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Argument: Water vapors do not have a "forcing" effect on climate change like human greenhouse gases
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]Argument summary
This argument recognizes that water vapor is the most prominent greenhouse gas in the atmosphere. But, it makes the point that this greenhouse gas has always been in the Earth's atmosphere and persists at relatively static levels. It is responsible for warming the earth indeed, but in a flat, sustainable way; not adding additional warming to the climate that it has fostered. Human greenhouse gases, on the other hand, are adding to the level of greenhouse gases in the atmosphere, subsequently "forcing" the increased warming of the earth, adding to the warm climate we enjoy, and causing what we call global warming and climate change.
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]Supporting evidence
- "How to talk to climate scientist. Climate scientists hide water vapor". February 22, 2006 - "Objection: Climate scientists never talk about water vapor, which is the strongest Greenhouse Gas, because it undermines their CO2 theory...
- Answer: There is no climate model or climate textbook that does not discuss the role water vapor plays in the Greenhouse Effect. It is the strongest Greenhouse gas, contributing 66% to 85% to the overall effect when you include clouds, 36% - 66% for vapor alone. It is however, not considered as a climate "forcing" because the amount of H2O in the air varies basically as a function of temperature. If you artificially increase the level of H2O in the air, it rains out immediately (in terms of climate response times), similarily, due to the abundance of sea surface, if you somehow removed water from the air it would quickly be replaced through evaporation. This has the interesting consequence that if one could somehow instantly remove all CO2 from the atmosphere, the temperature would begin to drop, causing percipitation to remove H2O from the air causing even further drops, in a feedback effect that would not end until no water was left unfrozen on the ground.
- CO2 put into the air by burning fossil fuels, on the other hand, has an atmospheric lifetime of centuries before natural sinks will significantly absorb any excess from the air. This is plenty of time to have substantial and even longer lasting effects of the climate system."
- "Water vapour: feedback or forcing?". Real Climate. April 6th, 2005 - "While water vapour is indeed the most important greenhouse gas, the issue that makes it a feedback (rather than a forcing) is the relatively short residence time for water in the atmosphere (around 10 days)."
- Stefan, commenting on "Water vapour: feedback or forcing?". Real Climate. April 6th, 2005 (a very good outline of this argument)] - "I’ve seen a slightly different '98%' argument made by climate skeptics (e.g., U. Berner in various popular brochures and articles): '98% of the greenhouse effect is natural and only 2% anthropogenic'. This is because the anthropogenic greenhouse gas forcing (~2.4 W/m2 until now, here IPCC is cited) is only ~2% of the total ('natural') greenhouse forcing.
- What these people usually leave out, of course, is that the natural greenhouse effect warms the surface temperature by ~33 ºC (all else being equal, notably albedo).
- A rough and linear back-of-envelope estimate of how large the 2% anthropogenic change in greenhouse effect might be: 2% of 33 ºC is ~0.7 ºC, very similar to what the more detailed and correct calculation gives.
- So, there is nothing wrong with this version of the 2% argument - except that it is often used in a context and in a way designed to mislead the public, i.e., it is used to suggest to a lay audience that 'global warming is 98% natural', so we are not responsible and can’t do anything about it."