On Monday, climate negotiators from around the world gathered in Marrakech, Morocco, to plan their next steps in the fight against global warming.

On Tuesday, American voters elected as president a man who has repeatedly called climate change a hoax.

“A third of the people here are walking around like zombies, like the walking dead, not sure what to do,” said UC Berkeley Professor Daniel Kammen, speaking from Morocco.

President-elect Donald Trump has vowed to back out of last year’s landmark Paris climate accord — the same agreement the negotiators in Marrakech are trying to implement. He has pledged to scuttle President Obama’s Clean Power Plan to cut carbon emissions from power plants, and he has promised to revive America’s flagging coal industry.

“Trump has said he’s against everything we’ve been making progress on, from the transition off of coal to being an international leader on climate,” Kammen said.

The Republican Trump’s surprise win over Democrat Hillary Clinton represents a potential U-turn in America’s climate and energy policies. And climate activists, whose mood Wednesday mixed anger with raw despair, fear that an already uphill fight to avert catastrophic warming has suddenly became far more difficult.

“The planet is in danger,” said Tom Steyer, the Bay Area billionaire who created a political organization, NextGen Climate, to mobilize young voters around global warming. “If they in fact do what they’re talking about — get rid of the Clean Power Plan, withdraw from the Paris agreement, cut the EPA in half — that is absolutely contrary to what is best for the planet.”

Some wondered Wednesday whether Trump might be open to persuasion, at least on supporting renewable power. The solar and wind industries, they noted, have been adding jobs in many of the states that backed Trump, such as Iowa and Texas.

“What we’re going to hope is that, over time, this evidence is going to become clear to the incoming administration,” said Andrew Steer, president of the World Resources Institute, a think tank focusing on economic and environmental issues. “And even if you don’t believe in climate change, it’s certainly smart to act on it.”

But Trump has already placed on his transition team a noted doubter of climate science — Myron Ebell of the Competitive Enterprise Institute conservative think tank. Ebell will oversee the transition at the Environmental Protection Agency, which Trump has threatened to eliminate.

Trump is said to be considering an oil company CEO — Harold Hamm, of Continental Resources — to become his energy secretary. Hamm was Trump’s top adviser on energy issues during the campaign.

The fossil fuel industries signaled Wednesday that they expect Trump to live up to his promises. He has called for making the United States the world’s leading producer of energy, by expanding drilling for oil and natural gas both onshore and off.

“We look forward to working with President-elect Trump’s administration to roll back many unlawful regulatory orders and to a federal government that is not beholden to the environmental lobby while ignoring the working class,” the Western Energy Alliance, which represents oil and gas production companies, said in a press release.

The alliance then listed some of the policies it expects from Trump: reviving the Keystone XL pipeline extension, approving the Dakota Access pipeline and increasing drilling on federal land.

One of Trump’s key campaign promises on energy — bringing back coal mining jobs — may not be feasible. While Trump and many Republicans blame Obama’s climate policies for coal’s demise, most analysts say the fuel can no longer compete with natural gas, which costs less.

“They can eliminate the Clean Power Plan, and they would still have to subsidize coal to keep those people’s jobs,” said Amy Myers Jaffe, a longtime energy market expert at UC Davis.

While most countries remain committed to fighting climate change, the United States is the world’s second-largest producer of greenhouse gases, behind only China.

Climate scientists have already warned that the pledges made in last year’s climate accord almost certainly won’t be able to limit warming in this century to 2 degrees Celsius (3.6 degrees Fahrenheit), the goal of negotiators. Should Trump make good on his promise to pull out of the agreement, that goal would be thoroughly out of reach.

Activists and climate experts held out hope Wednesday that some of the progress made under Obama would survive.

Solar and wind power prices have plunged and are continuing to fall, making them competitive with the cost of electricity from fossil fuel plants in many parts of the country. And many large corporations, from Apple to Walmart, have pledged to ramp up their own use of renewable power. Trump’s stance on solar and wind tax incentives is unclear.

“There are plenty of places where solar is the best resource, or where wind is the best resource, like the Midwest,” Jaffe said. “There are places that went for Trump where wind is really important.”

But activists also vowed to fight Trump should he move to gut climate regulations, even as they acknowledged his party will now control every branch of the federal government. And they urged him to reconsider.

“President-elect Trump must choose whether he will be remembered for putting America back on the path to climate disaster,” said Michael Brune, executive director of the Sierra Club.

David R. Baker is a San Francisco Chronicle staff writer. Email: dbaker@sfchronicle.com Twitter: @DavidBakerSF