On hot summer days, city streets can be much hotter than tree-lined streets. Cities around the world are now planting more trees to fight rising temperatures and air pollution.
From Seoul to Singapore, and from Milan to Paris, cities are building urban forests. The World Health Organization says 99 percent of people breathe air that is not clean enough. Clean air in cities is now very important for public health.
Cities are hotter than the countryside because of concrete and roads. Scientists call this the "urban heat island effect." Temperatures in cities can be 3 to 5 degrees Celsius higher. Trees give shade and release water into the air. This can lower the temperature by 2 to 8 degrees Celsius. One fully grown tree can take in about 22 kilograms of carbon dioxide each year.
Seoul is one of the cities leading this work. The city wants to create 3,000 hectares of new green space by 2030. The Seoullo 7017 project turned an old bridge into a park with more than 24,000 plants.
In Milan, two buildings called Bosco Verticale, or "Vertical Forest," were finished in 2014. They have more than 900 trees and 20,000 plants on their balconies. Paris is also planning to plant trees near famous places like the Gare de Lyon station.
Singapore has more than 300 kilometers of green paths that connect parks across the city. Trees can also cut building cooling costs by 20 to 50 percent and help people feel less stressed.