Several sustainable energy sources could reduce or eliminate our dependence on fossil fuels and nuclear energy. Some of these sources have been used for centuries but have been neglected since fossil fuels came into widespread use. Passive solar heat, fuelwood, windmills, and waterwheels, for instance, once supplied a major part of the external energy for human activities. With increased concern about the dangers and costs associated with conventional commercial energy, these ancient energy sources are being reexamined as part of a more sustainable future for humankind.

Biomass also may have some modern applications. In addition to direct combustion, biomass can be converted into methane or ethanol, which are clean-burning, easily storable, and transportable fuels. These alternative uses of biomass also allow nutrients to be returned to the soil and help reduce our reliance on expensive, energy-consuming artificial fertilizers.

Many of these sustainable energy sources depend on technology that is still experimental and too expensive to compete well with established energy industries. If the economies of mass production and marketing were applied, these new technologies could be made available more cheaply and more dependably. It may take special funding and other governmental incentives to make sustainable energy competitive. The subsidies for renewable energy sources have been especially meager in comparison to the billions of dollars spent on nuclear energy, large-scale hydropower, and fossil fuel extraction and utilization.