For two months last year, a smoky plume stretched around half the globe at the equator, drifting from East Africa to the international dateline in the western Pacific Ocean. The billowing smoke emanated from Indonesia, which wheezed its way through its worst fire season in more than 15 years.

The smoky girdle—detected by NASA satellites—was emblematic of a phenomenon with wide-reaching climate and health impacts, according to new research published in the Proceedings of the National Academy of Sciences.

Dry-season burning is commonplace in Indonesia: It remains the cheapest and most effective way to clear agricultural and logging waste. However, in exceptionally dry years—such as 2015, which was driven by El Niño conditions—this burning can threaten the extensive peat deposits underlying