The attorney from the U.S. Department of Justice had never seen anything like it: Under a Pacific Northwest drizzle last March, hundreds of people gathered on the steps of the federal courthouse in Eugene, Oregon, waving signs, chanting, and cheering. Some of those rallying had arrived as early as 4:30 A.M.; others had driven for hours to get there. A woman from Portland passed out homemade cookies decorated like Earth. "You've got quite a crowd here," the government counsel, Sean C. Duffy, told the plaintiffs' lead attorney, Julia Olson. "I don't think we're going to start on time."

Inside the courtroom, federal magistrate judge Thomas Coffin prepared to hear arguments in Kelsey Cascadia Rose Juliana et al. v. United States of America et al., a lawsuit alleging that the government's failure to meaningfully address climate change violates young people's constitutional rights to due process and "life, liberty, and property." On one side, Our Children's Trust, a scrappy public interest law group that had brought together 21 youths aged 8 to 19 to press the claim that inaction on climate change represents an intergenerational injustice. On the other side, the U.S. government, joined by Big Oil and other major industrial interests, making a motion to dismiss the case.

The lawsuit, filed in 2015, was at its earliest stage, and already it had become a proxy battle in the larger fight over climate. The Global Catholic Climate Movement, an international network including Pope Francis, had filed an amicus brief in support of the kids' case. The American Petroleum Institute and the National Association of Manufacturers were backing the government's position. Activist-author Naomi Klein called it "the most important lawsuit on the planet right now."

But even those sympathetic to the young plaintiffs' claims were skeptical of their chances for success. "It's definitely pushing the boundaries," Michael Burger, the executive director of the Sabin Center for Climate Change Law at Columbia Law School, said. "Getting the courts to say yes to this is going to be a tough sell."

Turns out the plaintiffs' attorneys did a good job selling their case. Less than a month after the hearing, Judge Coffin ruled that the lawsuit could move forward. The judge wrote: "The intractability of the [climate change] debates before Congress and state legislatures and the alleged valuing of short-term economic interest despite the cost to human life necessitates a need for the courts to evaluate the constitutional parameters of the action or inaction taken by the government. This is especially true when such harms have an alleged disparate impact on a discrete class of society."

Or, in plain English: Climate change may harm young people more than anyone else, and since the political system has failed to act on the issue, the young plaintiffs are due their day in court.

The plaintiffs were thrilled by the ruling. "Super happy, overjoyed, running around telling all of my friends and my family" is how 19-year-old Jacob Lebel of Roseburg, Oregon, described his reaction. "It's historic. It really is. There has been nothing like this. It really gave me hope."

The kids' cutting-edge lawsuit relies on a concept of law called the public trust doctrine. Based in British common law—and, before that, ancient Roman law—the doctrine holds that the government has a responsibility to steward for future generations shared resources such as ocean fisheries and navigable rivers. Public trust lawsuits are common in state courts, and the federal government has relied on the doctrine at times—for example, in pressing its case against BP for the Gulf of Mexico oil spill. But rarely have private citizens used the public trust doctrine to challenge the U.S. government. And no one has employed it to make an argument that the government must protect an "atmospheric trust" for all citizens.

Olson, the founder and general counsel of Our Children's Trust, got inspired to use the public trust doctrine after years of litigating left her feeling that many environmental lawsuits were little more than Whac-A-Mole. She worked with youth organizations across the United States to enlist the plaintiffs. Olson acknowledges the audaciousness of the suit but argues that the case rests firmly on constitutional principles. "When you read the case law over the past 150 years, there are a lot of intergenerational cases you can find," she says. "It's about being fair to those who come after us. I think people understand that concept."

Of course, for the plaintiffs themselves, climate change isn't just something that will harm "those who come after." It is, instead, a clear and present danger. "I was born in 1997, so I'll be more than 50 years old in 2050, when we'll have many more degrees of warming," says plaintiff Alex Loznak, a 19-year-old Oregon native and a student at Columbia University. His family owns an organic hazelnut and sustainable timber farm in the Umpqua Valley, and Loznak says it has lost revenue due to rising temperatures. "I think we have the moral authority to say that all of these horrible impacts will happen in our lifetimes."

The courts appear to be listening. In addition to Judge Coffin's favorable ruling, Our Children's Trust has seen victories at the state level. In a separate case last November, a judge in Washington ruled that the state's Department of Ecology has a "mandatory duty" to protect the air quality for future generations. And in May, after hearing a case brought by four teenagers, the Massachusetts Supreme Judicial Court ordered the state to follow through on its greenhouse gas reduction pledges.

The federal case will go to trial in early 2017. When it does, Olson and her legal team will present evidence that, as early as 1965, government employees expressed concern that, in their words, greenhouse gas emissions would lead to "apocalyptic" and "catastrophic" change. They'll also argue that government agencies colluded with the fossil fuel industry to suppress such warnings.

"It's going to be an important trial," Olson promises. "It's going to be the trial of the century."

This article appeared in the September/October 2016 edition with the headline "Kids Get Their Day in Court."