I agree with everything you say here but feel the ">20 years" hides something.

We can easily say 30 years, there are very good arguments to say ~50 years and to get truly technical we could say 70-100 years.

Yes, I didn't want to spread false information because I'm no expert either. ~20 years is the time span I consciously remember reading & hearing about it but I'm sure scientists have warned us for much longer.

I remember worrying about this as a child, and I'm in my forties now. It's been a long, slow process.

I was studying this in uni 30 years ago.

There are general-interest educational videos on global warming from the 1950s:

https://youtube.com/watch?v=T6YyvdYPrhY