Under President Donald Trump, the US Environmental Protection Agency is seeking to overturn its own 2009 "Endangerment Finding," which acknowledges that heat-trapping emissions are endangering public health and welfare.
It has long served as the legal basis for numerous federal regulations like emissions standards.
Lee Zeldin, the current EPA head, has said the standard -- which was upheld by the Supreme Court -- was based on flawed reasoning and caused economic harm.
But the letter said that "as climate scientists, public health experts, and economists, we can attest to the indisputable scientific evidence of human-caused climate change, its harmful impacts on people's health and well-being, and the devastating costs it is imposing."
"This explicit attempt to undermine or weaken these findings, as well as the critical regulations linked to them, is contrary to science and the public interest," it added.
The letter delivered by the Union of Concerned Scientists also skewered a recent report commissioned by the Trump-era US Department of Energy, reiterating criticism that the report is inaccurate, contradictory and a mischaracterization of data.
"The scientific evidence on human-caused climate change and its consequences was unequivocal in 2009 and, since that time, has become even more dire and compelling," read Tuesday's letter.
"We urge you to stop dismantling critical climate regulations and evading the EPA's responsibility by pushing disinformation about climate science and impacts," the signatories continued.
"Instead, we call on you to act with urgency to help address this pressing challenge by limiting heat-trapping emissions.
"People across the nation are relying on the EPA to fulfill its mission to protect public health and the environment."
Trump's fossil fuel agenda challenged in youth climate suit
Missoula, United States (AFP) Sept 16, 2025 -
Life, liberty and the right to a stable climate?
A group of young Americans say President Donald Trump is trampling their inalienable rights through an aggressive push for fossil fuels and a crusade against federal climate science -- and on Tuesday, a rural courtroom in Missoula, Montana will be their stage in a closely watched showdown.
Lighthiser v. Trump is emblematic of a growing global trend of legal action as a tool to push action on planetary warming amid political inertia -- or outright hostility.
"It's very intimidating to think about my future," lead plaintiff Eva Lighthiser recently told AFP in Washington, where she and other plaintiffs represented by the nonprofit Our Children's Trust recently traveled to lobby lawmakers.
The 19-year-old from Livingston, Montana, described smoke-choked skies, relentless floods, and her family's climate-driven relocation as "a lot to reconcile with, as somebody who's just entering adulthood."
Over two days of hearings, she and 21 co-plaintiffs -- all young adults or minors -- will testify about their health and other harms they have endured from the Trump administration's actions.
At issue are three executive orders that "unleash" fossil fuel development and curb the electric vehicle market; invoke emergency powers to accelerate drilling; and designate coal a "mineral," granting it priority status for extraction.
The plaintiffs also allege that scrubbing climate science from federal research has obscured the risks from global warming.
Their lawyers have called on several expert witnesses, including climate scientists, a pediatrician and even former senior White House official John Podesta, to weigh in on the legality of the directives at issue.
"This is really the first time plaintiffs have been able to put on live, cross-examined testimony against the federal government about how it is causing the climate crisis and injuring young people," Andrea Rogers, a lawyer with Our Children's Trust, told AFP.
- A long road -
The plaintiffs are seeking a preliminary injunction that could open the door to a full trial.
The federal government, joined by 19 conservative-leaning states and the territory of Guam, wants the case thrown out.
Most observers give the youths long odds. Judge Dana Christensen, an Obama appointee with a record of pro-environment rulings, is presiding.
But even if the plaintiffs notch a win, the case would then almost certainly land before the conservative-dominated Supreme Court.
"We don't have strong judicial precedent for there being a constitutional right to a clean environment at the federal level," Michael Gerrard, a professor of environmental law at Columbia University told AFP.
"They're trying to frame it as a matter of substance or due process, but that would require novel rulings from the courts to apply that to climate change," he continued, adding: "This Supreme Court is more about taking away rights than granting them, unless you're a gun owner."
Still, the legal team hopes momentum is building in the wake of recent state-level victories.
In 2023, a Montana judge sided with young plaintiffs who argued ignoring climate impacts when issuing oil and gas permits violated their constitutional right to a clean environment.
A year later, youth activists in Hawaii reached a settlement requiring the state to accelerate decarbonization of its transport sector.
But the record has proven bleak at the federal level.
The most prominent case was filed in 2015, Juliana v. United States, and eventually got dismissed after the Supreme Court refused to hear an appeal earlier this year.
The new suit argues that the government is violating due process by stripping citizens of fundamental rights, overstepping executive authority under laws like the Clean Air Act, and breaching its duty under the Fourteenth Amendment by knowingly worsening climate risks.
Gerrard said it would be intriguing to see whether the government will try to contest the factual claims brought by the plaintiffs, or focus instead on legal arguments.
The government is expected to argue these are policy questions for elected officials, not by courts.
But Rogers argued it was the government straying from its lane.
"Whether the executive branch is violating the constitutional rights of young people -- that's precisely the kind of question courts have resolved for decades."
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