Warmer air can hold more water vapor (the gaseous form of water) than cooler air . So, as the average air temperature increases due to climate change, the water-content of the atmosphere also increases .
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 ! Water vapor is actually responsible for about 50% of the greenhouse effect today [6,11].
This forms a “positive feedback loop” :
1) Greenhouse gas emissions increase the temperature of atmosphere 
2) Higher temperatures allow more water to evaporate, and the air holds more water vapor . Most of this evaporation is from oceans 
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 !
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 . Clouds of a certain thickness and height actually shield the Earth’s surface from sunlight by reflecting it before it hits the ground . This effect is outweighed, however, by the warming effect of other clouds : 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 .
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].
 https://www.ipcc.ch/site/assets/uploads/2018/03/TAR-07.pdf See: 220.127.116.11 Water vapour feedback
 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
 https://earthobservatory.nasa.gov/global-maps/MYDAL2_M_SKY_WV See: Main text
 https://iopscience.iop.org/article/10.1088/1748-9326/aae018/meta See: Abstract and conclusion
 https://www.ipcc.ch/site/assets/uploads/2018/03/TAR-07.pdf See: 7.1 Introduction; cloud radiation feedback
 https://www.pnas.org/content/pnas/110/45/18087.full.pdf See: Abstract and main text
 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)
 https://iopscience.iop.org/article/10.1088/1748-9326/aae018 See: Abstract
 https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2010JD014287 See: Conclusion
 https://www.ipcc-data.org/guidelines/pages/glossary/glossary_fg.html See : Greenhouse gases
 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
 https://iopscience.iop.org/article/10.1088/1748-9326/aae018/meta See: Introduction, paragraph 1
 https://www.ipcc.ch/site/assets/uploads/2018/03/TAR-07.pdf See : Executive summary, paragraph 3
 https://skepticalscience.com/water-vapor-greenhouse-gas.htm For a plain language explanation of the content in  see : paragraph 3
 https://www.ipcc.ch/site/assets/uploads/2018/03/TAR-04.pdf See : Executive summary, paragraph 2
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