Small vegetated patches greatly reduce urban surface temperature during a summer heatwave in Adelaide, Australia

Abstract

As the global climate warms, cities worldwide face more frequent and extreme heatwaves. These events can affect human health and decrease liveability. While the mitigating effects of vegetation on land surface temperature (LST) are well characterised at large spatial scales and during typical weather conditions, the cooling benefits that urban greening can provide at local scales, particularly during summer heatwaves, are poorly quantified.We quantified LST variation across an urban landscape to assess relationships with land use and vegetation cover from land unit (average ~ 460 m2) to suburb scale (~1.66 km2) under extreme summer heat conditions. High resolution (2 m), day- and night-time LST was measured at the peak of a heatwave event, after three consecutive days with air temperature exceeding 40 ˚C, by flying an aircraft fitted with a thermal imager over 90 suburbs in Adelaide, Australia. Daytime, night-time, and diurnal range LST distributions were related to measures of land use, vegetation cover (i.e., tree, grass) and urban morphology at both the suburb and land unit scale.At the suburb scale, vegetation cover did not affect LST. However, at the land unit scale, tree canopy cover, and to a lesser extent grass cover, decreased local LST by up to 6 ˚C during the day, but not at night. Overall night-time LST was poorly predicted by the land use and land cover predictors. Moving inland from the coast, small vegetation patches, mostly contained in yards and gardens, was associated with the greatest localised LST reductions in the hotter inland suburbs. LST within land units was further decreased during the day when vegetation was present within 30 m buffers around each land unit, suggesting a moderate landscape cooling effect on LST.Our results suggest that even small urban vegetation patches can be managed to provide substantial heat mitigation during increasingly frequent summer heatwaves, particularly around the residential environments where people live.