Complex sentence: Energy economic theories recognize that energy consumption and energy efficiency were important historical causes of economic growth and energy consumption remains highly correlated with economic growth, as is well noted in economic history and by the U. S. Dept. of Energy.
Split into simpler sentences: Energy economic theories recognize that energy consumption and energy efficiency were important historical causes of economic growth. Energy consumption has been shown to be highly correlated with economic growth, as is well noted in economic history and by the U. S. Dept. of Energy.