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CO2, methane and other gases contribute to the greenhouse effect.

But most of the greenhouse effect is caused by water vapour in the atmosphere. There is always a lot average moisture present, but it is limited by precipitation. It causes most of the total greenhouse effect, without it, the surface would just radiate heat away from Earth.

If there were no moisture in the air, that should make the average surface temperature much lower. (It is not physically possible, of course.)

What would that temperature be?

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  • $\begingroup$ This question asks what science would say would happen if the scientifically impossible happens. These kinds of questions don't make sense. The computer science analogy is nasal demons: "When the compiler encounters a given undefined construct it is legal for it to make demons fly out of your nose." $\endgroup$ – David Hammen Feb 20 at 14:17
  • $\begingroup$ it is an interesting question but removing the water will change the chemistry of the atmosphere,theCO2 level will go up metane will decrease and the other gasses will change a lot too so it is not possible to answer this question. $\endgroup$ – trond hansen Feb 20 at 14:53
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    $\begingroup$ I'm voting to close this question as off-topic because it's asking about a counter-factual fictional world. If you are building a fictional world, this question may be suitable for Worldbuilding $\endgroup$ – 410 gone Feb 20 at 16:57
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    $\begingroup$ Gedankenexperiment questions can be useful if asked in a thoughtful way, and if answers can be written based on science then they can be perfectly reasonable Stack Exchange questions. Voting to leave open, there's no need to block others from posting additional answers. $\endgroup$ – uhoh Feb 21 at 11:14
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As was pointed out in the question comments, there is no way to know what Earth's average temperature would do without water vapor in the atmosphere. However, we can estimate the effects of the water, and remove them. What follows is a rough approximation.

If there were no greenhouse effect, the average temperature of Earth would be about 0F (-18C) instead of 59F (15C). (https://www.giss.nasa.gov/research/briefs/ma_01/)

Water vapor roughly doubles the warming caused by carbon dioxide. (https://www.nasa.gov/topics/earth/features/vapor_warming.html)

Therefore I estimate that without water vapor, the average temperature would be about 30F (-1C).

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