Another one of my better posts.
Some scientists speculated about it at least as early as the 1920s.
Some environmentalists were noticing subtle changes by the 1950s; Rachel Carson was noting warming waters in the 1962's Silent Spring although it was an aside of noticing species migrating northward and was not the focus of the book.
1965-ish during the Lyndon B. Johnson administration a Presidential science advisory panel said carbon emissions were likely a problem, but it would take ~5 years until they would have the first math done to quantify it (had to wait for the moon shot and other government programs to keep pushing computer technology forward for the calculations).
1976-ish while George H. W. Bush was CIA director, the CIA put climate change on the list of major threats to the future of the US.
During the Reagan and early Bush administrations global agreements were reached on controlling ozone depleting gases and (primarily US) cap-and-trade was used to control sulfur emissions causing acid rain. Meanwhile environmentalists were full on Reagan derangement syndrome and espousing the James Watt was evil incarnate.
1988 George H. W. Bush put in his Presidential plank that action was needed on a global agreement on carbon emissions to control climate change.
The 1988 Democratic plank called for expanding coal production as fast as we could, in order to shut down nuclear plants as fast as possible -- you had some combination of carbon industry political donations, activists who grew up protesting nuclear weapons who expanded into protesting nuclear power, and it was before the Republican Southern Strategy started in the early 1970s had succeeded in re-aligning US politics -- so states like Kentucky and West Virginia were still solidly Democratic and important to the Democratic Party.
As the Bush administration went on, his Chief of Staff John Sununu had enough of fighting to get things like cap-and-trade on sulfur passed -- industry fought it, many environmentalists fought it because capitalism is evil and they wanted heavy, prescriptive regulations instead market forces. By the time that was done he was all, "Fuck it, all we get is political shit from both sides on environmental issues and we're not picking up any votes from the environmental voters."
1992 Bill Clinton becomes President, and well the Democrats were still clinging on to coal country for votes so they continued to lack an appetite to tackle climate change.
1994 the Southern Strategy finally popped. Led by Newt Gingrich, the Republicans picked up 54 seats in the House of Representatives. It was the first time in over 40 years the Democrats lost control -- and Democrats usually enjoyed ~10ppt advantage in the number of seats above their share of the popular vote.
The old Democratic Party had both right, centrist, and left wing factions; the Republicans more right and centrist factions. In 1994 the old Yellow Dog Democrats of the south who had voted for Democrats since the Civil War moved in mass to the Republican Party; and quickly the more liberal Rockefeller Republicans of the northeast quickly became an endangered species. (Plus the distinct flavor of the Republican Party of California committed political suicide with Propositions 187 (1994), 209, and 227 which despite their popularity at the time shifted the rising Latino vote and centrist whites firmly into Democratic hands in the following years).
This Southern Strategy that finally came together has created the current climate of tightly contested and close national elections (including popular for Congress), as well as firmly polarized each party as either right-wing or left-wing.
As early as 1970 the Southern Strategy was articulated (in the New York Times):
>From now on, the Republicans are never going to get more than 10 to 20 percent of the Negro vote and they don't need any more than that... but Republicans would be shortsighted if they weakened enforcement of the Voting Rights Act. The more Negroes who register as Democrats in the South, the sooner the Negrophobe whites will quit the Democrats and become Republicans. That's where the votes are. Without that prodding from the blacks, the whites will backslide into their old comfortable arrangement with the local Democrats.
The old parties that could combine each of their center and left wings on things like the Voting Rights Act, Environmental Protection Act, and Clean Water Act ceased to exist in that form.
It was replaced by a right leaning and left leaning party both of which spend their time bickering with each other; neither capable of forming an enduring, stable majority.
The rest of the world meanwhile for as much as they often like to bitch about America proved to be utterly incapable of meaningful action without American leadership.