Last week, I picked up a copy of the newly reissued 1971 Ursula Le Guin classic The Lathe of Heaven, which takes place in dystopic, post-collapse Portland, Ore., circa 2002 or so. It’s typical brilliance from Le Guin, of whom I can’t read enough, but I was interested to see that the novel begins by describing Mt. Hood devoid of snow due to the greenhouse effect. The climate is entirely different from that of the 1960s, with blue skies a thing of the past and rainfall patterns completely shifted.
It’s the earliest "popular literature" mention of global warming I’ve come across. Le Guin is often way ahead of her time (she invented Harry Potter and Hogwarts in 1968’s A Wizard of Earthsea, for example), though perhaps there are earlier instances of authors adding climate change to the collective body of literature.