They probably meant carbon emissions. > water vapour, which creates contrains and impacts cloud formation. There's huge uncertainty how large these effects are

They probably meant carbon emissions. > water vapour, which creates contrains and impacts cloud formation. There's huge uncertainty how large these effects are, but they're significant. Water vapor emissions aren't cumulative in the same way as CO2. It would be potentially an impact of what ever level of air travel you have, but it wouldn't accumulate over time like carbon emissions, and so could be sustainable if air travel is held at a fixed level.