The comments from UNEP executive director Erik Solheim came as the UN agency presented its latest "Emissions Gap" report, which gives a scientific assessment about how national efforts are affecting the greenhouse gas emission trend.
The report's release ahead of a crucial climate meeting next week in Bonn, Germany, aims to inject new momentum to the Paris accord and even strengthen it in 2020.
UN officials are increasingly citing the role of companies in the fight against climate change.
"There is one question that I get more often than any other question wherever I go on the planet and it is a very simple one. It is: 'What about Donald Trump?'" Solheim told a panel in Geneva by videoconference from Nairobi.
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"In all likelihood, the United States of America will live up to its Paris commitment, not because of the White House, but because of the private sector," he said. "All the big American companies are dedicated to go in the green direction."
"The Paris agreement boosted climate action, but momentum is clearly faltering," said Edgar Gutierrez-Espeleta, Costa Rica's environment minister who heads the 2017 UN Environment Assembly. "We face a stark choice: Up our ambition, or suffer the consequences."
The Paris accord aims to cap global temperature increases to 2 degrees Celsius (Fahrenheit) by the year 2100 compared to average world temperatures at the start of the industrial era
"Should the United States follow through with its stated intention to leave the Paris agreement in 2020, the picture could become even bleaker," UNEP said.
Lead author John Christensen of UN Environment noted some US states like California were "acting independently of what the White House decides." He said Trump's bid to peel back constraints on the coal industry won't mark a major shift because "it simply doesn't pay off."
But Christensen told the Geneva panel that the Trump administration's impact on efforts could affect "negotiation dynamics" with other countries and embolden "what I call 'noise-makers:' they are the ones that are opposed to the basic idea" of fighting climate change.
Bob Ward, an expert on climate change policy at the London School of Economics, agreed that the coal industry is "unlikely to be revived in the United States because it is being displaced by cleaner and cheaper sources of energy."
A new round of UN climate talks known as COP 23 starts in Bonn, Germany, yesterday, when countries will take stock of their achievements and prepare more ambitious national goals.
On the positive, UNEP highlighted "rapidly expanding mitigation action" and says carbon-dioxide emissions have remained stable since 2014, thanks partly to renewable energy use in China and India. It cautioned that other greenhouse gases like methane continue to rise, however.
"We are at the watershed moment: We have stopped the rise in CO2 (carbon dioxide) emissions there is every reason to believe we can bring them down," he said. "The train is on the right track, but our duty is to speed it up.
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