Both.I think either the change in the chemical balance of the atmosphere is creating this effect, OR -- there are just more aircraft in the sky.
Your use of "chemical" was incorrect. "Physical" would be better. The use of more aircraft in the same physical space each day makes that airspace WETTER, in that there are more water molecules (from combustion) in it, to the tune of a MILLION TONS of water.
Of course it will diffuse away from that layer (between 30,000 and 40,000 feet) on a daily basis, and will be renewed by the next day's flights.
But that increased wetness will surely precipitate heavier trails.
There are fifty times more aircraft in the sky than there were in the early sixties, so of course there are more trails.
You've been baiting a poster for no practical reason I can see.You been flying my plane all night.