Nature News: Electricity generation provides 18,000 terawatt-hours of energy a year, around 40% of humanity's total energy use. In doing so it produces more than 10 gigatonnes of carbon dioxide every year, the largest sectoral contribution of humanity's fossil-fuel derived emissions. Yet there is a wide range of technologies — from solar and wind to nuclear and geothermal — that can generate electricity without net carbon emissions from fuel.