Roundup is the most widely used weedkiller in the world. Its active ingredient, glyphosate, is sprayed on farms, roadsides, parks, and backyards, and traces of it now turn up in soil, rivers, rainfall, and even human urine. A decade ago, in 2015, the International Agency for Research on Cancer, the cancer arm of the World Health Organization and the world’s leading authority on what causes cancer, reviewed the published evidence and classified glyphosate as probably…