Can a treaty undo the world’s plastic problem?

Creating a global treaty on plastic isn’t easy, but the U.S. and other United Nations members are getting closer. They’re negotiating a legally binding agreement to dramatically reduce plastic pollution and waste, and the fourth round of talks ended this week. Their goal is to wrap it up this year, so where’s the holdup?

The most ambitious ideas are also the most controversial, especially whether the treaty will cap plastic production worldwide. Environmentalists and more than 90 countries...