Thermal Sensation and Carbon Emissions in Greener Urban Spaces: Case Study of Adelaide

Abstract

Urban resilience to climate change is intertwined with useability and comfort in outdoor spaces. To date, research indicates that frequency and quality of public life are affected by urban microclimate and outdoor thermal comfort. Concentrations of building mass and lack of sufficient greenery in cities are identified to contribute significantly to extended heat stress in the built environment, commonly known as the urban heat island (UHI) effect. Such accumulated urban heat stress affects citizens’ health, especially for more physiologically or socially vulnerable population. Previous studies have shown that urban greenery can moderate urban heat stress by up to 2°C via evapotranspiration and shading. A majority of existing studies are focused on the cooling effect of urban greenery without its potential links to peak energy demand and consequent infrastructure requirements and carbon emissions in cities. In the first section, this paper compares self-expressed thermal sensation vote (TSV) with corresponding predicted mean vote (aPMV) in different thermal environments in 12 public spaces of Adelaide CBD between July and October 2018. To formulate the outdoor thermal comfort, human subjects with various clothing and activity levels, exposed to different air temperatures, surface temperatures, relative humidity, wind speed and mean radiant temperature were asked for their TSVs (ASHRAE-55 seven-point thermal sensation scale was used). Findings indicate that thermal perceptions may have a step-change toward thermal comfort compared to the predicted values in public spaces with 10-30% greenery. Participants in greener public spaces expressed less thermal discomfort compared to what is expected from such thermal environment. In the second section, a scenario-based modelling is developed based on climate change projections and potential changes in urban greenery in 2030 and 2090. Results are then compared to energy demand data in different thermal environments in Adelaide to assess energy demand reduction potentials in a greener Adelaide. In the medium emission scenario (RCP4.5) the surface temperature of Adelaide is expected to increase between 0.3 °C and 1.0°C by 2030 compared to 2010 (RCP4.5 scenario). This temperature increase is expected to reach 2.4°C by 2090. An ideal urban landscape transformation scenario of having 30% tree canopy could decrease the ambient temperature in urban precincts by up to 1°C in winter and 3°C in summer. It is estimated that having heat resilient public spaces could save a total carbon emission of 140,000 tone CO2e in Adelaide annually compared to a business-as-usual scenario. Urban greenery may be used as a mean of increased urban life resilience to climate change by reducing ambient temperature, increasing urban population resilience to higher temperatures and decreasing the energy demand for air conditioning during summer.

Keywords: climate change resilience, thermal perception, urban green spaces, outdoor thermal comfort, Adelaide, energy demand

JEL Classification: Q01, Q41, R14