In a courtroom packed with environmental activists, federal judges wrestled Tuesday with whether climate change violates the constitutional rights of young people who have sued the U.S. government over the use of fossil fuels.

A Justice Department attorney warned three judges from the 9th U.S. Circuit Court of Appeals that allowing the case to go to trial would be unprecedented and open the doors to more lawsuits.

“This case would have earth-shattering consequences,” Assistant Attorney General Jeffrey Clark said.

He called the lawsuit “a direct attack on the separation of powers” and said the 21 young people who filed it want the courts to direct U.S. energy policy, instead of government officials.

The young people are pressing the government to stop promoting the use of fossil fuels, saying sources like coal and oil cause climate change and violate their Fifth Amendment rights to life, liberty and property.

The judges seemed to feel the enormity of the case, which the plaintiffs’ lawyer compared in scope to the U.S. Supreme Court’s Brown v. Board of Education ruling that mandated desegregation of schools in the 1950s.

If the case moves forward, the judiciary would be “dealing with different branches of government and telling them what to do,” said Judge Andrew Hurwitz, instead of issuing court orders telling officials to stop doing something deemed unconstitutional.

The dire threat to people, particularly the young, demands such action, said Julia Olson, chief legal counsel for Our Children’s Trust, which is representing the plaintiffs.