Therefore, we can handle the forcing from water vapor in terms of a feedback (W/m2/K) multiplied by a temperature change. It doesn’t need to be done that way, but it is far simpler. Increasing water vapor also reduces the lapse rate, so any surface warming should produced enhanced warming at higher altitudes, causing a greater increase in OLR than expected for a given increase in surface temperature. We could also call changes in the lapse rate another forcing from water vapor, since this does change the radiative balance at the TOA.

It really helps to keep two phenomena separate: forcing and feedback. The planet’s response at the TOA to any change in surface temperature – even one caused by internal variability is a feedback. If we could mix the ocean and lower the average surface temperature everywhere by many degC, we would have a temperature change without any forcing. And the climate feedback parameter (W/m2/K) times that temperature change would tell us how the radiative balance at the TOA would change, including the effect of less water vapor in a colder atmosphere. So it makes sense to include the effects of water vapor as a response to temperature change – a feedback – rather than a driver of temperature change.

This will be correct – of course – only if water vapor is controlled by temperature. If changes in agriculture change water vapor, this approach will be incorrect.

Isn’t water vapor controlled by evaporation, not temperature? The evaporation rate is proportional to wind speed and “undersaturation”, but evaporation can’t continue indefinitely without the vertical convection needed to produce precipitation. And convection doesn’t occur without an unstable lapse rate. As convection warms the upper troposphere, the lapse rate will not remain unstable unless heat is radiated across the TOA just as fast as it is convected upwards. (Modtran can demonstrate that net radiative cooling, OLR-DLR, doesn’t change much with surface temperature.)

]]>Take it as a sort of joke, but next time i might ask how the forcing from watervapor depends on the CO2 content.

]]>In the tropics, where it is both warmer and more humid, doubling reduces OLR by 3.3 W/m2. Warmer surface temperature means there is more OLR that can be absorbed and more humidity that can absorb it. In this case, temperature happens to be more important. Perhaps the opposite is true in polar regions.

]]>My question was rather simple:

The forcing from methane is dependent of other gasses in a complicated formula.so i wonder if the CO2 forcing really is independent of even the most powerfull GHG, water vapor?

How could it be that the forcing from changing CO2 content 5.35ln(C/Co) is independent of the other green house gasses especially H2O.

I find it strange that this very simple forcing formula is completely independent of

other GHG gasses that share the same radiation bands.

If the answer is that it is so, i would like a little explanation of how that could be.

If the answer is that it is dependent i would like a better formula.

I know it is very complicated because water vapor vary a lot with hight, but anyway some approximations would do.

I am not born and raised with english. Concider that if you find my wording a little strange.

]]>For example, by changing the water vapor scale from 1.00 to 1.01, I think you are modeling a 1% increase water vapor at all altitudes as a forcing. If you use a temperature offset of 1 K, you can compare constant absolute humidity to constant relative humidity. The difference appears to be water vapor feedback.

For clear skies, OLR is presumably calculated by integrating the Schwarzschild equation from the surface (at surface temperature, ? emissivity) to the TOA. For cloudy skies, OLR is calculated starting from the cloud top (temperature determined by lapse rate?) and integrating to space. MODTRAN offers a variety of cloud options. If you “look down” through clear skies from 2 km, you can compare OLR at 2 km to a hypothetical cloud emitting blackbody radiation upward from that altitude.

]]>It’s a complex and technical subject.

There’s some terminology that’s important.

“Forcing” = (in simple terms) change in the radiative balance of the planet as a result of changes in GHGs, like CO2.

“Feedback” = the resulting changes following radiative forcing, for example, the changes in water vapor and clouds and how they affect the climate.

So changes in CO2 are a forcing and the resulting changes in water vapor and clouds are a feedback.

But what you ask – as I read it – might not be what you mean to ask.

Depending on your appetite for learning about the important concepts, you might find these articles helpful:

Wonderland, Radiative Forcing and the Rate of Inflation

Wonderland and Radiative Forcing – Part Two

You might really be asking about whether water vapor overwhelms the CO2 effect?

Visualizing Atmospheric Radiation – Part Four – Water Vapor

– it might be best to start at the beginning of that series to get familiar with the concepts.

]]>The forcing from methane is dependent of other gasses in a complicated formula.so i wonder if the CO2 forcing really is independent of even the most powerfull GHG, water vapor?

A connected side question: Is the CO2 forcing dependent of the cloud cover?

Please state the assumptions you have to make to answer this rather complicated questions.

I have tried to find answers, but have failed so far. Hope You can help.

]]>