Updated July 30 with the U.S. Supreme Court denying the federal government's request to halt the children's climate lawsuit.

One of the world's top economists has written an expert court report that forcefully supports a group of children and young adults who have sued the federal government for failing to act on climate change.

Joseph Stiglitz, who was awarded the Nobel Memorial Prize for economics in 2001 and has written extensively about environmental economics and climate change, makes an economic case that the costs of maintaining a fossil fuel-based economy are "incalculable," while transitioning to a lower-carbon system will cost far less.

The government, he writes, should move "with all deliberate speed" toward alternative energy sources.

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Stiglitz has submitted briefs for Supreme Court cases—and normally charges $2,000 an hour for legal advice, the report says—but he wrote this 50-page report pro bono at the request of the attorneys representing the children. It was filed in federal district court in Oregon on June 28.

He is one of 18 expert witnesses planning to testify in the case, scheduled for trial later this year, the children's lawyers said.

New Government Attempt to Stop the Case

The children's climate lawsuit, filed in 2015, accuses the federal government of perpetuating policies that favor a fossil-fuel based energy system and of failing to adequately regulate greenhouse gas emissions. By doing so, the suit alleges, the government exposed the children to the dangers of climate change and has failed to manage natural resources, in the public trust, for future generations.

A federal district judge is scheduled to hear the case on Oct. 29. Both the Obama and Trump administrations have tried to have the case dismissed, but their efforts have been rejected by the courts.

In early July, attorneys for the Trump administration filed a motion for an emergency stay, which the children's attorneys responded to.

"Just to be clear, there is no emergency," said Julia Olson, executive director of Our Children's Trust, which is representing the children. "They're pulling out every frivolous motion they can to dodge the case."

On July 30, the U.S. Supreme Court rejected the federal government's request for an emergency stay, saying the request was premature. It wrote, however, that "the breadth of the respondents' claims is striking" and said the court "should take these concerns into account in assessing the burdens of discovery and trial, as well as the desireability of a prompt ruling on the government's pending dispositive motions."

Stiglitz: Action Is Feasible and Benefits Economy

Stiglitz, a Columbia University economics professor and former World Bank chief economist, concludes that increasing global warming will have huge costs on society and that a fossil fuel-based system "is causing imminent, significant, and irreparable harm to the Youth Plaintiffs and Affected Children more generally." He explains in a footnote that his analysis also examines impacts on "as-yet-unborn youth, the so-called future generations."

"There is a point at which, once this harm occurs, it cannot be undone at any reasonable cost or in any reasonable period of time," Stiglitz writes. "Based on the best available science, our country is close to approaching that point."

But, he says, acting on climate change now—by imposing a carbon tax and cutting fossil fuel subsidies, among other steps—is still manageable and would have net-negative costs. He argues that if the government were to pursue clean energy sources and energy-smart technologies, "the net benefits of a policy change outweigh the net costs of such a policy change."

"Defendants must act with all deliberate speed and immediately cease the subsidization of fossil fuels and any new fossil fuel projects, and implement policies to rapidly transition the U.S. economy away from fossil fuels," Stiglitz writes. "This urgent action is not only feasible, the relief requested will benefit the economy."

Stiglitz has been examining the economic impact of global warming for many years. He was a lead author of the 1995 report of the UN's Intergovernmental Panel on Climate Change, an authoritative assessment of climate science that won the IPCC the 2007 Nobel Peace Prize, shared with Al Gore.

If the Case Eventually Gets to the Supreme Court?

Olson said it's likely the case will end up in the Supreme Court eventually, but she's unconcerned about Kennedy's retirement and the expected shift to a more conservative court.

"This case is fundamentally a conservative case," she said. "It's about protecting individual liberties from government abuses of power, and that's very much in line with the conservative justices on the court."