Pexels In a first last month, renewables supplied more power to the U.S. than natural gas, a milestone in the shift to clean energy. However, rising power demand is complicating the transition away from fossil fuels by extending the lives of many aging coal power plants.Together, renewables — including solar, wind, hydropower, and bioenergy — were the biggest source of U.S. electricity in March, according to data from the think tank Ember. Along with nuclear power, they supplied more than half o...