The United Nations’ top court in a landmark advisory opinion says countries could be in violation of international law if they fail to take measures to protect the planet from climate change, and nations harmed by its effects could be entitled to reparations.

The International Court of Justice called global warming an “urgent and existential threat” at a closely watched case in The Hague.

THE HAGUE, Netherlands — The United Nations’ top court has issued a landmark advisory opinion on climate change, its 15-black-robed judges weighing in for the first time on what the court’s president called “an existential problem of planetary proportions that imperils all forms of life and the very health of our planet.”