Water vapour accounts for about 1.00% of the atmosphere as an overall average, it varies from nothing in the exceptionally cold and dry parts of Antarctica to a maximum of about 4.00% in the very hot and humid tropical climates.
By comparison, carbon dioxide, the next most abundant greenhouse gas accounts for 0.0395% of the atmosphere. All the other greenhouse gases account for about 0.0002%.
It’s harder to be specific when it comes to the contribution that each of these gases has on global warming and that’s because the warming potentials of each gas overlaps and because the presence of water vapour is needed to enhance the absorption capacity of the other gases.
Very approximately, water vapour causes 75% of global warming, carbon dioxide is 20% and the other greenhouse gases make up the remaining 5%.
Yes it is quite true, as several people above have said, water and clouds account for about 75% of the greenhouse effect, with the other greenhouse gases, primarily CO2, accounting for the other 25%. However, back in the 1890s Arrhenius realized that the concentration of water vapor in the air was limited by its saturated vapor pressure, which is dependent on the temperature. How then, could an increase in H2O increase the temperature when it was itself limited by the temperature? Carbon dioxide has no such limitation, so Arrhenius turned his attention to finding the climate’s sensitivity to carbon dioxide. Though Arrhenius’s model was simple and the calculations were laborious, he found that doubling the carbon concentration would increase the temperature of the earth by about 5°C. However, the increase in temperature would allow a greater concentration of water vapor in the air which would amplify the warming. Thus, the concentration of CO2 acts as a regulator of water vapor, and ultimately determines the planet’s long-term equilibrium temperature. Recent work using better data and models have found that the climate sensitivity to carbon dioxide is in the range of 3 to 4°C, and carbon dioxide has been proposed as the “control knob” for the Earth’s temperature. Still, water vapor and clouds contribute the most to greenhouse warming, and their contribution is considered to be a positive feedback to the increasing carbon dioxide concentration.
The article below examines the role of water vapor in global warming and some of the theories such as those by Lindzen, Spencer, and Pielke that claim otherwise.
Yes it is. It is also a much more powerful greenhouse gas than carbon dioxide, by a wide margin. The only way CO2 can cause anything near the warming predicted by the IPCC is through an amplification mechanism, i.e. by causing an increase in water vapor concentration.
The existence of the amplification has been postulated, but never validated.
The claims of high climate sensitivity to CO2 presume not only the above amplification, but the absence of other negative (stabilizing, self-correcting) feedbacks.
If the climate sensitivity to CO2 were as high as the warmists claim, any one of several periods in geologic history, where the CO2 concentrations ranged as high as 6500 ppm (about 450mya), would have caused the climate to go divergent, and 'slam to the stop' (runaway global warming), and stay there. Instead, in each of these periods, the temperature came back down from about 20-25C, and did so *before* the CO2 levels came down.
None of this behavior agrees with the anthropogenic global warming theory. James Hansen's 'tipping points' appear to not exist.
The empirical data suggest that the climate sensitivity is something close to 1.3 deg C/2xCO2, not the 2-4.5C that the IPCC claims in AR4.
From ex-warmist Nir Shaviv, PhD:
"If the cosmic ray flux climate link is included in the radiation budget, averaging the different estimates for the sensitivity give a somewhat lower result, namely, that λ=0.35±0.09°K/(W m-2). (Corresponding to ΔTx2=1.3±0.4°). Interestingly, this result is quite similar to the so called "black body" (i.e., corresponding to a climate system with feedbacks that tend to cancel each other)."
and...
"... once the CRF [cosmic ray flux] is included in the sensitivity estimate, the scatter in the different sensitivity estimates is much smaller. This is strong empirical evidence suggesting that the CRF is indeed affecting the climate."
Trevor is right about the atmospheric composition percentages. Water vapor varies from about 1-5%, considerably greater amounts than CO2.
Water vapor is also more important in terms of its effect as a greenhouse gas. This study measures each gases effect in W/m2 of radiative forcing (an imposed change on the planet's energy balance), and it calculates:
Water 60%
CO2 26%
O3 8%
CH4 & N20 6%
Of course the thing to remember is that we aren't changing the long term trends of atmospheric water levels in the same way we are with CO2.
I would also be interested in seeing any scientific sources for the suggestion that water vapor constitutes 95% of the greenhouse effect...Tomcat
Yes it is. The amount of water vapor in the atmosphere is dependent on temperature, through the Clausius-Clapeyron equation. That means that if another greenhouse gas increases, and raises the temperature, then it will also increase the amount of water vapor, further increasing the temperature.
EDIT: Tomcat, can you give a source for your 95-98% numbers? I've never heard anything like that from any SCIENTIFIC source--they sound like they came from Michele Bachmann or Rush Limbaugh. I know of at least one paper "Attribution of the present-day total greenhouse effect" by Schmidt et al that estimates a much lower value than that--only about 50%, rather than the astronomical numbers that you quote.
http://onlinelibrary.wiley.com/doi/10.10...
Another EDIT: So Lindzen is STILL claiming that? He was claiming that 20+ years ago, but I was under the impression that even he had backed off from that claim, after several studies found it significantly in error.
Yes it is, at current temperature levels it accounts for at least 95% possibly 98% of the greenhouse process. Recent finds indicate that changes in Stratospheric Water Vapor content may account for over half the global warming experienced during the last 30 years, which cast doubt on CO2's role in the current alleged energy imbalance of our climate which would put the role of water vapor in the green house process much higher. What is the dwell time of changes of H2O in the stratosphere?
Climate sensitivity is clearly set too high, the radiative transfer theory is flawed beyond imagination. The climate warmed from 1910 to 1945 at just as fast of a rate with no imbalance. I don't have time to publish a paper on the subject.
Lindzen frequently suggested 98%, currently we are receiving about 130 watts per square meter more energy from the sun than 6 months ago (Perihelion), do you see an increase of several degrees C in Earths mean temperature?
I have always had the idea that those fluffy white things that stop the sun getting through had a hand in both weather and climate. The good news is that there is an awful lot of them.
It sure is. But it won't fly because you can't tax it.
