"..Aircraft also emit water vapor at high altitude that forms condensation trails or "contrails." Contrails are visible cloud lines that form in cold, humid atmospheres and contribute to the warming impacts of aircraft emissions. Moreover, the persistent formation of contrails is associated with increased cirrus cloud cover, which also warms the Earth's surface.
Together, these high altitude emissions have a greater global warming impact than if the emissions were released at ground-level. A recent report by the UK Royal Commission on Environmental Protection found that the net effects of ozone, contrail and aviation-induced cloud cover is likely to triple the warming effect of aircraft-emitted CO2 alone..."
The UK environmental advisory body referred to in the above extract is actually called the "Royal Commission on Environmental Pollution". It was originally set up by the Wilson government in 1970, but the Coalition government thankfully abolished it in 2011.
I wouldn't be surprised if there was an environmental report somewhere which claimed that water vapour emitted at ground level increased the amount of cirrus cloud cover as well. But I'm not aware of any political climate policy taking on the idea of it being necessary to curb man-made water vapour emissions in some way.