What is a moist greenhouse effect? I've heard that this is where all the water on a planet is quickly driven into space. However, I cannot find much information on this- is it a relatively new (or obsolete) idea and what causes it? Also, any links regarding this would be appreciated.

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    $\begingroup$ Search for "moist greenhouse" using your favorite search engine. This will give you some Wikipedia links on related topics. On these WP pages, search for the text "moist greenhouse." You'll eventually find scientific citations on this topic. Or, more directly, use Google Scholar. $\endgroup$
    – BMS
    Sep 30, 2014 at 18:54
  • $\begingroup$ If the planet is large enough, it will not lose its hydrogen and the atmosphere will stay saturated with water vapor. $\endgroup$
    – CuriousOne
    Sep 30, 2014 at 19:28
  • $\begingroup$ (See also Earth Science Stack Exchange) $\endgroup$
    – gerrit
    Oct 1, 2014 at 14:56

2 Answers 2


What is a 'moist greenhouse effect'?

That's the Earth's fate in a billion years or so.

Right now, and for millions of years to come, the very sharp temperature inversion and the very cold temperatures at the tropopause keep the stratosphere very, very dry. This won't always be the case. The problem is that yellow dwarf stars such as our Sun output ever more energy as they age. The Sun will be shining about 10% brighter in a billion years than it does today. That extra energy from increased sunlight may be enough to push significant amounts of water vapor into the stratosphere and beyond.

That upper atmospheric water vapor will be subject to the more energetic portion of the solar spectrum, which has enough energy to dissociate water. The resultant hydrogen can leave the atmosphere. Our atmosphere currently loses about three kilograms of hydrogen per second to outer space. That loss rate will grow much larger a billion years from now, and the oceans will gradually disappear.

  • $\begingroup$ Interesting information about the dissociation of water molecules, the subsequent formation of hydrogen, and its escape from earth. The sun is much brighter for Venus today - so is that mechanism already at work on Venus? If I'm not mistaken, Venus is also a smaller planet (gravitational acceleration on the surface is < 9 m/s^2) so less gravity to hold the gases bound? Does this result in a very oxygen rich environment? Or does H2O2 become the predominant species? $\endgroup$
    – Floris
    Sep 30, 2014 at 23:15
  • $\begingroup$ Venus lost its water long, long, long ago. Since 96.5% carbon dioxide, and since carbon dioxide is 2/3 oxygen, one could say that Venus's atmosphere is a very oxygen rich environment. $\endgroup$ Sep 30, 2014 at 23:46
  • $\begingroup$ Funny how the moist "greenhouse effect" involves the loss of our strongest greenhouse gas. $\endgroup$
    – gerrit
    Oct 1, 2014 at 14:58
  • $\begingroup$ @gerrit - Yeah, it's a bit of a funny name. A moist greenhouse is distinct form a runaway greenhouse, where liquid water does not exist on the surface. A moist greenhouse is highly saturated from the surface well into the upper reaches of the atmosphere, rain can fall, and liquid water exists on the surface. $\endgroup$ Oct 1, 2014 at 15:30

From the book "Terraforming: The Creating of Habitable Worlds":

enter image description here

This seems pretty unambiguous. Water is a greenhouse gas. When the atmosphere gets warm, more water evaporates -> more greenhouse gas, more heating. Boom, no more life on earth.

  • $\begingroup$ Is there any reason why many other websites/books etc explain the Venusian atmosphere using the runaway greenhouse effect (rather than the 'moist' one)? $\endgroup$
    – user58953
    Sep 30, 2014 at 22:04
  • $\begingroup$ @user58953 - It's an issue of degree (pun intended). A moist greenhouse has water vapor reaching into the stratosphere and beyond but still has liquid water on the surface. A moist greenhouse starts when the surface temperature reaches about 65 °C. The water loss is slow but steady. A runaway greenhouse happens when the surface temperature becomes well over 100 °C. This will increase the boiling point (water will no longer be a trace gas). At some point, the greenhouse effect feeds on itself, eventually raising the surface tempature to over 400 °C (and then no water is left). $\endgroup$ Sep 30, 2014 at 22:29
  • $\begingroup$ There are several greenhouse gases: CO2, methane, water are among the most important. I think that the use of "moist greenhouse" refers to the presence of water in the atmosphere as a greenhouse gas (which ultimately leads to the runaway effect), but maybe I am interpreting it the wrong way... $\endgroup$
    – Floris
    Sep 30, 2014 at 23:12
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    $\begingroup$ @Floris - Water is by far the most potent greenhouse gas in the atmosphere right now. The difference between the greenhouse effect we see now (which is absolutely essential to life) and a "moist greenhouse" is that right now, what goes up must come down rules the day. With a moist greenhouse, significant amounts of water vapor break through the tropopause, get photodissociated, and then the hydrogen goes up and out. $\endgroup$ Oct 1, 2014 at 15:09

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