Gavin Schmidt:
RealClimate: Water vapour: feedback or forcing?: While water vapour is indeed the most important greenhouse gas, the issue that makes it a feedback (rather than a forcing) is the relatively short residence time for water in the atmosphere (around 10 days).... I did a GCM experiment where I removed all the water in the atmosphere.... At Day 0 there is zero water, but after only 14 days, the water is back to 90% of its normal value, and after 50 days it’s back to within 1%. That’s less than 3 months. Compared to the residence time for perturbations to CO2 (decades to centuries) or CH4 (a decade), this is a really short time.
Only the stratosphere is dry enough and with a long enough residence time (a few years) for the small anthropogenic inputs [to water vapor] to be important. In this case (and in this case only) those additions can be considered a forcing....
When surface temperatures change (whether from CO2 or solar forcing or volcanos etc.), you can therefore expect water vapour to adjust quickly to reflect that. To first approximation, the water vapour adjusts to maintain constant relative humidity.... [T]he total amount of water vapour will increase adding to the greenhouse trapping of long-wave radiation. This is the famed ‘water vapour feedback’....
How do we know that the magnitude of this feedback is correctly simulated? A good test case is the response to the Pinatubo eruption. This caused cooling for up to 3 years after the eruption – plenty of time for water vapour to equilibriate to the cooler sea surface temperatures. Thus if models can simulate the observed decrease of water vapour at this time, it would be a good sign that they are basically correct. A good paper that demonstrated this was Soden et al (2002) (and the accompanying comment by Tony DelGenio). They found that using the observed volcanic aerosols as forcing the model produced very similar cooling to that observed. Moreover, the water vapour in the total column and in the upper troposphere decreased in line with satellite observations, and helped to increase the cooling by about 60% – in line with projections for increasing greenhouse gases.
To be sure there are still some lingering uncertainties.... However, given the Pinatubo results, the models are probably getting the broader picture reasonably correct.
