The world’s top diplomat, António Guterres, the United Nations secretary general, has lately been unusually blunt in his broadsides against fossil fuel producers. He has accused them of “profiting from destruction.” He has urged governments to stop funding coal and to put the brakes on new oil and gas projects. “History is coming for the planet-wreckers,” he has said.

But who are these “planet wreckers?” He doesn’t name them.

Not China, the world’s coal behemoth. Not Britain or the United States, who both have ambitious climate laws but continue to issue new oil and gas permits. Not the United Arab Emirates, a petrostate where a state-owned oil company executive is hosting the upcoming United Nations climate negotiations — a move that activists have decried as undermining the very legitimacy of the talks.

The contradictions show not only the constraints on Mr. Guterres, a 74-year-old politician from Portugal who has made climate change his centerpiece issue, but also the shortcomings of the diplomatic playbook on a problem as urgent as global warming.

“The rules of multilateral diplomacy and multilateral summitry are not fit for the speedy and effective response that we need,” said Richard Gowan, who decodes the rituals of the United Nations for the International Crisis Group.

The 2015 Paris climate accord asks only that countries set voluntary targets to address climate pollution. The agreements that come out of annual climate negotiations routinely get watered down, because every country, including champions of coal, oil and gas, must agree on every word and comma.

The secretary general can cajole but not command, urge but not enforce. He doesn’t name specific countries, though nothing in the United Nations Charter prevents him from doing so.

Despite his exhortations, governments have only increased their fossil fuel subsidies, to a record $7 trillion in 2022. Few nations have concrete plans to move their economies away from fossil fuels, and many depend directly or indirectly on revenues from coal, oil and gas. The human toll of climate change continues to mount.

“He has interpreted his role as a sort of truth teller,” said Rachel Kyte, a former United Nations climate diplomat and a professor at the Fletcher School at Tufts University. “The powers available to him as secretary general are awesome but limited.”

This week, on the sidelines of the United Nations General Assembly meeting, he is deploying a bit of a diplomatic wink-nod. At a Climate Ambition Summit he is hosting on Wednesday, he is giving the mic only to those countries that have done as he has urged, and only if they send a high-level leader, to show that they take the summit seriously. “A naming and shaming device that doesn’t actually require naming and shaming anyone,” Mr. Gowan said.

Diplomatic jockeying around who will get on the list has been intense. More than 100 countries sent in requests to speak, and Mr. Guterres’s aides have in turn requested more information to prove they deserve to be on the list. What have you done on coal phaseout, some have been asked. How much climate funding have you offered? Are you still issuing new oil and gas permits? And so on.

“It’s good to see Guterres trying to hold their feet to the fire,” said Mohamed Adow, a Kenyan activist.

Mr. Guterres has waited until the last possible minute to make public the list of speakers.

Expect the awkward.

John Kerry, the United States climate envoy, is expected to attend but not speak. (Mr. Guterres is giving the mic only to high-level national leaders.) It’s unclear whether the head of the Chinese delegation this year, Vice President Han Zheng, will have a speaking role. The European Commission president, Ursula von der Leyen, has secured the mic. Britain’s prime minister, Rishi Sunak, isn’t coming to the General Assembly conclave at all. Sultan al-Jaber, the head of the Emirati oil company, and host of the next climate talks, is scheduled to speak.

Mr. Guterres will also invite companies with what he calls “credible” targets to reduce their climate emissions to participate. Expect to count them with the fingers of one hand.

Mr. Guterres, who had led the United Nations refugee agency for 10 years before being selected for the top job, didn’t always make climate change his centerpiece issue.

In fact, he didn’t talk about it when he was chosen to head the United Nations in 2016. Climate was seen as the signature issue of his predecessor, Ban Ki-moon, who shepherded through the Paris Agreement in 2015. Mr. Guterres spoke instead about the war in Syria, terrorism, and gender parity in the United Nations. (His choice disappointed those who had pressed for a woman to lead the world body for the first time in its 70-year history.)

In 2018 came a shift. At that year’s General Assembly, he called climate change “the defining issue of our time.” In 2019, he invited the climate activist Greta Thunberg to the General Assembly, whose raw anger at world leaders (“How dare you?” she railed at world leaders) spurred a social media clash with President Donald J. Trump, who was pulling the United States out of the Paris Accord.

Mr. Guterres, for his part, studiously avoided criticism of the United States by name.

By 2022, as oil companies were raking in record profits in the aftermath of the Russian invasion of Ukraine, he amped up his language. “We need to hold fossil fuel companies and their enablers to account,” he told world leaders at the General Assembly. He called for a windfall-profit tax, urged countries to suspend subsidies for fossil fuels and appointed a committee to issue guidelines for private companies on what counts as “greenwashing.”

This year, he stepped into the contentious debate between those who want greenhouse gas emissions from oil and gas projects captured and stored away, or “abated,” and those who want to keep oil and gas tucked in the ground altogether. “The problem is not simply fossil fuel emissions. It’s fossil fuels, period,” Mr. Guterres said in June.

The reactions from the private sector are mixed, said Paul Simpson, a founder and former head of CDP, a nongovernmental group that works with companies to address their climate pollution. Some executives privately say Mr. Guterres is right to call for a swift phaseout of fossil fuels, while others note that most national governments still lack concrete energy transition plans, no matter what he says.

“The question really is, how effective is the United Nations?” Mr. Simpson said. “It has the ability to get governments to focus and plan. But the U.N. itself doesn’t have any teeth, so national governments and companies must act.”