Nuclear power plants have been used since the 1950s to produce a steady low-carbon supply of electricity, without creating local air pollution. In 2020, nuclear power plants in over 30 countries generated 10% of global electricity. Nuclear power's lifecycle greenhouse gas emissions (including the mining and processing of uranium), are similar to the emissions from renewable energy sources. Reducing the time to build and cost to operate nuclear power have been goals for decades, but progress has been limited. As of 2020, costs per unit of electricity produced for nuclear are about 4 times as high as for utility-scale solar and onshore wind (not including the costs of energy storage), and about 3 times as high as for base load natural gas.