The public land in dispute was leased by the Wyoming-based company in 2007. It's located in a sparsely populated area dominated by ranching and mining.

Environmentalists sued after the expansion was approved, claiming climate change hadn't been fully considered. That led to a 2016 order for officials to re-examine the environmental impacts, and then another lawsuit when that study was completed.

Cavan agreed with the plaintiffs — WildEarth Guardians and the Montana Environmental Information Center — that officials from Interior's Office of Surface Mining had again "failed to take a hard look at greenhouse gas emissions."

The judge said the agency needed to consider the damages those emissions could inflict on society, as well as the human health impacts from other pollutants emitted by coal-burning power plants.

"They kept trying to diminish the impact of their actions," WildEarth Guardians' Jeremy Nichols said of federal officials. "This is the judge saying, 'You have to take responsibility for your action.'"

Department of Interior spokeswoman Faith Vander Voort declined to comment on Cavan's ruling.

Mining in the expansion area has been ongoing since 2012.