Science Forums 2

[Chris C]

A few points--

turtle,

Global Warming by definition is just ΔT, not absolute T, so you need to look at drivers of climate change which can initiate a change from state 1 to state 2, and water vapor cannot do that. Then, you can look at 'feedbacks' (or what changes as a reaction to climate change which amplifies or dampens the original forcing). The realclimate article is an easy-to-understand way of saying that the "saturation vapor pressure" goes up or down as a function of temperature. Water vapor is a rather weak absorber near the planck function for Earthlike temperatures, and is more or less controlled by CO2 as WV impact is temperature dependent so if you remove all the CO2 then the water vapor impact will decrease exponentially since it condenses in a cooler climate. Gaseous H20 has a very short residence time in the atmosphere (~10 days; CO2- centuries to millennia), and also, there is a lot less WV as you go up to the higher, colder, lower pressure part of the atmosphere where the Earth's heat balance is determined. It takes two to tango, so even though WV is helping keep the planet warmer than CO2 is, that impact is very much CO2-dependent, and the CO2 impact is still non-negligible (about 30 W/m^2 of the 150 W/m^2 of outgoing energy that gets absorbed by the greenhouse effect).

As for numbers, WV actually makes up about 2/3 of the longwave radiation absorbed by the atmosphere (table 3; http(www).cgd.ucar.edu/cas/Trenberth/trenberth.papers/KiehlTrenbBAMS97.pdf ). I think the impacts in units W/m^2 by quantifying the role in the greenhouse effect or change in greenhouse effect is the best way to look at it. For example, doubling the atmospheric CO2 has a radiative forcing of 4 W/m^2, and so at equilibrium you go from around 150 W/m^2 absorbed by the atmosphere to about 170 W/m^2. A change from 280 to 380 ppmv is not 0.00X percent, and this is where bad logic of small percentages should stop. A good first step is recognizing that ~99% of the atmosphere (O2, N2, Argon) do not contribute to infrared absorption and emission so they can be removed from our numbers already.