Scientists and engineers are continually improving the efficiency of renewable energy systems. Humans have been harnessing the wind for thousands of years, and are now cleanly producing electricity with it. Air flowing through turbines or spinning blades generates power that can be used to pump water or generate electricity. Wind energy is now the world's fastest growing energy source and has become one of the most rapidly expanding industries. The wind industry is modifying equipment to address this issue but also points out that countless wild creatures are killed every year as part of the acquisition and distribution of conventional energy sources.
Hydroelectric power is another source of renewable energy.
Hydroelectric dams, however, are no longer considered environmentally benign sources of power. Fisheries and other wildlife habitat have been severely impacted on many dammed rivers. Most of the world's hydroelectric dams are historically recent, but all reservoirs eventually fill up and require very expensive excavation to become useful again. At this time, most of the available locations for hydroelectric dams in the U.S. are already developed. Humans have been burning biomass materials since the dawn of time, and it is still the principal fuel used in many parts of the world.