2019 Volume 75 Issue 3 Pages 144-152
In urban areas around the world, air temperature increases due to global warming and urbanization are adversely affecting human health and living environments. We used a numerical weather forecasting model to quantitatively simulate the effectiveness of citizen-driven urban forestry, the voluntary activity of citizens to plant and maintain trees in their residential areas, as a method for reducing summer air temperatures in the Tokyo metropolitan area, the largest urban area in Japan. We calculated the daily maximum air temperature for 10 days in the summer of 2007, and simulated the effects of five different levels of increased urban forest area per capita according to population distribution using albedo, heat capacity, and evapotranspiration. Increasing the forest cover by 3.3 m2 per capita reduced the daily maximum air temperature by only 0.014 °C, although the effect increased linearly with higher forest cover per capita. If forest planting is increased to about 30 m2 per capita, the air temperature reduction in the center of Tokyo could reach 0.4 to 0.5 °C, comparable to the increase in urban air temperature over the past hundred years due to global warming. The results suggest that the collective actions of individual citizens to increase urban forest cover can produce a significant effect on the mitigation of high air temperatures due to the urban heat island effect and climate change.