For what it's worth, contrails only form when conditions are right at certain altitudes, so if we had renewable fuels we could theoretically move aircraft outside the contrail altitude window. It would piss off controllers and burn slightly more fuel sometimes but it could be done.
This became a minor fake controversy recently when Google changed their carbon footprint measures on flight to stop estimating these non carbon effects.
The reason they did so was they were just using a rule of thumb that applies to all flights and they want to be able to encourage airlines to adopt the known practices that can avoid these extra emissions (shifting time of flights is the big one I think).
There's no standard yet for estimating them though, but they are working on one:
I believe it's not actually contrails specifically but rather the effects of releasing exhaust (CO2) at a high altitude (which would happen whether the contrails form or not).
CO2 is just the basic evil everyone talks about. Water is a lot more evil (much broader range of wave lengths of light absorbed than the comparatively tiny range of CO2), if it is in the air.
Water changes it behavior depending on the relative amount in the air. A sufficiently high amount will act on visible light (contrails, clouds, fog, ...), but even in its "transparent" form it messes with wave lengths outside of the visible range.
Contrails are created by local changes of temperature at the wing tips (turning the invisible water into visible, temporarily) and by adding water as part of the exhaust. Burning CxHy leaves some H2O, visible or not.
I would have guessed that the visible form is contributing more to cooling than the (probably more common) invisble form. Possibly a matter of when they turn invisible, which might be more likely in the morning than at night (those produced during one day staying visible during the night helping with heating but vanishing next day, not helping with cooling).
Modelling water properly is quite surely complicated, with bigger effects than CO2. Not only for contrails.
Contrails are ice from water in the exhaust, unrelated to the wingtips. You will sometimes see vortices at high angles of attack on takeoff or landing, but those don’t turn into contrails.
It’s true, though. Water causes roughly 30°C of global warming. We can’t get much if any more from water vapor, but there are several positive and negative feedbacks from clouds that are quite interesting. High clouds are ice, which allow transmission of visible light, but absorb in the infrared.
