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In textbooks, while water vapour and carbon dioxide are said to be the greenhouses gases which are most abundant in the atmosphere, when a table is given showing the top greenhouse gases emitted by humans, water vapour is omitted (carbon dioxide and methane are the top two).

However, when methane is oxidised to release energy from fossil fuels, the products are carbon dioxide and water (i.e. water vapour), therefore, aren't humans responsible for the concentration of water vapour present in the atmosphere? Why is water vapour generally omitted from the tables?

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    $\begingroup$ Most likely because our atmosphere contains already huge amounts of it. This is different for CO2. $\endgroup$ – Chris Apr 8 '16 at 21:47
  • $\begingroup$ Your title confuses me. Is your question if water vapour is produced by fossil fuel combustion (answer: yes) or if water vapour is a greenhouse gas (no, not really, see answer EdM)? $\endgroup$ – RHA Apr 10 '16 at 6:24
  • $\begingroup$ @RHA the terminology here is tricky. The ACS page I linked suggests that water vapor accounts for over half of the total greenhouse effect, so it should be called a greenhouse gas. It's not, however, a greenhouse gas that's being affected by human activity in a way that increases the greenhouse effect, unlike carbon dioxide. $\endgroup$ – EdM Apr 10 '16 at 12:37
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    $\begingroup$ I'm voting to close this question as off-topic because it is not about biology. It appears to be about climatology or some such subject. $\endgroup$ – David Aug 7 '17 at 18:10
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Althugh water vapor contributes to greenhouse warming of the atmosphere, changes in atmospheric water vapor tend to follow rather than drive changes in temperature. Also, in the form of clouds, atmospheric water can have a cooling effect on earth temperature.

Clouds illustrate why water vapor, unlike carbon dioxide and methane, is not thought of as a driver of recent changes in earth temperature but rather is more of a follower. Unlike those other two gases, if you keep trying to put water vapor gas into the atmosphere it will convert to liquid water at some point, leaving only a constant level of water in the form of gas. That's what happens when a cloud forms; what you see and what can shield you from the sun is a collection of water droplets or ice crystals. Large amounts of water vapor are also regularly put into the atmosphere by evaporation of water from the earth's surface, dwarfing the contribution from human respiration. Eventually the atmosphere gets locally saturated with water vapor and it falls back to earth as rain or snow in the hydrologic cycle.

In principle methane and carbon dioxide could similarly condense out of the gas phase, but not at temperatures that occur in our atmosphere. You need to go far out into cold reaches of the Solar System to find naturally occurring methane lakes or solid carbon dioxide. There is no equivalent of rain to remove excess of those gases from our atmosphere. Carbon dioxide can dissolve in water to leave the atmosphere, but at the cost of acidifying the water. Ocean acidification is another serious result of adding carbon dioxide to the atmosphere.

The amount of vapor you can add before conversion to liquid water increases with the temperature. So as land temperature increases the rate of evaporation of water into the atmosphere can increase, and as atmospheric temperatures increases the amount of water vapor that the atmosphere can hold also increases. So in principle water vapor might thus serve to amplify the greenhouse warming from other gases, although its transition to liquid or ice in the atmosphere might tend to counteract that. This is a major issue being investigated in attempts to understand the interplay of factors that determine earth temperature.

A focus on carbon dioxide has an important practical aspect. If you control the excess release of carbon dioxide due to fuel combustion, you necessarily also control the associated excess release of water vapor.

In summary, human activity ends up having much less direct influence on changes in atmospheric water vapor than it does on changes in carbon dioxide or methane. That's why the latter two gases rightfully receive more attention as controllable greenhouse gases.

The American Chemical Society also discusses this issue.

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The answer from the ACS foes not make sense. They contradict themselves in order to support the main party line that CO2 is the problem. Water vapor is a far more potent greenhouse gas than CO2.The ACS concedes it makes up 60% of the entire earth's greenhouse effect. According to the chemical reaction of combustion of greenhouse gases twice as much water vapor is created as CO2. Thus, if there was a substantial greenhouse effect at all it must be due to water vapor. The data does not support it at all so it is left out of all EPA, IPCC and other political discussions of climate science.

The two main contention of the ACS are invalid. First, CO2 rise also follows temperature, as they report is the case for water vapor. This is separate from anthropogenic sources of the gases, though! The reason is that CO2 solubility in the ocean decreases with temperature rise, thus the ocean releases CO2 into atmosphere from warming. The other claim is pure speculation that the water vapor saturates the air and forms clouds. They do not present any evidence of increased clouds due to combustion, which would actually cool the surface temperature by blocking sunlight, the main source heating the earth.

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    $\begingroup$ Water vapor has a positive and a negative green house effect. Clouds cancel out the sun. Additionally: The concentration of water in the athmosphere should be more or less constant, while the concentration of CO2 is rising... $\endgroup$ – Chris Aug 7 '17 at 15:13
  • $\begingroup$ Natural feedback mechanisms control the amount of water vapor in the atmosphere. Added water vapor vapor becomes rain in short order, increasing sea level by a miniscule amount. OTOH, CO2 is a driver because there are no feedback mechanisms that regulate its concentration, at least on any timescale relevant to humans. $\endgroup$ – jamesqf Aug 7 '17 at 19:58

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