A Canadian federal court has ruled in favor of environmental groups that sued in opposition to a massive planned oil-sands mine in Alberta. The 120-square-mile strip mine had recently been approved by a joint federal-provincial panel that found the project’s estimated annual greenhouse-gas emissions of 3.7 million tons to be insignificant. Yet no justification was given for the finding. “The panel dismissed as insignificant the greenhouse-gas emissions without any rationale,” the judge wrote, ruling that the panel must justify its conclusion. Environmentalists hoped the ruling would force stricter reviews of similar projects in the future. If it’s ultimately approved, the mine could produce 300,000 barrels of oil sands a day.