David Fraser
Senior Member.
Are we not introducing water that was not in the cycle and consistently increasing the levels?Disagree!
No, in the context of anthropomorphic climate change, water vapour is not a pollutant in the same way that CO2 is.
We (human civilisation) are not tapping into a vast and ancient reservoir of water, that has been locked out of the water cycle for millennia, and introducing it into the current water cycle. That is not what's happening.
Water vapour just happens to be one of the 3 important greenhouse gases.
Water has a relatively short life time in the in the atmosphere - 9 days, whereas CO2's life time is estimated at 30 to 95 years.
http://en.wikipedia.org/wiki/Greenhouse_gas#Atmospheric_lifetime
The EPA even tried to have water vapour classifed as a pollutant back in 2007 or so.
http://www.epa.gov/climatechange/endangerment/comments/volume9.html
It was rejected but not without objections.
As far as I understand it much of the evidence was based on the IPCC exclusion.
Comment (9-29):
A number of commenters (e.g., 2885, 3509.1, 3577.1, 3702.1, 3747.1) question the exclusion of water vapor from the definition of air pollution because it is the most important GHG responsible for the natural, background greenhouse effect.
Many commenters argue that water vapor has a greater contribution to the greenhouse effect than any other substance. Several commenters (e.g., 1924, 0639.1) note that water vapor is produced by combustion, and question its exclusion (one commenter noted that it constitutes 26% to 45% of the products of combustion).
Similarly, a commenter (3397) objects to the categorization of anthropogenic water vapor emissions as insignificant, and contends that they are the same percentage as CO2 emissions. The commenter indicates that EPA provides no reference for the implication that water vapor is not “long-lived” in the atmosphere. Furthermore, the commenter notes that the Millennium Ecosystem Assessment has estimated that 750 cubic kilometers (750 billion meter tons) of water vapor are released annually by human activities, or 6% of the global total. The commenter notes that one-third of this annual amount is fossil water that is not recharged to the ground water. The commenter requests that EPA at least explain why anthropogenic emissions of water vapor should be considered insignificant.
One commenter states that this exclusion demonstrates that the finding is arbitrary and capricious, because water vapor is the most abundant GHG and therefore the CAA compels the inclusion of water vapor in the definition of air pollution. The commenter claims that the inclusion of non-202(a) compounds SF6 and perfluorocarbons (PFCs) weakens the argument that direct water emissions from motor vehicles are negligible. The commenter requests more discussion of water vapor, including localized variations due to irrigation and contrails. Furthermore, the commenter states that if direct emissions of water vapor are excluded from the endangerment finding, then it is inconsistent to include water vapor from feedbacks for the other GHGs.