As a quick refresher: the original climate deal, the Kyoto Protocol, was signed in 1992, by then everyone was well on board with the science. In fact, the climate negotiations were spun off from the convention on reducing CFCs (Vienna Convention of 1985), which are important greenhouse gases as well as being damaging to the ozone layer. It was thought (correctly) at the time that because CFCs were produced and used in a limited number of places, it would be much easier to resolve that problem and move the wider greenhouse gas negotiations into a separate treaty, which took another 7 years until 1992.
I also think the same of Carl Sagan. For all his wonderful and critical achievements, I bet if he could see how things turned out, he would have unquestioningly devoted his life to spreading awareness about climate change here on Earth instead of focusing on astronomy and nuclear war.