Natural gas and renewables are highly complementary energy sources. Together, they are the best way to reduce greenhouse gas emissions. Because energy storage technology is not yet viable for fast-ramping energy generation, natural gas provides renewable energy like wind and solar with the flexible back-up power required to quickly respond to fluctuations in the power grid's supply and demand. Grid-scale battery technologies may not become available until 2040, and natural gas will provide crucial support to renewables until then.