WASHINGTON — President Trump has rolled back environmental regulations, pulled the United States out of the Paris climate accord, brushed aside dire predictions about the effects of climate change, and turned the term “global warming” into a punch line rather than a prognosis.

Now, after two years spent unraveling the policies of his predecessors, Mr. Trump and his political appointees are launching a new assault.

In the next few months, the White House will complete the rollback of the most significant federal effort to curb greenhouse-gas emissions, initiated during the Obama administration. It will expand its efforts to impose Mr. Trump’s hard-line views on other nations, building on his retreat from the Paris accord and his recent refusal to sign a communiqué to protect the rapidly melting Arctic region unless it was stripped of any references to climate change.

And, in what could be Mr. Trump’s most consequential action yet, his administration will seek to undermine the very science on which climate change policy rests.

Mr. Trump is less an ideologue than an armchair naysayer about climate change, according to people who know him. He came into office viewing agencies like the Environmental Protection Agency as bastions of what he calls the “deep state,” and his contempt for their past work on the issue is an animating factor in trying to force them to abandon key aspects of the methodology they use to try to understand the causes and consequences of a dangerously warming planet.

As a result, parts of the federal government will no longer fulfill what scientists say is one of the most urgent jobs of climate science studies: reporting on the future effects of a rapidly warming planet and presenting a picture of what the earth could look like by the end of the century if the global economy continues to emit heat-trapping carbon dioxide pollution from burning fossil fuels.

The attack on science is underway throughout the government. In the most recent example, the White House-appointed director of the United States Geological Survey, James Reilly, a former astronaut and petroleum geologist, has ordered that scientific assessments produced by that office use only computer-generated climate models that project the impact of climate change through 2040, rather than through the end of the century, as had been done previously.

Scientists say that would give a misleading picture because the biggest effects of current emissions will be felt after 2040. Models show that the planet will most likely warm at about the same rate through about 2050. From that point until the end of the century, however, the rate of warming differs significantly with an increase or decrease in carbon emissions.

The administration’s prime target has been the National Climate Assessment, produced by an interagency task force roughly every four years since 2000. Government scientists used computer-generated models in their most recent report to project that if fossil fuel emissions continue unchecked, the earth’s atmosphere could warm by as much as eight degrees Fahrenheit by the end of the century. That would lead to drastically higher sea levels, more devastating storms and droughts, crop failures, food losses and severe health consequences.

Work on the next report, which is expected to be released in 2021 or 2022, has already begun. But from now on, officials said, such worst-case scenario projections will not automatically be included in the National Climate Assessment or in some other scientific reports produced by the government.

“What we have here is a pretty blatant attempt to politicize the science — to push the science in a direction that’s consistent with their politics,” said Philip B. Duffy, the president of the Woods Hole Research Center, who served on a National Academy of Sciences panel that reviewed the government’s most recent National Climate Assessment. “It reminds me of the Soviet Union.”

In an email, James Hewitt, a spokesman for the Environmental Protection Agency, defended the proposed changes.

“The previous use of inaccurate modeling that focuses on worst-case emissions scenarios, that does not reflect real-world conditions, needs to be thoroughly re-examined and tested if such information is going to serve as the scientific foundation of nationwide decision-making now and in the future,” Mr. Hewitt said.

However, the goal of political appointees in the Trump administration is not just to change the climate assessment’s methodology, which has broad scientific consensus, but also to question its conclusions by creating a new climate review panel. That effort is led by a 79-year-old physicist who had a respected career at Princeton but has become better known in recent years for attacking the science of man-made climate change and for defending the virtues of carbon dioxide — sometimes to an awkward degree.

“The demonization of carbon dioxide is just like the demonization of the poor Jews under Hitler,” the physicist, William Happer, who serves on the National Security Council as the president’s deputy assistant for emerging technologies, said in 2014 in an interview with CNBC.

Mr. Happer’s proposed panel is backed by John R. Bolton, the president’s national security adviser, who brought Mr. Happer into the N.S.C. after an earlier effort to recruit him during the transition.

Mr. Happer and Mr. Bolton are both beneficiaries of Robert and Rebekah Mercer, the far-right billionaire and his daughter who have funded efforts to debunk climate science. The Mercers gave money to a super PAC affiliated with Mr. Bolton before he entered government and to an advocacy group headed by Mr. Happer.

Climate scientists are dismissive of Mr. Happer; his former colleagues at Princeton are chagrined. And several White House officials — including Larry Kudlow, the president’s chief economic adviser — have urged Mr. Trump not to adopt Mr. Happer’s proposal, on the grounds that it would be perceived as a White House attack on science.

Even Stephen K. Bannon, the former White House strategist who views Mr. Happer as “the climate hustler’s worst nightmare — a world-class physicist from the nation’s leading institution of advanced learning, who does not suffer fools gladly,” is apprehensive about what Mr. Happer is trying to do.

“The very idea will start a holy war on cable before 2020,” he said. “Better to win now and introduce the study in the second inaugural address.”

