I'd like to say something noble like warn Amelia Earhart, or hookup Adolph with some Bob Ross videos. But if I'm being honest, I would probably be selfish. I would tell past me to not fuck up quite a few things that past me royally fucked up.
Fun bit of trivia. In some Christian churches, regular attendees have a nickname for the folks who only show up on Christmas and Easter. They call them the "Hollies and the Lilies".
Life is cheap on the internet, because people feel far removed (and/or "above it"). Social media "engagement" algorithms divide and isolate people from each other.
(I think as far as Lemmy is concerned, it's just spillover / remnant behaviors from that stuff. There's no engagement algorithm here other than what we bring in ourselves.)
Here are a some studies on it from people a lot smarter than me. (Note these are more about general toxicity and hate speech and not zeroed in on your exact question, but they may be helpful).
Meat consumption is huge, especially red meat. I would be happy to add that to the list of easy things for harm reduction.
I really don't think all the big dumb pickup trucks are needed, though. I understand some people like contractors and landscapers really do need them. But I also see parking lots for office workers filled with the stupid things.
That's also true. It's less of a problem in pedestrian-heavy walkable cities and towns. But in the average American city or town covered in stroads where car is king, it's a big problem.
Humans are bad at it, too. If you've ever ridden a bike or motorcycle, you quickly learn that car and truck drivers simply aren't looking for 2 wheelers. And therefore they don't see them. (I think this reinforces your point).
I can't remember how I did it but I finally figured out a way to turn that shit off. My phone is pretty old, though. They probably took that away on the newer phones.
I know it won't fix it, and this is hardly a comprehensive list. But I feel like there are some stupid-easy things Americans could do to reduce the harm. Just a few that come to mind:
Don't buy a big vehicle like a giant pickup truck unless you really need it. (A pavement princess for your ego is not a need).
Book air travel as little as possible. Again, only if you truly need it.
If you are growing something that requires a lot of water in an area that doesn't get very much rain...stop doing that. (If you have some kind of closed loop water system, that's an exception. But how many actually have that.)
Telecommute if you can. If you can do your job from home and your boss won't let you, it's time to hunt for another job.
Also, I recommend watching Ricky Royal's play-through on YouTube. His channel is "Box of Delights". The play-through is from several years ago but is very well done.
"How long do you plan on cheating on your current employer?"
"Wait, is that you in Groucho glasses, boss?"