The Supreme Court has taken the unexpected step of pausing a landmark climate change lawsuit brought by 21 young people, which had been trudging through the legal system since 2015.

Late last week, as Brady Dennis reports for the Washington Post, Chief Justice John G. Roberts Jr. ordered a stay in the suit Juliana v. US, which was supposed to go to trial on October 29 at the United States District Court in Oregon. The Chief Justice’s order marks yet another surprising turn in what has proven to be an unusual and contentious legal case. Michael Gerrard, an environmental law professor at Columbia University, tells Dennis that it is “extremely rare, if not unprecedented” for Supreme Court justices to enjoin a trial that is still making its way through the lower courts.

“Ordinarily they’ll wait for the lower court to rule,” Gerrard adds.

It is not clear how long the stay will be in effect, according to John Schwartz of the New York Times.

The lawsuit’s plaintiffs, who range in age from 11 to 22, argue that U.S. government policies have contributed to climate change, thereby denying them their “fundamental” right to a healthy environment. Young people, they say, are the ones who will have to contend with the potentially catastrophic effects of global warming should nothing change, and so their lawsuit seeks to compel the government to limit its support for fossil fuel extraction and support policies that will curb greenhouse gas emissions. The U.S. government, the president and various other official departments and agencies are named as defendants.

Both the Obama and Trump administrations have tried to put a stop to the lawsuit, arguing that the court is not the right place to deal with this matter. The case is “an attempt to redirect federal environmental and energy policies through the courts rather than through the political process,” as Trump administration lawyers wrote when they made their last-minute effort to persuade the Supreme Court to intervene in the case, according to Dennis. But lower courts had previously allowed the lawsuit to go ahead.

A main source of contention is whether or not a safe climate should be considered a civil right. “I have no doubt that the right to a climate system capable of sustaining human life is fundamental to a free and ordered society,” Judge Ann Aiken, who was scheduled to hear the case on the 29th, wrote in a 2016 ruling that dismissed a motion to halt the lawsuit.

But others disagree. Jeffery Wood, a Justice Department attorney, recently said during a law conference that the right to a safe climate “simply does not exist,” reports Vox’s Umair Irfan.

The fact that the Supreme Court has stepped in to stop the lawsuit, even if temporarily, is “certainly a signal that the court is uncomfortable with the underlying legal theory of the Juliana case,” Ann Carlson, a professor of environmental law at the University of California Los Angeles, tells Irfan.

What happens next remains to be seen. According to Schwartz of the Times, the Supreme Court can dismiss the litigation, but it is unlikely to do so “at this early stage.” On Monday, lawyers for the plaintiffs filed a 103-page response arguing that the application to stay the case should be “swiftly denied,” in part because “a stay of trial in the district court will disrupt the integrity of the judiciary’s role as a check on the political branches and will irreparably harm these childen.”