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My 4th grade teacher (1978) was brain washing us into being tree huggers. Conserve water while brushing our teeth was "magic". Turn down the heat. Recycle. Etc.

I recall hippie flavored propaganda in the school library asking if we wanted Earth to become a Red Planet (industrial) a Green Planet (farming) or Blue Planet (natural ecosystems), or some combination.

My (much older) cousin was a physicist studying submarine acoustics. He's always known, patiently explained the whatnots to us younger skeptics.

I worked at a civil engineering firm while still in high school (1980s). All the adults seemed to know what was happening, even if we didn't yet know all the causes. I vividly remember my boss (from Canada) telling me his childhood neighborhood lake was no longer safe to play hockey on.

I volunteered thru the 1990s at an org trying to save the PNW salmon. The staff scientists knew our local snow packs were shrinking. (Need the melt for salmon runs.) Now our mountains are bare most of the year, of course. Terrifying.

Maybe because I read a lot of sci-fi, I've always "known". Most memorable to me have been John Brunner's Shockwave Rider, The Sheep Look Up, and Ernest Callenbach's Ecotopia. But of course, post-apocalyptic is a popular storyline, so there have been zillions others.

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PS, for the down voter(s), one of the minor plot lines from the TV show Dallas was Bobby Ewing's renewable energy efforts, in opposition to older brother JR Ewing's doubling down on carbon. In debating with Bobby in front of Daddy Ewing, JR's money quote was "We've been running out of oil since the first oil well." So, ya, climate change was part of the (most) popular culture too.