The one I remember for my country was that by year 2000 (prediction made in 1980s) half of the forest will be gone because of acid rain. I remember as a kid magazines with illustrations showing large dead forest everywhere. Lack of ozone layer was also supposed to kill most of the planet. Fortunately when the danger is identified people start to act, this make long term predictions very difficult.

I am 50 years old, a science fiction fan, and an amateur follower of actual science. Generally speaking, the sci-fi authors are the best at making predictions. https://archive.nytimes.com/www.nytimes.com/books/97/03/23/l...