I don't think that's true. I think most everyone notices. I think they:

* Don't care
*Care, but care about something else, like money, more
*Think there's still plenty of time and it's not going to be that bad for them.
*Are looking to the wrong people/things to fix the problem; for example, they think technology is going to save the day.
*Don't know what to do about it
*Don't think enough other people truly care to make a difference
*Actually want it to happen because they want extinction for the human race or they don't think there's enough good in the systems we have already so they want the whole thing scrapped, whatever the cost
*Aren't interested in wasting time trying to convince the masses. They see it as a waste of time. They're preparing as best they can for their own families and small groups.
*Are trying the best they can with what they've got. They're pretty much being drowned out and have been for decades.

Honestly, at this point anyone over 40 will have witnessed enough changes within their own lifetime to have enough of a sense from that lived experience that things are changing. Even if you don't believe it's human caused rather than just natural fluctuations, you know that climate bobbles like the "little ice age" caused a lot of problems to peoples of that era all over the globe.

You'd have to be willfully ignorant or belligerently obtuse to not notice at this point.