Fossil fuels – coal, petroleum and natural gas – have provided humans with energy for centuries; however, their heightened use in recent decades has produced acid rain, ozone pollution, climate change and other environmental problems. Further, fossil fuels are finite, meaning that alternative energy sources will eventually be needed. Renewable energy is generated from natural resources such as sunlight, wind, rain, tides and geothermal heat. In contrast to fossil fuels, renewable energy does not deplete resources. While alternative energy is currently more expensive than fossil fuel, developing more efficient forms of renewable energy is a major area of research and innovation. This is occurring among governments, companies and individuals, as all are affected by the prospect of a limited energy supply.