Trees and Urban Heat Island (UHI)

One of the many benefits of the urban forest is its ability to ameliorate urban microclimate, cool large or small areas, and benefit human health. Cities tend to have higher ambient air temperatures than rural areas due to the high heat absorption capacity of various building materials (GoC, 2022). Concrete, asphalt, and cement absorb sunlight and trap heat much more effectively than trees, parks, and fields, leading to higher air temperatures in built-up areas (USEPA, 2024). Cities also produce their own heat, which is released by vehicles, air conditioners, and machinery (Climate Atlas of Canada, n.d.). Trees and green spaces can improve urban climate, reduce surface and air temperatures, cool the environment, improve the comfort of citizens by providing shade, and mitigate the effects of urban heat islands through evapotranspirative cooling (Yin et al., 2024; Schwaab et al., 2021). The combined effects of evapotranspiration and shading can reduce summer temperatures by 1–5°C (USEPA, 2008).

The urban heat island effect in cities across the globe is amplified by climate warming. Urban forests and green infrastructure are recognized as nature-based solutions and natural capital investments for addressing climate change impacts (IFC, 2024). Canada, like many other urbanized countries, is facing challenges related to climate change and the urban heat island effect. These issues include the impacts of heat on human health, infrastructure, biodiversity, and wildlife. Urban areas in British Columbia and Quebec have experienced extreme heat, leading to an increase in heat-related illnesses and mortality during heat waves in recent years (Poitras et al., 2018; Beugin et al., 2023). The extent and distribution of urban tree canopy in Canadian cities can significantly benefit both human and environmental health, especially during the summer months. As a result, urban trees and green spaces are becoming increasingly valuable as climate change continues to drive extreme weather events, such as heat waves and temperature fluctuations (Health Canada, 2020).

UHI: Trees and Air Cooling

Many studies have documented across the globe that during the summer peak, air temperature in large cities with heat-absorbing surfaces and lack of green space could be as much as 10-15°C hotter than surrounding areas, while at night, the difference can be up to 12°C (Joint Research Centre, 2022; Mentaschi et al., 2022). This higher air temperature across urban areas is referred to as the urban heat island effect (UHIE). The UHIE phenomenon also impacts many cities across Canada. For example, it reached the highest daytime value of 7.25°C for Vancouver and the highest nighttime UHI intensity at 4.36°C for Toronto (Duan, Agrawal, Sanchez-Azofeifa, and Welegedara, 2024). When gray infrastructure absorbs heat from the sun, this heat is retained and slowly released even after the sun goes down, which keeps city temperatures higher during the night (USEPA, 2024).

Urban forests, trees, and urban greenery can help reduce UHIE by cooling city air temperatures through sunlight absorption, evapotranspiration, and interception of particulate matter. Evapotranspiration, the process that adds water to the air through evaporation from plants and surrounding soil, can reduce ambient air temperatures by 1-5°C (USEPA, 2024). Studies have found that greener urban areas are cooler on average than less-green urban areas, with urban forests having daytime temperatures about 1.5°C cooler than surrounding areas during summer months (Knight et al., 2021). Additionally, by intercepting greenhouse gases and particulate matter associated with air pollution from dust, car exhaust, and wildfires, urban trees use their leaves and needles to filter the air and reduce ground-level temperature by offsetting greenhouse gas emissions and reducing smog in cities (Knight et al., 2021).

Urban tree canopies provide much-needed shade and can reduce the amount of sunlight absorbed by gray infrastructure like buildings and roads (SFI, 2024). They help decrease the severity of this UHI effect by intercepting sunlight before it reaches buildings and roads. Additionally, the shade provided by urban trees can help decrease cooling-related energy costs by up to 7% in summer months by reducing the amount of sunlight absorbed by building exteriors, reducing cooling energy costs (Nowak, 2017). In addition to urban forests, green roof technologies can reduce roof surface temperature by up to 20°C, further asserting the benefits of vegetation and green spaces (USEPA, 2024). 

Canadian municipalities such as Kingston, Vancouver, and Surrey have successfully implemented diverse urban forestry initiatives to combat high temperatures and climate change. For example, Kingston’s urban forest has helped to combat the urban heat island effect by improving thermal comfort and reducing energy consumption associated with cooling (Guilbault, 2016). The City of Vancouver analyzed local climate zones to optimize tree planting locations, ensuring that urban trees contribute effectively to maintaining outdoor thermal comfort (Aminipouri et al., 2019); this approach highlights the value of local and site-specific urban forest planning strategies. Additionally, the City of Surrey has engaged residents in urban heat readiness by developing a conversation guide emphasizing the role of urban trees in mitigating heat waves and improving community resilience (City of Surrey, 2021). These are some examples of novel approaches to utilizing urban forestry as a tool to combat climate change and enhance human health in urban areas in Canada.

UHI: Human Health

As the climate changes in Canada, the role of trees in cooling urban areas and supporting human health becomes increasingly important. Urban forests serve as a crucial climate change mitigation measure. With rising summertime temperatures, the ability of trees to cool the air and provide shade is an essential resource for public health in Canada.

Heat waves and excessive temperatures yearly contribute to many illnesses and deaths in Canadian cities; these high temperatures can induce heat cramps, respiratory difficulties, heat stroke, and even heat-related mortality (GoC, 2020; Chen et al., 2016). By increasing daytime temperatures and reducing nighttime cooling, the urban heat island effect is responsible for over 45 deaths in Canada annually (StatCan, 2024). Young children under the age of 5, older people over the age of 65, people with chronic illnesses, homeless people, and low-income, low canopy cover communities are particularly at risk when it comes to heat-related illnesses and mortality (Climate Atlas of Canada, n.d.; Whittingham et al., 2022).  

Urban trees can reduce the severity of these health hazards through air cooling and shade provision, and the City of Toronto became the first Canadian municipality to develop a policy specifically related to urban trees and heat. In collaboration with many city departments and NGOs such as Parks, Forestry and Recreation, Child Services, Tree Canada, and LEAF, Toronto Public Health formed an interdisciplinary team to develop the first Shade Policy in Canada (City of Toronto, 2007, 2010). This initiative, led by the Toronto Cancer Prevention Coalition between 2005-2015, is the first of its kind. It represents an important step towards preparing for a warmer climate with increased frequency and duration of extreme heat events. 

The Toronto Cancer Prevention Coalition’s shade policy initiative is a testament to the importance of shade in protecting against skin cancer. Integrating shaded areas, especially where the shade is created by trees, into urban parks, streets, schools, and facilities draws an important connection between urban forestry, urban planning, and public health (Sivarajah, Thomas & Smith, 2020). This policy officially recognized the value of shade trees in cities, especially large trees with dense canopies, in providing shade and lowering air temperatures, and created a policy framework to incorporate shade provision into planning, bylaws, and climate change and energy action plans (City of Toronto, 2010).