But at a White House meeting on May 1, at which the skeptical advisers made their case, Mr. Trump appeared unpersuaded, people familiar with the meeting said. Mr. Happer, they said, is optimistic that the panel will go forward.

The concept is not new. Mr. Trump has pushed to resurrect the idea of a series of military-style exercises, known as “red team, blue team” debates, on the validity of climate science first promoted by Scott Pruitt, the E.P.A. administrator who was forced to resign last year amid multiple scandals.

At the time, the idea was shot down by John F. Kelly, then the White House chief of staff. But since Mr. Kelly’s departure, Mr. Trump has talked about using Mr. Happer’s proposed panel as a forum for it.

For Mr. Trump, climate change is often the subject of mockery. “Wouldn’t be bad to have a little of that good old fashioned Global Warming right now!” he posted on Twitter in January when a snowstorm was freezing much of the country.

His views are influenced mainly by friends and donors like Carl Icahn, the New York investor who owns oil refineries, and the oil-and-gas billionaire Harold Hamm — both of whom pushed Mr. Trump to deregulate the energy industry.

Mr. Trump’s daughter Ivanka made a well-publicized effort to talk him out of leaving the Paris accord in 2017. But after being vanquished by officials including Mr. Bannon, Mr. Pruitt, former Attorney General Jeff Sessions and the former White House counsel Donald F. McGahn II, there is little evidence she has resisted his approach since then.

The president’s advisers amplify his disregard. At the meeting of the eight-nation Arctic Council this month, Secretary of State Mike Pompeo dismayed fellow diplomats by describing the rapidly warming region as a land of “opportunity and abundance” because of its untapped reserves of oil, gas, uranium, gold, fish and rare-earth minerals. The melting sea ice, he said, was opening up new shipping routes.

“That is one of the most crude messages one could deliver,” said R. Nicholas Burns, who served as the NATO ambassador under George W. Bush.

At the National Security Council, under Mr. Bolton, officials said they had been instructed to strip references to global warming from speeches and other formal statements. But such political edicts pale in significance to the changes in the methodology of scientific reports.

Mr. Reilly, the head of the Geological Survey, who does not have a background in climate change science, characterized the changes as an attempt to prepare more careful, accurate reports. “We’re looking for answers with our partners and to get statistical significance from what we understand,” he said.

Yet scientists said that by eliminating the projected effects of increased carbon dioxide pollution after 2040, the Geological Survey reports would present an incomplete and falsely optimistic picture of the impact of continuing to burn unlimited amounts of coal, oil and gasoline.

“The scenarios in these reports that show different outcomes are like going to the doctor, who tells you, ‘If you don’t change your bad eating habits, and you don’t start to exercise, you’ll need a quadruple bypass, but if you do change your lifestyle, you’ll have a different outcome,’” said Katharine Hayhoe, the director of the Climate Science Center at Texas Tech University and an author of the National Climate Assessment.

Not all government science agencies are planning such changes. A spokesman for the National Oceanic and Atmospheric Administration, asked if its scientists would limit the use of climate models, wrote in an email, “No changes are being considered at this time.”

The push to alter the results of at least some climate science reports, several officials said, came after November’s release of the second volume of the National Climate Assessment.

While the Trump administration did not try to rewrite the scientific conclusions of the report, officials sought to play it down — releasing it the day after Thanksgiving — and discredit it, with a White House statement calling it “largely based on the most extreme scenario.”

Still, the report could create legal problems for Mr. Trump’s agenda of abolishing regulations. This summer, the E.P.A. is expected to finalize the legal rollback of two of President Barack Obama’s most consequential policies: federal regulations to curb planet-warming pollution from vehicle tailpipes and power plant smokestacks.

Opponents say that when they challenge the moves in court, they intend to point to the climate assessment, asking how the government can justify the reversals when its own agencies have concluded that the pollution will be so harmful.

That is why officials are now discussing how to influence the conclusions of the next National Climate Assessment.

“They’ve started talking about how they can produce a report that doesn’t lead to some silly alarmist predictions about the future,” said Myron Ebell, who heads the energy program at the Competitive Enterprise Institute, an industry-funded research organization, and who led the administration’s transition at the E.P.A.

A key change, he said, would be to emphasize historic temperatures rather than models of future atmospheric temperatures, and to eliminate the “worst-case scenarios” of the effect of increased carbon dioxide pollution — sometimes referred to as “business as usual” scenarios because they imply no efforts to curb emissions.

Scientists said that eliminating the worst-case scenario would give a falsely optimistic picture. “Nobody in the world does climate science like that,” said Michael Oppenheimer, a professor of geosciences and international affairs at Princeton. “It would be like designing cars without seatbelts or airbags.”

Outside the United States, climate scientists had long given up on the White House being anything but on outlier in policy. But they worry about the loss of the government as a source for reliable climate research.

“It is very unfortunate and potentially even quite damaging that the Trump administration behaves this way,” said Johan Rockström, the director of the Potsdam Institute for Climate Impact Research in Germany. “There is this arrogance and disrespect for scientific advancement — this very demoralizing lack of respect for your own experts and agencies.”