Climate Change
Understanding the impact of increased water vapor on global climate is
very complicated because climate involves numerous natural and anthropogenic
(human-induced) factors with direct and indirect effects. Furthermore, the
earth has had
major climate swings over its history.
Nevertheless, water vapor is an important factor in global climate and we
explore the two major effects and a discussion on each.
Greenhouse Effect (Warming)
Water vapor is present in all parts of the atmosphere. Particularly when it is present in the upper atmosphere, water vapor is a very powerful greenhouse gas. Greenhouse gases allow sunlight to pass through them, but when the light hits the earth and is reflected back toward space as infrared radiation, they absorb it. This effect is depicted in the diagram below.
http://www.enchantedlearning.com/ggifs/Greenhouseeffect.GIF
Water vapor is the most significant greenhouse gas, contributing approximately
50% of the total greenhouse effect. (Carbon dioxide is second most
important, contributing approximately 15%). Increased water vapor in
the atmosphere would increase the "greenhouse effect" by trapping additional infrared radiation, which would increase the
earth's temperature.
Cirrus clouds affect the earth in a similar way as
greenhouse gases. These clouds are optically thin and allow shortwave
sunlight to pass through them, but trap the earth's reflected longwave
radiation. Aircrafts produce cirrus clouds by their water vapor in
their exhaust, called contrails (condensation trails).
Contrail-induced cirrus clouds from aircraft cover an average of 0.1% of the sky
at any given time. The projected contribution to global warming is
approximately 0.1-0.2 degrees Celsius per
decade, with high uncertainty1.
Increased Cloud Formation (Cooling)
As noted on the Humidity page, increased water vapor in the atmosphere can increase cloud formation. Clouds cause daytime temperatures to be cooler by reflecting the sun's shortwave radiation, and cause nighttime temperatures to stay warmer by trapping the earth's reflected longwave radiation. This is shown in the diagram below.
http://earth.usc.edu/~geol150/weather/images/cloudreflect.jpg
Most clouds cause a significant overall cooling effect because they block so much of the sun's heat from reaching the ground. Cirrus clouds are the exception - they appear to trap more heat than they reflect.
I bet you're starting to think that this is getting
really complicated. Warmer temperatures lead to increased
evaporation which leads to even warmer temperatures, but also more clouds
which leads to cooler temperatures. So which is it -- warmer or
cooler?? The latest scientific consensus is that the the greenhouse
effect will continue to cause global warming, albeit with increased clouds and
precipitation which may lead to offset some, but not all of the temperature
increase.2
And The Effect of Water Vapor from Combustion Is.....
In order to reach a conclusion, let's compare the amount of water vapor produced from combustion to natural sources of humidity:
Combustion
water vapor: As calculated on the
Oil to Water page, humanity releases 8 billion kg of water vapor from
combustion into the atmosphere each day.
Natural
water vapor: Each day, the sun evaporates about 1 trillion tons of water worldwide. This
converts to 900 trillion kg per day. (Wow!)
Water vapor evaporated
naturally amounts to 10,000 times that produced by combustion globally. Or
said another way, all of the water vapor from combustion is only 0.01%
that of natural evaporation. This is likely to be insignificant, or a
"drop in the bucket" if you will. It should be noted that other aspects of
combustion, such as carbon dioxide and aerosol formation, may have a significant
effect on climate change.
In large cities, the water vapor from combustion is much more concentrated than the global average calculated here. Let's see if water vapor could affect weather in a regional urban area.
1 Travis et
al., J. Climate, 17, 1123-1134, 2004.
2 2001 IPCC
Climate Change Report:
http://www.ipcc.ch/pub/un/syreng/spm.pdf
