Hundreds of scientists, many of whom say they are dismayed at seeing their work undone, are departing.

Among them is Matthew Davis, a biologist whose research on the health risks of mercury to children underpinned the first rules cutting mercury emissions from coal power plants. But last year, with a new baby of his own, he was asked to help support a rollback of those same rules. “I am now part of defending this darker, dirtier future,” he said.

This year, after a decade at the Environmental Protection Agency, Mr. Davis left.

“Regulations come and go, but the thinning out of scientific capacity in the government will take a long time to get back,” said Joel Clement, a former top climate-policy expert at the Interior Department who quit in 2017 after being reassigned to a job collecting oil and gas royalties. He is now at the Union of Concerned Scientists, an advocacy group.

Mr. Trump has consistently said that government regulations have stifled businesses and thwarted some of the administration’s core goals, such as increasing fossil-fuel production. Many of the starkest confrontations with federal scientists have involved issues like environmental oversight and energy extraction — areas where industry groups have argued that regulators have gone too far in the past.

“Businesses are finally being freed of Washington’s overreach, and the American economy is flourishing as a result,” a White House statement said last year. Asked about the role of science in policymaking, officials from the White House declined to comment on the record.

The administration’s efforts to cut certain research projects also reflect a longstanding conservative position that some scientific work can be performed cost-effectively by the private sector, and taxpayers shouldn’t be asked to foot the bill. “Eliminating wasteful spending, some of which has nothing to do with studying the science at all, is smart management, not an attack on science,” two analysts at the conservative Heritage Foundation wrote in 2017 of the administration’s proposals to eliminate various climate change and clean energy programs.

In some cases, the administration’s efforts to roll back government science have been thwarted. Each year, Mr. Trump has proposed sweeping budget cuts at a variety of federal agencies like the National Institutes of Health, the Department of Energy and the National Science Foundation. But Congress has the final say over budget levels and lawmakers from both sides of the aisle have rejected the cuts.

For instance, in supporting funding for the Department of Energy’s national laboratories, Senator Lamar Alexander, Republican of Tennessee, recently said, “it allows us to take advantage of the United States’ secret weapon, our extraordinary capacity for basic research.”

As a result, many science programs continue to thrive, including space exploration at NASA and medical research at the National Institutes of Health, where the budget has increased more than 12 percent since Mr. Trump took office and where researchers continue to make advances in areas like molecular biology and genetics.

Nevertheless, in other areas, the administration has managed to chip away at federal science.

At the E.P.A., for instance, staffing has fallen to its lowest levels in at least a decade. More than two-thirds of respondents to a survey of federal scientists across 16 agencies said that hiring freezes and departures made it harder to conduct scientific work. And in June, the White House ordered agencies to cut by one-third the number of federal advisory boards that provide technical advice.

The White House said it aimed to eliminate committees that were no longer necessary. Panels cut so far had focused on issuesincluding invasive species and electric grid innovation.

At a time when the United States is pulling back from world leadership in other areas like human rights or diplomatic accords, experts warn that the retreat from science is no less significant. Many of the achievements of the past century that helped make the United States an envied global power, including gains in life expectancy, lowered air pollution and increased farm productivity are the result of the kinds of government research now under pressure.

“When we decapitate the government’s ability to use science in a professional way, that increases the risk that we start making bad decisions, that we start missing new public health risks,” said Wendy E. Wagner, a professor of law at the University of Texas at Austin who studies the use of science by policymakers.

Skirmishes over the use of science in making policy occur in all administrations: Industries routinely push back against health studies that could justify stricter pollution rules, for example. And scientists often gripe about inadequate budgets for their work. But many experts say that current efforts to challenge research findings go well beyond what has been done previously.

In an article published in the journal Science last year, Ms. Wagner wrote that some of the Trump administration’s moves, like a policy to restrict certain academics from the E.P.A.’s Science Advisory Board or the proposal to limit the types of research that can be considered by environmental regulators, “mark a sharp departure with the past.” Rather than isolated battles between political officials and career experts, she said, these moves are an attempt to legally constrain how federal agencies use science in the first place.

Some clashes with scientists have sparked public backlash, as when Trump officials pressured the nation’s weather forecasting agency to support the president’s erroneous assertion this year that Hurricane Dorian threatened Alabama.

But others have garnered little notice despite their significance.

This year, for instance, the National Park Service’s principal climate change scientist, Patrick Gonzalez, received a “cease and desist” letter from supervisors after testifying to Congress about the risks that global warming posed to national parks.

