CO2 seems much worse pollutant than water long term, because it stays in the air. Water at high altitudes condenses, reflects some radiation back to space and then falls back down.
Could you give a source for that? My understanding after some search is that effect of high-altitude water is hard to evaluate. I agree that at high altitudes contrails may evaporate and the water then may not fall down... Is it possible that jet exhaust water vapor accumulates in stratosphere? That would be terrible.
It is hard to evaluate, but of course being hard to evaluate does not mean it's not a real thing.
The effect (not counting contrails) is about double the effect of CO2 emissions alone. Including contrails, it may be even higher. There are mitigation strategies to deal with these non-CO2 radiative forcing effects, though it's very complicated.
> At pressures greater than 500 hPa (i.e. below roughly 6 km), the forcing [of water vapor] is assumed to be negligible
So flying below 6km would help a lot for impact of water vapor. However, given this was not yet acted upon, IPCC is probably right that
> the effect of water vapour emissions is likely to be
a significant, or even the dominant, contributor to their climate forcing.
This is even worse given the projections of aviation expansion. Right now the water does not seem to be a big problem and it could stay that way - water lifetimes even in stratosphere is counted in years, as opposed to centuries for CO2 - but if aviation expands, then it becomes more serious than CO2.
