(Part of the How to Talk to a Global Warming Skeptic guide)
Objection: Climate scientists never talk about water vapor -- the strongest greenhouse gas -- because it undermines their CO2 theory.
Answer: Not a single climate model or climate textbook fails to discuss the role water vapor plays in the greenhouse effect. It is the strongest greenhouse gas, contributing 36% to 66% to the overall effect for vapor alone, 66% to 85% when you include clouds. It is however, not considered a climate "forcing," because the amount of H2O in the air basically varies 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). Similarly, due to the abundance of ocean on the earth's surface, if you somehow removed all the water from the air, it would quickly be replaced through evaporation.
This has the interesting consequence that if you could somehow instantly remove all CO2 from the atmosphere, the temperature would begin to drop, causing precipitation to remove H2O from the air, causing even further drops, in a feedback effect that would not end until no liquid water was left, only ice sheets and frozen oceans.
CO2 put into the air by burning fossil fuels, on the other hand, stays in the atmosphere for centuries before natural sinks finish absorbing the excess. This is plenty of time to have substantial and long-lasting effects on the climate system. As the climate warms in response to CO2, humidity rises and increased H2O concentration acts as a significant amplifier of CO2-driven warming, basically doubling or tripling its effect.
An article from RealClimate -- "Water vapor: feedback or forcing?" -- has a good discussion of this subject.
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Comments View as Flat
vaporman Posted 7:17 am
17 Jun 2007
water pumped from underground aquifers
for all the talk of "sequestering" CO2 how can we ignore the "de-sequestration" of water from underground aquifers? Here's the bottom line-- the high-end estimate of annual CO2 discharge is 25 billion tons per year. However, the annual rate of groundwater "de-sequestration" i.e. the net amout of water pumped to the surface and not returned underground is 160 billion tons per year. Even though all of this water is now in the atmopheric system, i.e. the lower troposphere, let us just assume that half, or 80 billion tons a year ends up as extra atmospheric water vapor. since water vapor has about ten times the "greenhouse effect" as CO2, this is the CO2 equvalent of 800 Billion tons!! Or about 32 times the impact of CO2 increase!! Water aquifer info is from various sources, including USGS and USAID.
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bobbyfontaine Posted 1:37 am
25 Jun 2007
pollution changes water vapor
water vapor does not condense normally into rain when polluted so there is more water vapor in the atmosphere because it has been made toxic and gaseous
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warreno Posted 11:32 pm
05 Jul 2007
Low physical concentration limits to water vapor
There is a very strict, and pretty low partial pressure limit to water in the atmosphere, as well as to limits on suspension of ice/water particles in the air. This is true even for slight rises of temperature. Suffice it to say that any amount of water removed from the ground goes into the ocean or possibly into ice. The main effect is localized freshening of sea water. The atmosphere really does not count as a storage in the water cycle.
Water vapor is essentially an system amplifier of heat forcing, where as C02 makes the forcing term larger.
I'm trying to think of a good mechanical analogy.
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barry schwarz Posted 5:32 pm
15 Jun 2008
water vapour discussed at length 2001 2007 IPCC
[Transferred in part (and slightly amended) from my post on the subject at illconsidered blogspot]
Water vapour gets a lot of mention in the IPCC documents (Intergovernmental Panel on Climate Change).
If you want to corroborate this, you can open the pdf files for each of the IPCC chapters at the links below, type 'vapour' (note the European/Australian spelling) in the search box, and click the appropriate button to search through each document (different versions of pdf have different search buttons).
IPCC 2001:
http://www.grida.no/climate/ipcc_tar/wg1/index.htm
Don't bother with the HTML pages on the left of that web page. They're just summaries of the chapters. Open the pdf files on the right hand side of that page and have a good search.
IPCC 2007:
http://ipcc-wg1.ucar.edu/wg1/wg1-report.html
I chose 3 chapters for 2007, the three I thought most likely would discuss water vapour (2, 3 and 8, from memory).
I also ran a search on 'carbon dioxide' and 'CO2' and compared the results.
Again from memory, there was 273 hits for 'water vapour', and 295 for 'CO2' and 'carbon dioxide' from those three chapters, including the studies referenced at the bottom of the page.
I chose 3 chapters from 2001 and did the same thing. There were tons of hits for water vapour in that one, too. I then checked all the chapters but didn't bother to count, as it was obvious that water vapour was discussed again and again. Some chapters only mention it a little.
Also, both the 2001 and 2007 reports had sections specifically dedicated to water vapour.
This says nothing about the focus or quality of the science, of course, but it completely dispels the myth behind this simple statement:
"Climate scientists dodge the subject of water vapor"
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