Urban heat stress reduction through optimised tree cover scenarios

Rapid urbanisation exacerbates urban heat stress, posing significant risks to human well-being under a warming climate. This research investigates nature-based solutions for urban heat stress mitigation, implemented within a climate service framework and using a mid-sized Brazilian city as a case study. We employed the SOlar and LongWave Environmental Irradiance Geometry (SOLWEIG) model to simulate mean radiant temperature and the Universal Thermal Climate Index (UTCI) for winter and summer at high spatial (2 and 10 m) and temporal (1 hr) resolutions. Our case study sought to: (i) characterise the hourly, seasonal, and weather-dependent variations in urban heat stress; (ii) identify the neighbourhoods and population groups most vulnerable to extreme heat across key Local Climate Zones; and (iii) develop strategies for maximising tree canopy cover to improve thermal comfort. We explored the potential of green infrastructure enhancement through four scenarios (no trees; current tree cover; and two levels of increased tree cover) utilising a developed algorithm that optimises tree cover by replacing smaller trees with larger specimens and strategically planting new, mature trees. Our spatial analysis of UTCI distribution highlighted that the city centre (where a significant fraction of the elderly population resides), along with its adjacent neighbourhoods, experienced the highest percentage of summer hours exceeding strong and very strong heat stress thresholds, which poses significant health risks to the population. Notably, during the summer, the city centre recorded 194 hr above the very strong heat stress threshold (> 38 °C). Doubling the canopy area in the city centre can lead to UTCI reductions of up to 1.2 °C compared to the baseline (current tree cover). Further expanding the canopy to 2.8 times the original area resulted in an additional decrease of 0.9 °C, bringing the total reduction to 2.1 °C. A closer analysis of two blocks in the city centre revealed UTCI reductions of up to 1.0 °C and 3.1 °C when considering two scenarios with a progressive increase in the number of trees, compared to the baseline during the hottest hours of the day (12:00–14:00). This study demonstrates the effectiveness of our framework as a practical, accessible, customisable and cost-efficient tool for cities to assess and implement tree-based heat mitigation strategies.