by Dan McCabe
One key objective of sustainable urban planning is to limit the urban heat island (UHI) effect, the increased local temperature in highly built areas due to differences from the natural environment in the absorption and reflection of solar energy at the surface. Previous research has displayed the value of large urban parks in controlling temperature in cities, but less is known about the effect of smaller green spaces. In order to investigate how vegetation and construction levels impact UHI severity, Rotem-Mindali et al. (2015) used ten years of remotely sensed data from two NASA satellites to analyze the relationship between different land uses and land surface temperature (LST) in Tel Aviv, Israel. The authors compiled information on local LST and Normalized Difference Vegetation Index (NDVI), a measure of vegetation cover, and used it to search for a correlation between land use type and mean surface temperature for summer nights. In their analysis, they found an enormous difference of 13°C in mean temperature among different locations in Tel Aviv. There was a strong correlation between land use type and LST, with the most vegetated regions experiencing much lower average temperatures than highly built regions.
Tel Aviv is Israel’s largest metropolitan area and contains a diverse array of land use types. For the purposes of their analysis, the authors separated these into four categories: “green” residential areas characterized by prevalent vegetation in private gardens, other residential spaces, industrial regions, and small- to medium-sized public parks. Five plots of each type, all close to the city center, were selected for analysis. In addition to the results mentioned earlier, one of the study’s most significant and unprecedented findings was that residential areas with substantial vegetation are in some ways more effective at offsetting UHI than small parks. Green residential districts exhibited mean LST 0.5°C less than these parks, but more significantly, while the thermal benefits of the parks diminish rapidly in their immediate surroundings, the UHI offset due to green residential areas is more spatially constant. The authors acknowledge that this result is not simply a testament to the superiority of private gardens for heat mitigation, because parks tend to be surrounded by parking lots and larger streets that generate a large thermal load. However, some of the advantage of residential green areas may be attributable to a more efficient allocation of plant life. The dense planting of trees and shrubs in residential areas and more uniform distribution of vegetation may enhance their mitigation capabilities.
The authors’ finding that residential private gardens are highly effective at UHI mitigation is encouraging for the future of urban planning for climate management. In a dense and established urban fabric, it can be difficult to develop or incorporate large new green spaces that promote a more moderate local climate. This research, however, finds that the increased prevalence of urban gardens in residential areas can provide similarly significant benefits. Thus, the development of more green neighborhoods in cities can generate “cooling islands” to help promote more moderate urban temperatures that reduce energy demand and improve quality of life.
Rotem-Mindali, O., Michael, Y., Helman, D., Lensky, I. M., 2015. The role of local land-use on the urban heat island effect of Tel Aviv as assessed from satellite remote sensing. Applied Geography 56, 145-153.