The question of data comes up often. Adm. Luiz Henrique Caroli, Brazil’s senior I.M.O. representative, said he does not believe the studies showing rising emissions. Brazil wants to cut emissions, he said, but not before further study on the economic effect.

“We want to do that, this reduction, in a controlled way,” he said in an interview.

The Cook Islands, another Pacific archipelago, make a similar argument. Like the Marshalls, they face rising seas and an uncertain future. But the more immediate concerns are jobs and cost of living, said Joshua Mitchell, of the country’s foreign office. “Existential questions have to be balanced against the priorities of the country in the moment,” he said.

Megan Darby, a journalist for Climate Home News, said she was suspended from maritime meetings after quoting a Cook Islands diplomat.

The I.M.O. almost never puts environmental policies to a vote, favoring instead an informal consensus-building. That effectively gives vocal opponents blocking power, and even some of the agency’s defenders acknowledge that it favors minimally acceptable steps over decisive action.

So, when delegates finally set goals in 2018, Mr. de Brum’s ambition had been whittled away.

The Marshall Islands suggested a target of zero emissions “by the second half of the century” — meaning by 2050. Industry representatives offered a slightly different goal: Decarbonization should occur “within” the second half of the century, a one-word difference that amounted to a 50-year extension.

Soon, though, the delegates agreed, without a vote, to eliminate zero-emissions targets entirely.

What remained were two key goals:

First, the industry would try to improve fuel efficiency by at least 40 percent. This was largely a mirage. The target was set so low that, by some calculations, it was reached nearly the moment it was announced.

Second, the agency aimed to cut emissions at least in half by 2050. But even this watered-down goal is proving unreachable. The agency’s own data say emissions may rise by 30 percent.

When delegates met last October — five years after Mr. de Brum’s speech — the organization had not taken any action. Proposals like speed limits had been debated and rejected.

What remained was what several delegates called the “refrigerator rating” — a score that, like those on American appliances, identified the clean and dirty ships.

European delegates insisted that, for the system to work, low-scoring ships must eventually be prohibited from sailing.

China and its allies wanted no such consequence.

So Sveinung Oftedal of Norway, the group’s chairman, told France and China to meet separately and compromise.

Delegates worked across time zones, meeting over teleconferences because of the Covid-19 pandemic. Shipping industry officials said they weighed in through the night.

The Marshallese were locked out.

“We’re always being told ‘We hear you,’” Mr. Ishoda said. “But when it comes to the details of the conversation, we’re told ‘We don’t need you to contribute.’”

Ultimately, France ceded to nearly all of China’s requests, records show. The dirtiest ships would not be grounded. Shipowners would file plans saying they intended to improve, would not be required to actually improve.

German delegates were so upset that they threatened to oppose the deal, likely triggering a cascade of defections, according to three people involved in the talks. But European Union officials rallied countries behind the compromise, arguing that Europe could not be seen as standing in the way of even limited progress.

“At I.M.O., that is as always the choice,” said Damien Chevallier, the French negotiator. “We have the choice to have nothing, or just to have a first step.”

All of this happened in secret. The I.M.O.’s summary of the meeting called it a “major step forward.” Natasha Brown, a spokeswoman, said it would empower customers and advocacy groups. “We know from consumer goods that the rating system works,” she said.

But the regulation includes another caveat: The I.M.O. will not publish the scores, letting shipping companies decide whether to say how dirty their ships are.

Ms. Kabua, the Marshallese minister, is under no illusions that reclaiming the diplomatic seat will lead to a climate breakthrough.

But if it works, she said, it might inspire other countries with private registries to do the same. Countries could speak for themselves rather than through a corporate filter.

Regardless of the outcome, the political winds are shifting. The European Union is moving to include shipping in its emissions-trading system. The United States, after years of being minor players at the agency, is re-engaging under President Biden and recently suggested it may tackle shipping emissions itself.

Both would be huge blows to the I.M.O., which has long insisted that it alone regulate shipping.

Suddenly, industry officials say they are eager to consider things like fuel taxes or carbon.

“There’s much more of a sense of momentum and crisis,” said Mr. Platten, the industry representative. “You can argue about, ‘Are we late to it,’ and all the rest. But it is palpable.”

Behind closed doors, though, resistance remains. At a climate meeting last winter, recordings show that the mere suggestion that shipping should become sustainable sparked an angry response.

“Such statements show a lack of respect for the industry,” said Kostas G. Gkonis, the director of the trade group Intercargo.

And just last week, delegates met in secret to debate what should constitute a passing grade under the new rating system. Under pressure from China, Brazil and others, the delegates set the bar so low that emissions can continue to rise — at roughly the same pace as if there had been no regulation at all.

Delegates agreed to revisit the issue in five years.