Urban heat threats public health and sustainability. Frequency, duration and intensity of extreme heatwaves will likely increase under climate change and will further exacerbate thermal maxima induced by urbanization. It is known that trees and vegetation can reduce land surface temperature (LST) across natural and semi-natural landscapes. However, little empirical evidence exists on how smaller urban green spaces, such as yards and home gardens, as well as the trees and vegetation therein, can reduce urban LST during extreme weather events.
Adelaide is one the hottest cities in Australia and is increasingly affected by climate change. High resolution thermal imagery (2 m) was collected using an aircraft over 110 Adelaide’s suburbs during an extreme summer heatwave. Hyperspectral imagery and photogrammetry were used to derive a tree canopy and vegetation map. All the residential front, corner and backyards (n= 182, 891) in the urban landscape were geolocated by using a novel GIS algorithm. Diurnal and nocturnal LST was summarized within each urban land use (e.g., infrastructures, residential) and land cover (e.g., vegetation, roofs, concrete, etc.).
Results/Conclusions
Backyards had the lowest mean diurnal LST during the heatwave after open spaces (36.64 ± 0.02 SEM and 35.84 ± 0.13˚C, respectively). However, maximum nocturnal LST in backyards (24.94 ± 0.01˚C) was significantly lower compared to open spaces (26.46 ± 0.09˚C). Front and corner yards (n = 80,921 and 18,952, respectively) had significantly higher diurnal and nocturnal LST than backyards (n = 83,018). Residential yard cover was three times larger than open space cover (n = 1,982) and significantly more widespread across the urban landscape. Yards contributed to distributed reduction in LST. However, yard clustering had little effect in compounding the reduction of LST. The presence of trees and vegetation significantly decreased LST, though their temperature day-night differential was lower compared to other artificial land covers.
This research suggests that yards and trees are critical and undervalued assets for urban climate change adaptation and urban resilience to extreme weather events. Importantly, yards and trees provide cooling benefits close to people and their home environments. Under the push for urban densification to improve sustainability, urban planners will need to design urban forms with enough space for yards and trees to effectively manage urban heatscapes under climate change.