If Volkswagen was hoping that its $10 billion buyback settlement with U.S. officials would bring some closure to months of hand-wringing over diesel emissions, its timing couldn't have been worse. A recent flood of news showed that what was once a single company's scandal has grown into a global regulatory crisis. As world leaders gathered in New York to sign the Paris climate accord on Friday, a cloud of doubt settled over one of the most hotly debated areas of environmental regulation.

As the face of auto-emissions cheating, VW has taken the brunt of the scandal's impact: It has budgeted $18.2 billion to cover the cost of repairs and buybacks associated with its U.S. settlement and recalls in Europe. Its stock is down some 40 percent over the last year.

But the emissions problem is now more than just a VW scandal. Mitsubishi has admitted that it overstated fuel efficiency in more than half a million cars, drawing a raid by Japanese authorities and an investigation by U.S. regulators. Only the second company to admit active cheating on vehicle performance specifications, the Japanese automaker has lost about 40 percent of its market value in the last week of trading.