THE HAGUE, Netherlands —

The United Nations’s top court announced Wednesday that if countries fail to take measures to protect the planet from climate change, they could be in violation of international law.

The International Court of Justice was delivering an advisory opinion in a landmark case about nations’ obligations to tackle climate change and the consequences they may face if they don’t, calling it an “urgent and existential” threat to humanity.

“Failure of a state to take appropriate action to protect the climate system ... may constitute an internationally wrongful act,” court President Yuji Iwasawa said during the hearing.

The non-binding opinion, which runs to over 500 pages, is seen as a potential turning point in international climate law.

The court also said a “clean, h

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