latitudes.

How Does Water Vapor Affect Global Warming?

Water vapor influences climate change in three main ways.

(a) It acts as a positive climate feedback amplifying the effect of other radiative forcings.
(b) It forms clouds that can exert both a warming and a cooling effect.
(c) It helps to create the hydroxyl radical (OH), which removes certain greenhouse gases from the atmosphere.

Water Vapor is a Climate Feedback

Water vapor is recognized as having a strong influence on rising temperatures, but it is not regarded as a direct cause of climate change, and for good reason. It has been around in the atmosphere for millions of years without causing any problems.

Basically, the movement of water vapor in and out of the atmosphere is a natural process – regulated exclusively by temperature – over which we have no control. 3

The direct causes of climate change – known as “climate forcings” (or “radiative forcings”) – are the factors that disturb Earth’s radiative equilibrium, thus forcing Earth’s temperature to rise or fall. These can be natural or man-made.

Natural climate forcings include variations in the amount of sunlight reaching Earth and large volcanic eruptions. Man-made climate forcings mostly concern emissions of greenhouse gases (such as CO2, methane, nitrous oxide or the F-gases), or industrial aerosols.

However, in addition to climate forcings, there are a number of “climate feedbacks” or secondary influences. These feedbacks are not independent drivers of global warming, they are processes that merely amplify or reduce the effect of a particular climate forcing.

Water vapor creates a positive feedback, making any temperature changes larger than they would otherwise be. Just remember, the amount of water vapor in the atmosphere is dictated by temperature. The hotter it gets, the more vapor in the atmosphere.

Here’s how the feedback works:

  • Carbon dioxide is emitted from a coal-fired power plant, causing the atmosphere to heat up.
  • This warming causes more water to evaporate into the air.
  • This extra water vapor traps more heat, causing the atmosphere to heat up even more.
  • As a result, more water evaporates which results in more heat, and so on.
  • This loop continues until the water vapor condenses and falls to Earth as rain or snow.
  • The point is, without the initial warming effect from the CO2, the amount of water vapor would have remained unchanged.

How Much Extra Warming is Generated by This Climate Feedback?

Scientific studies indicate that water vapor feedback roughly doubles the amount of global warming caused by carbon dioxide. So, if CO2 causes a one-degree Celsius change, water vapor will cause the temperature to go up another one degree. When other feedback loops are included, the total warming could rise to as much as three degrees Celsius. 4

Water Vapor Has No Official Global Warming Potential

Every greenhouse gas is given a rating of its “Global Warming Potential” (GWP) – its heat-trapping power compared to that of carbon dioxide. Every gas, that is, except water vapor.

This is due to its short life span: it generally remains in the atmosphere for only a matter of days (before precipitating out), while other greenhouse gases, such as methane (12 years), nitrous oxide (120 years) and carbon dioxide (30-35,000 years), remain in the atmosphere for much longer.

Diagram effects of clouds and water vapor on climate
Image Credit: NASA

Water Vapor Forms Clouds That Warm and Cool the Planet

Water vapor also influences climate through its formation of clouds, the dense collections of water droplets that drive so much of our weather.

During the day, for instance, the white clouds exert a cooling influence on Earth’s surface by reflecting sunlight back into space, thus reducing the amount of solar energy that is absorbed by the surface. This reflective capacity or “albedo effect” is capable of reflecting as much as 90 percent of sunlight – the same as fresh snow.

As a rule of thumb, lower, thicker clouds reflect the most sunlight. 5

Conversely, as darkness falls, clouds exert a warming influence by trapping radiant heat trying to escape into space from the surface of the planet, and re-radiating part of it back down to the surface. Think of it as a night-time greenhouse effect. (A cloudy night is always warmer than a cloudless night.)

So, clouds both warm and cool the Earth’s surface. The question is, which is greater – the warming or the cooling?

Previously, climate scientists used to think that clouds had a net cooling effect of about 5°C (9°F). 6

Then, in its Fifth Assessment Report (2013), the IPCC characterized the net effect as being “between near-zero and moderately positive.” In other words, the warming effect of clouds predominates. However, this is unlikely to be the last word on the subject.

Satellite view of Hurricane Michael clouds
The ring of cumulonimbus clouds surrounding the eye of category-5 Hurricane Michael as it heads towards Florida on October 10, 2018. Image: Photo taken by astronauts onboard the International Space Station.

Water Vapor Powers Hurricanes

Tropical hurricanes/cyclones are giant engines that use warm, moist air as fuel. This is because water vapor contains the solar heat that caused it to change from a liquid into a gas. So, when water evaporates from (say) the surface of the equatorial ocean, it’s like invisible energy rising up into the air.

At some point, the vapor condenses back into liquid droplets, and releases its heat content into the surrounding air. This rising heat – re-fuelled by more vapor rising from warm ocean water – drives the rapidly forming clouds up to 16 kms (10 miles) high. As the clouds rise upwards, the Earth’s rotation causes them to spin. The clouds merge into a vast mass of rotating energy which spins and grows, fed by the ocean’s heat and water evaporating from the surface. And before long a hurricane is born.

In the worst storms, the amount of heat absorbed by water vapor can be 200 times larger than the entire global production of electricity at any given moment. 7

We ourselves depend upon the energy content of water vapor. Its use as steam in steam turbines, for instance, continues to be a major component in global energy production. Around 90 percent of the world’s electric power is produced by steam-electric power plants. 8

Water Vapor and the Hydroxyl Radical (OH)

As well as being a powerful greenhouse gas and a constituent of clouds, water molecules also affect the climate by reacting with other chemicals in the atmosphere. For example, water vapor, along with ozone, helps to create the highly reactive hydroxyl radical (OH). 9

OH is often referred to as the “detergent” of the troposphere because it reacts with many greenhouse gases, decomposing them in the process, as a first step to their removal. For example, it plays an important role in the elimination of methane, as well as the removal of certain F-gases and other pollutants. 10

References

  1. “A Dictionary of Earth Sciences (3rd ed) Edited by Michael Allaby. Oxford University Press. ISBN-13: 9780199211944 []
  2. “Humidity.” Britannica. []
  3. But see: “The Role of Anthropogenic Water Vapor in Earth’s Climate.” []
  4. “Explaining how the water vapor greenhouse effect works.” Skeptical Scientist []
  5. “Albedo.” Dr. Timothy Bralower and Dr. David Bice, College of Earth and Mineral Science, The Pennsylvania State University. []
  6. “Cloud Climatology.” NASA Goddard Institute for Space Studies. International Satellite Cloud Climatology Project. (ISCCP) []
  7. “Water Vapor Fuels Hurricanes.” PBS Learning Media. (Video) []
  8. “Energy resources: occurrence, production, conversion, use.” Wendell H. Wiser (2000). Birkhauser. p. 190. ISBN 978-0-387-98744-6. []
  9. “Greenhouse gas ‘detergent’ recycles itself in atmosphere.” NASA Study []
  10. “Observing Water Vapor.” WMO []
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