"The oil industry is volatile, as we know. But I think climate-related risk is finally, actually starting to play in here." Related : The Oil Industry Was Warned About Climate Change in 1968 S&P's decision focused on rising red ink rather than mercury. ExxonMobil's debt "has more than doubled in recent years," as the company spent big on new projects when crude prices were high, the ratings agency wrote. Meanwhile, stock buybacks and dividend payments to shareholders "substantially exceeded internally generated cash flow." ExxonMobil is the biggest US oil company and the world's largest publicly traded producer. Having a AAA rating allowed it to borrow money at dirt-cheap interest rates, and the company and its corporate ancestors had held that rating since 1930. But the entire industry has been struggling with a worldwide glut that has driven prices down from more than $100 a barrel two years ago to as low as $26 in January. It's trading around $45 a barrel this week.