Donald Trump’s new term as US president poses a grave threat to the planet if it blows up the international effort to curb dangerous global heating, stunned climate experts have warned in the wake of his decisive election victory.

Trump’s return to the White House is widely expected to result in the US, yet again, exiting the Paris climate agreement and may even remove American involvement in the underpinning United Nations framework to deal with the climate crisis.

While campaigning for president, Trump has called climate change “a big hoax”, scorned wind energy and electric cars and vowed to gut environmental rules and the “green new scam” of the Inflation Reduction Act, a major bill passed by Democrats to support clean energy projects.

Trump’s agenda, analysts have found, risks adding several billion tonnes of extra heat-trapping gases to the atmosphere, further imperiling goals to stave off disastrous global heating that governments are already failing to meet. Michael Mann, a climate scientist at the University of Pennsylvania, said that the US is now a “failed democracy” and that “we now pose a major threat to the planet.”

The election result will send shockwaves through annual UN climate talks that start in Azerbaijan on Monday. “The election of a climate denier to the US presidency is extremely dangerous for the world,” said Bill Hare, a senior scientist at Climate Analytics, who warned a Trump administration would likely “damage efforts” to keep the world from heating by more than 1.5C above pre-industrial levels, a Paris target that now appears even further out of reach.

While Joe Biden’s administration will send a delegation to the Cop29 summit next week, this will be overshadowed by an incoming Trump government that threatens to disengage with other major carbon emitters, such as China, to address the climate crisis. “The nation and world can expect the incoming Trump administration to take a wrecking ball to global climate diplomacy,” said Rachel Cleetus, policy director at the Union of Concerned Scientists.

Across Europe, climate activists and politicians who support stronger action to cut pollution reacted with despair to the news of Trump’s win. “This is a dark day in the US and globally,” said Thomas Waitz, an Austrian MEP and co-chair of the European Green party.

Luisa Neubauer, a German climate activist from the Fridays for Future movement, who went door-knocking for Harris, compared the feeling to a bad breakup. “A decision over parts of the near future has been made and most of us didn’t have a say in it,” she said. “And for a moment it feels like the world is going to end. It’s not. But the heartbreak is real.”

But they also urged supporters of climate action to not give up.

Areeba Hamid, joint executive director of Greenpeace UK, said it was “an election won with corporate cash, big polluter backers and disinformation” but that a global movement was already fighting to rein in the damage.

“We simply don’t have any more time to waste,” she added. “Whatever a Trump presidency chooses to do on global climate action, we know that damage can be contained if the grown-ups in the room speak up.”