Water is a greenhouse gas!

Water is a greenhouse gas!

Warmer air can hold more water vapor (the gaseous form of water) than cooler air [2]. So, as the average air temperature increases due to climate change, the water-content of the atmosphere also increases [1].

Water vapor is a greenhouse gas; it warms the atmosphere in a similar way to CO2 [12,13]. In fact, it is the most abundant greenhouse gas in the atmosphere [5]! Water vapor is actually responsible for about 50% of the greenhouse effect today [6,11].

This forms a “positive feedback loop” [8]:

1) Greenhouse gas emissions increase the temperature of atmosphere [13]

2) Higher temperatures allow more water to evaporate, and the air holds more water vapor [2]. Most of this evaporation is from oceans [15]

3) Water vapor absorbs and releases infrared radiation (see our “greenhouse effect” post for details) [4,6]

4) The temperature of the atmosphere increases, going back to step 2 [8]!

This water vapor feedback loop at least doubles the amount of warming caused directly by CO2 emissions [16,17].

In addition, water vapor contributes to the formation of clouds: when rising air expands and cools “cloud particles” form, which are mostly made from liquid water or ice [14]. Clouds of a certain thickness and height actually shield the Earth’s surface from sunlight by reflecting it before it hits the ground [7]. This effect is outweighed, however, by the warming effect of other clouds [9]: clouds are thought to contribute about 25% of the present-day greenhouse effect, compared to the 50% attributed to water vapor, and 20% attributed directly to CO2 [11].

Importantly: reducing CO2 emissions is a much more effective way of reducing warming caused by water vapor, than by reducing water vapor emissions (such as watering crops) directly [5,18].


[1] https://www.ipcc.ch/site/assets/uploads/2018/03/TAR-07.pdf See: Water vapour feedback

[2] https://www.researchgate.net/figure/Air-water-vapor-saturation-curve-as-a-function-of-temperature_fig1_275533794 See: Figure 1

[3] https://www.ipcc.ch/site/assets/uploads/2018/03/TAR-07.pdf See: 7.2.1 Physics of the Water Vapour and Cloud Feedbacks, especially paragraph 3

[4] https://earthobservatory.nasa.gov/global-maps/MYDAL2_M_SKY_WV See: Main text

[5] https://iopscience.iop.org/article/10.1088/1748-9326/aae018/meta See: Abstract and conclusion

[6] https://www.acs.org/content/acs/en/climatescience/greenhousegases/properties.html Paragraph 5

[7] https://www.ipcc.ch/site/assets/uploads/2018/03/TAR-07.pdf See: 7.1 Introduction; cloud radiation feedback

[8] https://www.pnas.org/content/pnas/110/45/18087.full.pdf See: Abstract and main text

[9] https://www.ipcc.ch/site/assets/uploads/2018/02/WG1AR5_Chapter07_FINAL-1.pdf See: Executive Summary; Water Vapour, Cloud and Aerosol Feedbacks (cloud feedback from all cloud types +0.6W/m2)

[10] https://iopscience.iop.org/article/10.1088/1748-9326/aae018 See: Abstract

[11] https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2010JD014287 See: Conclusion

[12] https://www.ipcc-data.org/guidelines/pages/glossary/glossary_fg.html See : Greenhouse gases

[13] https://www.acs.org/content/acs/en/climatescience/greenhousegases.html

[14] https://www.ipcc.ch/site/assets/uploads/2018/02/WG1AR5_Chapter07_FINAL-1.pdf See : 7.1.1, Clouds and Aerosols in the Atmosphere, paragraph 2

[15] https://iopscience.iop.org/article/10.1088/1748-9326/aae018/meta See: Introduction, paragraph 1

[16] https://www.ipcc.ch/site/assets/uploads/2018/03/TAR-07.pdf See : Executive summary, paragraph 3

[17] https://skepticalscience.com/water-vapor-greenhouse-gas.htm For a plain language explanation of the content in [16] see : paragraph 3

[18] https://www.ipcc.ch/site/assets/uploads/2018/03/TAR-04.pdf See : Executive summary, paragraph 2

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