A day after the United Nations issued its most urgent call to arms yet for the world to confront the threat of climate change, President Trump boarded Air Force One for Florida — a state that lies directly in the path of this coming calamity — and said nothing about it.

It was the latest, most vivid example of Mr. Trump’s dissent from an effort that has galvanized much of the world. While the United Nations warned of mass wildfires, food shortages and dying coral reefs as soon as 2040, Mr. Trump discussed his successful Supreme Court battle rather than how rising seawaters are already flooding Miami on sunny days.

The president’s isolation is not just from the world: In California, New York, Massachusetts and other states, governments and companies are pushing ahead with regulations and technological innovations to reduce greenhouse gas emissions.

That bottom-up activism is a source of hope for those who have watched in despair since last year when Mr. Trump declared he would pull the United States out of the Paris climate accord. But experts say it is no substitute for the world’s largest economy, and second-largest emitter of carbon dioxide, turning its back on the fight.

“You have this enormous discrepancy between the White House and, essentially, everyone else,” said Johan Rockström, the director of the Potsdam Institute for Climate Impact Research in Germany. “The leadership in Washington is really moving against the whole agenda.”

The United Nations report paints a far more dire picture of the immediate consequences of climate change than previously thought and says that avoiding the damage requires transforming the world economy at a speed and scale that has “no documented historic precedent.”

It describes a world of worsening food shortages and poverty; more wildfires; and a mass die-off of coral reefs as soon as 2040 — a period well within the lifetime of much of the global population.

Among climate-change scientists, there were increasing fears that Mr. Trump’s withdrawal from the Paris accord, which at first seemed a lonely act of defiance, may embolden other countries to leave it as well.

In Brazil, voters are on track to elect a new president, Jair Bolsonaro, who has vowed to withdraw his country, the world’s seventh-largest emitter of greenhouse gases, from the pact.

Mr. Trump’s announcement last year prompted a show of solidarity from the other 194 countries that signed the accord, not to mention American political and business leaders who rallied under the slogan, “We are still in!” But to populists like Mr. Bolsonaro, Mr. Trump’s demands that the United States be given a better deal could prove appealing.

“To the extent that we get these narrow-minded, so-called nationalist, populist leaders, we could have a big problem,” said John P. Holdren, who served as President Barack Obama’s chief science adviser. “Brazil, with its huge area of forests, is going to suffer terribly from climate change.”

Beyond the domino effect, Mr. Holdren, who is now a professor at Harvard’s Kennedy School, said there were other immediate costs to what he called “the squandering of U.S. leadership on an acute global issue.”

Mr. Trump, who has mocked the science of human-caused climate change, cut the American contribution to a global fund that supports climate mitigation and assistance efforts in developing countries by two-thirds, to $1 billion. He has tried to cut government funding of climate-related research — an effort that Congress has so far resisted.

The White House issued no public response to the United Nations report, which was issued Monday in South Korea at a meeting of the Intergovernmental Panel on Climate Change, a group of scientists convened by the United Nations to guide world leaders.

“Not today,” said Bill Shine, the White House communications director. “It’s a Kavanaugh night.”

After Mr. Trump returned Monday from Orlando, Fla., where he spoke to a convention of police chiefs and referred to the hurricane now approaching that state, he attended a White House ceremony to swear in Judge Brett M. Kavanaugh as an associate justice of the Supreme Court.

Following the ceremony, Lindsay E. Walters, a deputy press secretary, said, “The United States is leading the world in providing affordable, abundant and secure energy to our citizens, while protecting the environment and reducing emissions through job-creating innovation.”

She noted that carbon dioxide-related emissions declined 14 percent in the United States from 2005 to 2017, while they rose 21 percent globally during the same period.

On Saturday, an American delegation in South Korea joined more than 180 countries in accepting the report’s summary for policymakers, but a statement from the State Department added that it “does not imply endorsement by the United States of the specific findings or underlying contents of the report.”

The report “is quite a shock, and quite concerning,” said Bill Hare, an author of previous Intergovernmental Panel on Climate Change reports and a physicist with Climate Analytics, a nonprofit organization. “We were not aware of this just a few years ago.”

The authors found that if greenhouse gas emissions continue at the current rate, the atmosphere will warm by as much as 2.7 degrees Fahrenheit, or 1.5 degrees Celsius, above preindustrial levels by 2040.

The Paris accord set a goal of preventing warming of more than 3.6 degrees above preindustrial levels — long considered a threshold for the most severe social and economic damage from climate change. But the heads of small island nations, fearful of rising sea levels, had also asked scientists to examine the effects of 2.7 degrees of warming.

Without aggressive action, many effects that scientists once expected to happen further in the future will arrive by 2040, and at the lower temperature, the report shows.

“It’s telling us we need to reverse emissions trends and turn the world economy on a dime,” said Myles Allen, an Oxford University climate scientist and an author of the report.

To prevent 2.7 degrees of warming, the report said, greenhouse emissions must be reduced by 45 percent from 2010 levels by 2030, and by 100 percent by 2050. It also found that use of coal as an electricity source would have to drop from nearly 40 percent today to 1 to 7 percent by 2050.

“This report makes it clear: There is no way to mitigate climate change without getting rid of coal,” said Drew Shindell, a climate scientist at Duke University and an author of the report. Mr. Trump has vowed to increase the burning of coal.

“It makes me feel angry when I think about the U.S. government,” Mr. Shindell said. “My kids feel like it’s their future being destroyed.” He watched as the grounds of his son’s high school in Durham, N.C., and the roads around it flooded last month after Hurricane Florence.

Dr. Allen said there was little question the report will be ignored in Washington. “The current administration doesn’t seem interested in it all,” he said, although he added that as a scientist, he takes the long view.

“One way or another,” he said, “the facts do win out.”

Mr. Trump encouraged scientists recently when he nominated Kelvin Droegemeier, a well-respected meteorologist who is an expert on extreme weather, to lead the White House Office of Science and Technology Policy. The post had been vacant since Mr. Trump took office.

But it is unlikely that Mr. Droegemeier will change the president’s views on climate change, and other influential aides are hardly going to challenge him.

For example, the Trump administration’s counterterrorism strategy, released last week, made no mention of climate change as a cause for extremism. The Obama administration regularly cited it in threat assessments because of its effect on migration and the competition for food and water.

“I don’t think climate change is a cause of international terrorism,” said the national security adviser, John R. Bolton.

For all that, scientists said that they saw a few rays of sunshine amid the clouds. A Democratic takeover of the House would raise the odds that Congress would continue blocking cuts to research. And despite his criticism of the Paris accord as “very unfair” to the United States, Mr. Trump has left the door open to staying in the deal, if the terms were improved.

“I have been of the opinion all along that we can definitely see the U.S. back in the Paris agreement, even under Trump,” Mr. Rockström said.

Legally, he noted, the United States cannot formally withdraw from the pact until 2020, and the agreement’s terms are voluntary.

“He can sit there in the White House and draw up his own plan,” Mr. Rockström said.