Colorado Attorney General Phil Weiser has filed a court brief with the U.S. Supreme Court in a lawsuit brought by Boulder city officials seeking to hold oil companies liable for their environmental impact.

The case began in 2018, when the City and County of Boulder filed a lawsuit in Colorado District Court against Suncor Energy and ExxonMobil, alleging that the companies knowingly violated numerous state environmental and consumer protection laws.

The Commissioners allege that ExxonMobil and Suncor knew for decades that burning fossil fuels and releasing greenhouse gases into the atmosphere would change the planet’s climate. Yet, the companies allegedly misrepresented or concealed their internal findings from the public. As a result, Boulder County and the city of Boulder have had to spend money to address droughts, wildfires and other manifestations of extreme weather, they said.

The oil companies have leaned on a decision out of the New York-based U.S. Court of Appeals for the Second Circuit. In that lawsuit, where New York City sued ExxonMobil and other energy corporations, the Second Circuit ruled that the city would be “jeopardizing our nation’s foreign policy goals” by deploying state-level legal claims against longstanding national and international energy agreements.

The litigation went back and forth between state and federal courts and was ultimately denied for review by the U.S. Supreme Court in 2023.

The oil companies moved to dismiss the suit in state court the following year, arguing that federal law preempted state and local law because greenhouse gas emissions cross state and international lines. After state district courts denied the motion, Exxon Mobil and Suncor petitioned the Colorado Supreme Court for an interlocutory appeal. The State Supreme Court affirmed the district court’s preemption order on a 5-2 vote.

Suncor and ExxonMobil petitioned the U.S. Supreme Court to hear the case again. This time, the Court agreed.

“Our federal system depends on state and federal courts each fulfilling their proper role,” Weiser said. “This case involves the basic question whether Colorado courts can consider common law claims that fossil fuel companies engaged in wrongful behavior that contributed to climate change. At this stage of the case, it would be an overreach and improper for the Supreme Court to prevent Colorado courts from considering the merits of this litigation.”

In the brief to the Court, Weiser and the other attorneys general argue that states should not lose their authority to apply state law just because a case involves issues that cross state lines. They also expressed concerns about the potential precedent the case could set for other state common law claims, warning that allowing federal courts to intervene before state court proceedings are complete could impact a wide range of cases.

“State courts already have well-established legal rules for determining whether claims should proceed,” the attorneys general said. “Those decisions should be made through the normal judicial process rather than a broad new federal rule that prevents state law claims from proceeding in state courts.”

The Supreme Court will hear the case, Suncor Energy v. County Commissioners of Boulder County, in October, with a ruling expected by January.

Reporter Michael Karlik contributed to this article.