Negotiators closed a deal setting rules for carbon markets at the United Nations COP26 climate talks on Saturday, potentially unlocking trillions of dollars for protecting forests, building renewable energy facilities and other projects to combat climate change.

The final deal adopted by nearly 200 countries will implement Article 6 of the 2015 Paris Agreement, allowing countries to partially meet their climate targets by buying offset credits representing emission cuts by others.

Companies, as well as countries with vast forest cover...