It was 8 degrees in Minneapolis on a recent January day, and out on Interstate 394, snow whipped against the windshields of drivers on their morning commutes. But inside the offices of Cargill, the food conglomerate, Greg Page, the company’s executive chairman, felt compelled to talk about global warming. Mr. Page is a member of the Risky Business Project, an unusual collection of business and policy leaders determined to prepare American companies for climate change. It’s a prestigious club, counting a former senator, five former White House cabinet members, two former mayors and two billionaires in the group. The 10...