President Trump announced Thursday that he plans to withdraw the U.S. from the Paris climate agreement, a nonbinding 2015 deal that commits nearly the entire world — 195 nations — to taking action on climate change. This marks the second time in as many decades that the U.S. has torpedoed a major international climate agreement and, more importantly, it cements America’s reputation for having a highly polarized environmental policy, a reputation we’ve been building for nearly 30 years.

Environmentalism in the U.S. used to be a fairly bipartisan issue. The Environmental Protection Agency, founded in 1970, was a product of the Nixon administration. But somewhere around 1990, everything changed. And it changed quickly, in ways that had big impacts on our ability to negotiate international environmental accords like the Paris agreement. (The Obama administration framed the Paris accord as an agreement, which the executive branch can enact alone, not as a treaty, which would require Senate approval, but we’re comparing it to international treaties because that’s the framework it fits in best and because some experts have argued that it ought to be considered a treaty.)

One of the teams that spotted this 1990 shift is a group of German and Norwegian researchers, who published a 2012 paper analyzing U.S. participation in multilateral environmental agreements. They concluded that this was the tipping-point year by looking at the content of international treaties that were signed by U.S. presidents only to end up in senatorial limbo — treaties that the president signs don’t have the force of law until two-thirds of the Senate gives its approval. The chamber can also choose not to vote at all, leaving the treaty officially “pending,” sometimes for decades. For example, the International Labor Organization Convention No. 87 Concerning Freedom of Association and Protection of the Right to Organize has been patiently twiddling its thumbs waiting for a vote since 1949.

Looking at what kinds of treaties received this snubbing revealed a pattern: Before 1990, the researchers found a single environment-related treaty that had been left pending. Between 1990 and May 2012, when their paper was published, nine such treaties were left hanging. And that’s not counting treaties that were never submitted to the Senate, a group that includes the Kyoto Protocol — the world’s previous attempt at a comprehensive climate change plan.

The authors of that 2012 paper blamed this shift on the growing polarization of American political parties on environmental issues. After 1990, they wrote, presidents could no longer count on bipartisan support for environmental agreements. And, increasingly, those presidents began to sign treaties that they knew would never pass the Senate because they knew that even if the treaty was never approved, the signature alone would please their political base — a fact the authors believe helped to increase the number of signed treaties that have gotten lost in limbo.

And this isn’t the only research to find a significant shift in political polarization on environmental issues after 1990. The League of Conservation Voters is an activist group that gives politicians annual ratings on a point scale of 0 to 100, based on how they voted on a slate of environmental issues. According to research published last year, Republicans were always less likely than Democrats to vote for the bills the league liked — the closest the two parties’ average score ever came in the Senate was a margin of 10 points in 1979 and 1980. But the difference between the parties got bigger after 1988. That year, Senate Republicans’ average score was 37, and the Democratic average was 56. By 1992, the gap had widened to an average score of 19 for Senate Republicans and 63 for Democrats.

Public opinion followed the same pattern. A 2014 paper analyzing data from the General Social Survey through 2012 found that between 1980 and 1990, Republicans (and ideological conservatives) increasingly agreed that the government spent “too little” on the environment. In 1990, Democrats and Republicans were in almost perfect agreement on the issue. But after that year, Republican support for increased spending quickly plummeted, with the percentage who said the government spent “too little” falling from nearly 75 percent to less than 55 percent in just three years.

You can even see the shift in the rhetoric of former President George H.W. Bush. In June 1989, the Republican president presented an ultimately successful, bipartisan proposal for updating the Clean Air Act, in which he castigated “political paralysis” on the issue and touted the benefits of strict enforcement. But by the time the 1992 general election rolled around, he was calling Al Gore “Ozone Man.”

So what the hell happened in 1990 that sent Republicans and Democrats in wildly different directions on the environment? Aaron McCright, a sociology professor at Michigan State and one of the authors of the 2014 paper, argues that the change was tied to the fall of the Soviet Union.

His team is working on a follow-up to its previous research in an effort to understand why the political winds shifted when they did. To study what happened, he read every issue of the conservative magazines National Review and Human Events from 1970 to 2014 and coded references to environmental issues. The paper is still in the works, but McCright told me that in the late 1980s, there was a distinct uptick in anti-environmental sentiment.

That timing correlates with the decline of the Soviet Union, and McCright said the rhetoric about environmentalism began to be tied to that as well. “You start seeing essays about the environmental movement wherein people attacking it will start talking about [how] the failed Marxists are now the greens,” he told me. “The ‘watermelon’ slur comes up” — it was used to refer to someone who was “green on the outside but red on the inside.” Like a ripple in a pond, this shifting attitude spread out to change the votes of conservative lawmakers and the opinions of Republican voters, he said.

McCright thinks that, as communism became less of a threat to free-market capitalism, conservative thinkers began to see the regulations that went along with environmentalism as a bigger problem — especially as the scope of those regulations became more international. Environmentalism came to be seen as a tool for controlling markets and limiting freedom. “And that has really taken hold in the Republican Party,” he said. “To the point that … well, you’ve been living in America. You know what’s going on.”