A group of young Americans who have spent nearly four years trying to compel the federal government to take action on climate change found themselves back in court on Tuesday, arguing that their unprecedented lawsuit should move forward.

The Trump administration, like the Obama administration before it, was there to argue once again that the lawsuit should be tossed out before it ever goes to trial, both because the plaintiffs do not meet the legal requirements to bring such a suit and because "there is no fundamental constitutional right to a 'stable climate system.' "

The lawsuit, filed in 2015 by 21 young people who argue that the failure of government leaders to combat climate change violates their constitutional right to a clean environment, had been scheduled to go to trial last fall before a district judge in Oregon.

But it was delayed at the last minute while the Supreme Court considered an emergency request from the government.

In early November, the court refused to grant the Trump administration's plea to stop the case before trial, instead sending it back to the US Court of Appeals for the 9th Circuit.

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That is where the case, known as Juliana v United States, got its latest moment in court on Tuesday afternoon.

"It is a case that is a dagger at the separation of powers," Jeffrey Bossert Clark, an assistant attorney general for the Justice Department, argued before a panel of three appeals court judges. He added, "This is a suit that is designed to circumvent a whole bunch of statutes."

Mr Clark reiterated the arguments that the government has long made in the case: that the lawsuit amounts to an "end run" around the authorities of Congress and federal agencies and that the remedies the plaintiffs are seeking would be unwieldy, unrealistic and unwarranted.

But Julia Olson, the attorney for the plaintiffs and executive director of Our Children's Trust, insisted that her clients - a majority of whom are now old enough to vote - had been deprived of their fundamental rights as a result of government policies that fuel global warming.

The group's goal is to compel the government to scale back its support for fossil fuel extraction and production and to support policies aimed at reducing greenhouse gas emissions.

"When our great-grandchildren look back on the 21st century, they will see that government-sanctioned climate destruction was the constitutional issue of this century," Ms Olson said.

"We must be a nation that applies the rule of law to harmful government conduct that threatens the lives of our children, so that they can grow up free and pursue their happiness. That is what the founders intended."

The three judges presiding over Tuesday's hearing grilled both attorneys on the particulars of their arguments, and they seemed to wrestle with whether the courts could make such sweeping demands of the government.

"Look, you're arguing for us to break new ground," judge Andrew Hurwitz told Ms Olson at one point. "You may be right. I'm sympathetic to the problems you point out. But you shouldn't say this is just an ordinary suit. . . . You're asking us to do a lot of new stuff, aren't you?"

"We're asking the court to apply bedrock constitutional law and principles to a wholly new set of facts," Ms Olson replied.

In a congressionally-mandated report last fall, the federal government detailed how the effects of climate change, including deadly wildfires, increasingly debilitating hurricanes and heat waves, are already battering the United States - and how the risk of more such catastrophes is worsening.

Produced by 13 federal departments and agencies, along with outside researchers, the National Climate Assessment stretched more than 1,000 pages.

Its authors argued that climate change "is transforming where and how we live and presents growing challenges to human health and quality of life, the economy, and the natural systems that support us."

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The Supreme Court's three-page order in November, which sent the case back to the 9th Circuit, noted the government's assertion that the "suit is based on an assortment of unprecedented legal theories, such as a substantive due process right to certain climate conditions, and an equal protection right to live in the same climate as enjoyed by prior generations."

The justices acknowledged that the 9th Circuit had previously turned down the government but said those decisions came when there was a "likelihood that plaintiffs' claims would narrow as the case progressed."

That no longer seems the case, the unsigned opinion said, suggesting the possibility that the 9th Circuit might see things differently now.

The order also left open the possibility that the government could ultimately return to the Supreme Court.

In briefs to the Supreme Court, solicitor general Noel Francisco wrote that "the assertion of sweeping new fundamental rights to certain climate conditions has no basis in the nation's history and tradition - and no place in federal court."

Even before Tuesday, lawyers in two administrations had made similar arguments in lower courts. But again and again, judges allowed the case to proceed.

In an interview last fall, Ms Olson was optimistic that the young plaintiffs will get the trial they seek.

"We've been confident throughout this case that we would get to trial, and I believe we will get to trial," she said.

"We have overcome everything the government has thrown at us. It is not luck. It is the strength of the case and the strength of the evidence and the strength of the legal arguments we are making."