Human society depends on reliable energy supplies, and global energy demand continues to grow over the long term. Meeting future demand will require energy systems that are secure, economically viable and environmentally sustainable. Reducing dependence on fossil fuels is also important for limiting greenhouse-gas emissions and improving long-term energy security. Renewable energy sources are expected to play an increasingly important role alongside other low-carbon energy technologies. Solar and wind power are currently expanding most rapidly, while hydropower, biomass and geothermal energy continue to contribute to the global renewable energy mix.