Throughout most of human history, biomass from plants was the main energy source, which was burned for heat and to feed animals used for transportation and plowing. Nonrenewable sources began replacing most of renewable energy use in the United States in the early 1800s, and by the early-1900s, fossil fuels were the main sources of energy. Use of biomass for heating homes remained a source of energy but mainly in rural areas and for supplemental heat in urban areas. In the mid-1980s, use of biomass and other forms of renewable energy began increasing largely because of incentives for their use, especially for electricity generation. Many countries are working to increase renewable energy use as a way to help reduce and avoid carbon dioxide emissions.