They might be responsible for global warming in a relatively unknown way.
When all aircraft were grounded the day of the WTC attacks of 9/11/2001 and the following few days, meteorologists noticed that the average US temperatures decreased a few degrees.
Since then there have been investigations into contrails reflecting and retaining heat in much the same way as the CO2 greenhouse effect.
Do you live in a large city, or somewhere out in the country, not necessarily "off the grid"? If you lived for long times in
cities, you aren't used to feel the "normal" effects of weather and night-day differences of temperature anymore. The cover of buildings and streets dampens the difference. Out in the country during clear skies you feel how cold it can get. In a city not so much. I experienced this by mistake some time ago while riding out with a bicycle for about 80km in light shorts and a sleeveless shirt assuming i could take a train back. Which was wrong. I could even feel the difference in temperature different sorts of pavement made, or if i rode between open fields with just earth, or crops, or forest to both sides of the way. Anyways, as i finally got back near home i could see the haze dome from afar, and under it it wasn't so cold anymore.
Long story short: no clouds equals cold nights. Contrails are a form of cloud and contribute a little to the aforementioned dampening effects.
Except they are tiny compared to a cloud. The amount of sky they "cover" is a tiny percentage for a short time and even then they're not very substantial unlike regular clouds which can be que thick and dense. It just doesn't seem plausible.
And yet, no EPA emissions regulations exist for aircraft at all.
I am glad to see aircraft efficiency improve regardless.