“I saw it as attempted intimidation,” said Dr. Gonzalez, who added that he was speaking in his capacity as an associate adjunct professor at the University California, Berkeley, a position he also holds. “It’s interference with science and hinders our work.”

Curtailing Scientific Programs

Even though Congress hasn’t gone along with Mr. Trump’s proposals for budget cuts at scientific agencies, the administration has still found ways to advance its goals.

One strategy: eliminate individual research projects not explicitly protected by Congress.

For example, just months after Mr. Trump’s election, the Commerce Department disbanded a 15-person scientific committee that had explored how to make National Climate Assessments, the congressionally mandated studies of the risks of climate change, more useful to local officials. It also closed its Office of the Chief Economist, which for decades had conducted wide-ranging research on topics like the economic effects of natural disasters. Similarly, the Interior Department has withdrawn funding for its Landscape Conservation Cooperatives, 22 regional research centers that tackled issues like habitat loss and wildfire management. While California and Alaska used state money to keep their centers open, 16 of 22 remain in limbo.

A Commerce Department official said the climate committee it discontinued had not produced a report, and highlighted other efforts to promote science, such as a major upgrade of the nation’s weather models.

An Interior Department official said the agency’s decisions “are solely based on the facts and grounded in the law,” and that the agency would continue to pursue other partnerships to advance conservation science.

Research that potentially posed an obstacle to Mr. Trump’s promise to expand fossil-fuel production was halted, too. In 2017, Interior officials canceled a $1 million study by the National Academies of Sciences, Engineering, and Medicine on the health risks of “mountaintop removal” coal mining in places like West Virginia.

Mountaintop removal is as dramatic as it sounds — a hillside is blasted with explosives and the remains are excavated — but the health consequences still aren’t fully understood. The process can kick up coal dust and send heavy metals into waterways, and a number of studies have suggested links to health problems like kidney disease and birth defects.

“The industry was pushing back on these studies,” said Joseph Pizarchik, an Obama-era mining regulator who commissioned the now-defunct study. “We didn’t know what the answer would be,” he said, “but we needed to know: Was the government permitting coal mining that was poisoning people, or not?”

While coal mining has declined in recent years, satellite data showsthat at least 60 square miles in Appalachia have been newly mined since 2016. “The study is still as important today as it was five years ago,” Mr. Pizarchik said.

The Cost of Lost Research

The cuts can add up to significant research setbacks.

For years, the E.P.A. and the National Institute of Environmental Health Sciences had jointly funded 13 children’s health centers nationwide that studied, among other things, the effects of pollution on children’s development. This year, the E.P.A. ended its funding.

At the University of California, San Francisco, one such center has been studying how industrial chemicals such as flame retardants in furniture could affect placenta and fetal development. Key aspects of the research have now stopped.

“The longer we go without funding, the harder it is to start that research back up,” said Tracey Woodruff, who directs the center.

In a statement, the E.P.A. said it anticipated future opportunities to fund children’s health research.

At the Department of Agriculture, Secretary of Agriculture Sonny Perdue announced in June he would relocate two key research agencies to Kansas City from Washington: The National Institute of Food and Agriculture, a scientific agency that funds university research on topics like how to breed cattle and corn that can better tolerate drought conditions, and the Economic Research Service, whose economists produce studies for policymakers on farming trends, trade and rural America.

Nearly 600 employees had less than four months to decide whether to uproot and move. Most couldn’t or wouldn’t, and two-thirds of those facing transfer left their jobs.

In August, Mick Mulvaney, the acting White House chief of staff, appeared to celebrate the departures.

“It’s nearly impossible to fire a federal worker,” he said in videotaped remarks at a Republican Party gala in South Carolina. “But by simply saying to people, ‘You know what, we’re going to take you outside the bubble, outside the Beltway, outside this liberal haven of Washington, D.C., and move you out in the real part of the country,’ and they quit. What a wonderful way to sort of streamline government and do what we haven’t been able to do for a long time.”

The White House declined to comment on Mr. Mulvaney’s speech.

The exodus has led to upheaval.

At the Economic Research Service, dozens of planned studies into topics like dairy industry consolidation and pesticide use have been delayed or disrupted. “You can name any topic in agriculture and we’ve lost an expert,” said Laura Dodson, an economist and acting vice president of the union representing agency employees.

We live in an era where ignorance and stupidity are valued more than evidence and scientific inquiry.

If this continues, our society will go backwards, and our climate–the air we breathe, the water we drink, the oceans and the climate that sustains life–will be jeopardized.

We are an endangered species, endangered by ignorance.