The Green and Alternative Energy concentration. Alternative energy refers to energy sources that have no undesired consequences, for example fossil fuels or nuclear energy. Alternative energy sources are renewable and are thought to be "free" energy sources. They all have lower carbon emissions, compared to conventional energy sources. These include biomass energy, wind energy, solar energy, geothermal energy, and hydroelectric energy sources. Combined with the use of recycling, the use of clean alternative energies such as the home use of solar power systems will help ensure man's survival into the 21st century and beyond. By 2050, one-third of the world's energy will need to come from solar, wind, and other renewable resources, according to British Petroleum and Royal Dutch Shell, two of the world's largest oil companies. Climate change, population growth, and fossil fuel depletion mean that renewables will need to play a bigger role in the future than they do today.