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Water Vapor Causing Climate Warming
karvind writes "According to BBC, new studies suggest that water vapor rather than carbon dioxide in the atmosphere is the main reason why Europe's climate is warming. The scientists say that rising temperatures caused by greenhouse gases are increasing humidity, which in turn amplifies the temperature rise. This is potentially a positive feedback mechanism which could increase the impact of greenhouse gases such as CO2. Even though 2005 will probably be warmest year, climatologists still differ in opinion"
There seem to be a few things missing in this discussion:
s .1896.climate.pdf
1. The fact that most of the warming associated
with global warming is directly forced by water
vapor is well established, going back at least
as far as Arrhenius's 1895 paper often credited
with "discovering" global warming.
(original paper at:
http://geosci.uchicago.edu/~archer/PS134/arrheniu
)
i.e. this result is CONSISTENT with our understanding
of global warming.
2. Increases in atmospheric water vapor are tightly tied
to temperature. The saturation specific humidity
(the amount of water air will hold) increases
exponentially with temperature (an implication of the
Clasius-Claperyon relationship). Thus when you increase the
temperature of the atmosphere by dT (by, for example, adding
some CO2), more water vapor evaporates into the atmosphere,
amplifying the warming.
3. This effect, known as the water vapor feedback, has been in
our climate models from the beginning (at least as far back
as 1895), and produces results consitent with observations.
4. The cited Geophysical Research Letters paper uses observations
to estimate the strength of the water vapor feedback and
finds that it is strong (even stronger than most models
predict). It is also a step in the process
of understanding climate change on a regional level.
Z