Scientists in the 19th century first introduced the concept of the greenhouse effect — the phenomenon by which certain gases created by humans trap heat in the Earth's atmosphere — just as humans began to ramp up production and consumption of fossil fuels. By the 1960s, the concept had migrated from the pages of academic journals to the minds of policymakers. A 1965 report from President Lyndon Johnson's scientific advisory committee warned that carbon dioxide emissions could trap heat in the atmosphere, though the prospect remained a far-off threat.

That attitude changed in the 1980s. As the science of climate change developed, media outlets began bringing scientists' concerns into mainstream conversation. For the first time, in 1981, climate change made the front page of the New York Times. And, in widely televised 1988 testimony, NASA scientist James Hansen went before the Senate Energy Committee to say it was "time to stop waffling" on the issue.

"Global warming has reached a level such that we can ascribe with a high degree of confidence a cause and effect relationship between the greenhouse effect and observed warming,'' he said, according to a Times report. "It is already happening now."

Politicians on both sides of the aisle took note, with Bush and his Democratic opponent, Michael Dukakis, both promising an aggressive approach to the issue. Once in office, though many environmentalists argue that he pushed for a weaker version than was eventually created, Bush helped launch the international framework for addressing climate change — the United Nations Framework Convention on Climate Change — that would later lead to important agreements on the issue.