Evaluating heat vulnerability and the impact of urban street tree planting on radiant heat exposure: examples from Vancouver’s neighborhoods

Extensive impervious surface cover, anthropogenic heat, building structure and lack of vegetation contribute to the formation of distinct urban microclimates where higher air and surface temperature as well as lack of shade intensify outdoor heat exposure and thermal discomfort for humans. The objectives of this thesis are to explore the determinants of heat vulnerability across Vancouver’s neighborhoods and assess the impact of increasing street tree cover on extreme radiant heat exposure in different neighborhoods classified into local climate zones (LCZs) under present and future climate. To achieve these goals, first, the determinants of heat vulnerability in Vancouver’s neighborhoods were identified and population groups most vulnerable to extreme heat exposure were mapped by spatially superimposing multiple layers of socio-economic, environmental, and infrastructural data. Secondly, the influence of added street trees on radiant heat exposure across six different LCZs was investigated under present climate. This was done by employing the SOlar and LongWave Environmental Irradiance Geometry (SOLWEIG) model. The radiant cooling effect of increased street tree cover during the hottest day on record for Vancouver (July 29, 2009) was modeled by quantifying the spatiotemporal changes to mean radiant temperature (Tmrt). Results indicated a 2.1–4.2 °C reduction in spatially-averaged Tmrt during the hottest period of day. Lastly, this thesis sought to explore how changes in temperature and solar radiation under future climate projections would change Tmrt in Vancouver over the 2070-2100 period and the extent to which these changes could be mitigated by increased street tree cover. To this scope SOLWEIG was driven with downscaled climate projections using Representative Concentration Pathways (RCP) 4.5 and 8.5. Results showed that days with extreme radiant heat exposure were predicted to increase three- to five-fold under RCP 4.5 and 8.5, respectively. The addition of street trees can mitigate the increase in Tmrt under RCP 4.5 but is not sufficient to compensate for the Tmrt increase under RCP 8.5. The results of this thesis provide valuable insights to city decision-makers and urban planners regarding effective heat mitigation and adaptation interventions and guide future research seeking to simulate the effect of heat mitigation measures under current and future climates.