Urban Heat Islands and the Cooling Power of Trees
Heat kills hundreds of thousands of people worldwide each year, with cities facing particularly severe problems. Urban areas experience what scientists call the urban heat island effect, where city temperatures rise higher than surrounding rural areas. This happens because cities contain large amounts of concrete and asphalt that absorb sunlight and release it as heat. On average, cities are about 1 degree Celsius warmer at the surface and 0.5 degrees warmer in the air compared to nearby rural areas.
Climate change makes this problem worse. Earth's average temperature has already increased by 1.1 degrees Celsius, and scientists expect another 0.4 degrees of warming by 2035. Heat waves are becoming more frequent, increasing from two per year in 1971 to four per year in 2022. Without action, life-threatening heat could affect up to three-quarters of all people by 2100.
Trees provide natural cooling through two main processes. First, they release water vapor through their leaves, which removes heat from the air. Second, they create shade that prevents sunlight from heating up pavement and buildings. These effects can reduce air temperature by 1 to 2 degrees Celsius near the trees. In the United States alone, urban trees prevented approximately 1200 heat-related deaths in 2016.
Researchers studied 8919 large cities worldwide to understand how much trees help with heat. They found that current tree cover reduces the urban heat island effect by nearly half, or about 48.6 percent. Right now, trees in cities around the world provide 0.151 degrees Celsius of cooling on average.
However, climate change will increase summer temperatures in cities by about 1.5 degrees Celsius by 2050. Current tree cover can only offset about 10 percent of this warming. Even if cities plant the maximum possible number of trees, they would only offset about 20 percent of expected warming, meaning trees alone cannot solve the heat problem.