In 2012, the South Korean city of Seoul (10 million inhabitants, 23% of GDP) launched the One Less Nuclear Power Plant Initiative thanks to considerable efforts by the city’s former mayor Park Won-soon. The initiative set a reduction target for energy consumption and carbon emissions. Building on the success of activities linked to the initiative, the city began a second phase, which involved all the relevant areas of municipal action and brought citizens on board, particularly with planning and implementation. Seoul cut its GHG emissions by 5% between 2005 and 2017 as a result. In the same period, South Korea recorded a 26.8% increase in GHG emissions.