A big shift to renewable energy and efficiency could produce 4.2 million new environmentally friendly jobs over the next three decades. The U.S. Conference of Mayors study says such "green" jobs could become the "fastest growing segment" of the U.S. economy over the next few decades.

The study says about 750,000 people work today in what can be considered "green" jobs. That includes the makers of wind turbines and more energy-efficient products and scientists who research alternative fuels. The report says that by 2038, another 4.2 million green jobs are expected to be added. That would account for 10 percent of new job growth over the next 30 years.

Miami Mayor Manny Diaz, the conference's president, says the report makes "a very compelling economic argument for investing in the green economy."