I remain in questioning mode over the influence the massive increase in polar air travel in the last 30 years. Up until 1989 high altitude jet aircraft rarely flew north of 80 degrees latitude because such actions were pointless. Starting in 1990 or 1991 the FSU opened air corridors between North America and South Asia through Siberia to civilian air travel. As a consequence there are now direct flights from central North American international airports, Detroit, Chicago, Saint Louis, Cleveland...that pass directly through the 80 North circle of air space at 25,000 to 35,000 feet on routes to India, Pakistan, Hong Kong, even Bangkok. Every one of those flights releases thousands of gallons of water equivalent as water vapor in the polar Stratosphere which is as low as 12,000 feet in the dead of winter. Sure, one or two flights is insignificant, but over the last 28 years there have been a dozen a day 365 days a year. The water vapor they release at altitude is quite hot at release which makes it buoyant compared to the surrounding air masses so it tends to rise even higher than the release altitude.
Normally water vapor in the Stratosphere meets one of two fates, if it encounters dust it will eventually accumulate on the dust thick enough to make the dust fall down into the troposphere where it rejoins the water cycle. However in clean air, something the Arctic is noted for, water vapor, H2O remains lighter than N2, Nitrogen molecules. Over long periods of time this means the water vapor can gradually rise and hang around for a very very long time. Noctiluscent clouds are what you get when some of that rare dust enters the Polar Stratosphere and encounters that very cold water vapor. Vapor condenses onto nearly microscopic dust making extremely small ice crystals that act like a rainbow for passing light, refracting the light in all colors of the visible spectrum. Typically these clouds only form very early in spring because once the sunlight starts refracting through the crystals it transfers a tiny percentage of its energy into heating the ice and the crystals turn back to vapor not long after the clouds form.
So we know Noctiluscent clouds formed even before jet travel existed and we know the numbers are increasing. We also know that methane which persists long enough in the atmosphere rises up into the Ozone layer where it is broken down into CO2 and H2O which is the most likely source of the moisture in pre-jet travel clouds in the polar stratosphere. However we also know that modern jet travel is dumping small quantities of persistent water vapor into the polar air mass daily, and they have been doing so for nearly three decades at an ever growing rate.
There are many opportunities for methane to break down long before it rises up into the ozone layer. However with modern jet travel humans are directly injecting water vapor into the mid and upper bounds of the polar stratosphere. So how do we determine which effect is the more significant? Both are part of the equation, you can't just ignore one and blame the other for the whole situation.