The city operates with a commission-based government guided by a mayor and four commissioners as well as Metro, the only directly elected metropolitan planning organization in the United States. Portland was the first city to enact a comprehensive plan to reduce carbon dioxide emissions. In 2018, a national survey ranked Portland as the 10th greenest city in the nation. Its climate is marked by warm, dry summers and cool, rainy winters. This climate is ideal for growing roses, and Portland has been called the "City of Roses" for over a century.