Urban heat islands and extreme summer heat waves
September 10, 2025
Written in collaboration with Marie-France Stendal
Autumn is just around the corner. But since memory has a tendency to fade, we feel it is worth revisiting today the many heat waves that marked the summer now coming to an end.
As climate change continues to intensify, these heat waves are expected to become longer, more frequent, and more severe in the years ahead. According to a Statistics Canada study published in July 2025 (Canadian Social Survey (CSS) – Quality of life and energy consumption behaviours), nearly 70% of Québec households now own an air conditioner or a heat pump. Unfortunately, while offering short-term relief, this solution only worsens the phenomenon of urban heat islands (UHI).
What is an urban heat island (UHI)?
An urban heat island is an area where summer temperatures are significantly higher than those in surrounding rural regions. Urban heat islands result from a combination of factors: the density of buildings and paved surfaces that absorb and retain heat, the lack of vegetation that would otherwise provide shade and natural cooling through evapotranspiration, heat emissions generated by human activity, and urban geometry that restricts air circulation and limits heat dissipation.
During heat wave nights in some central Québec neighbourhoods, it is no longer uncommon to record temperatures more than 12°C higher than those in nearby rural areas. During these extreme heat events—when the body struggles to recover—Health Canada considers the situation to pose a major risk to public health, given the observed increases in mortality rates.
The Institut national de santé publique du Québec has developed detailed mapping of urban heat islands to support professionals working in public health, land-use planning, environmental management, and urban forestry, as well as those involved in climate change adaptation. This mapping aims to help protect existing cool zones and guide future urban development strategies.
In the image, our Québec City head office is highlighted on the urban heat island map produced by the Institut national de santé publique du Québec.
How can we respond to extreme heat waves?
Building resilience
First and foremost, it is essential to build resilience and to ground our decisions in long-term thinking and collective well-being. For example, while 1.6 billion air conditioning units were in use worldwide in 2016, that number is expected to reach 5.6 billion by 2050 (The future of cooling, International Energy Agency). The environmental impacts of air conditioning are significant. Beyond increasing energy demand and greenhouse gas emissions associated with their operation, air conditioners simply transfer heat from indoor spaces to the outdoors, directly raising urban temperatures in the process. This self-reinforcing cycle of warming and exponentially growing cooling demand alone is believed to be responsible for an increase in global temperatures of approximately 0.5°C (Les grands reportages, Pas de clim pour le climat), when compared to the global objective set by the Paris Agreement, which aims to limit climate warming to below 1.5°C relative to pre-industrial levels.
To strengthen our resilience, could we accept indoor temperatures that do not drop below 25°C through air conditioning, with the goal of reducing energy demand and external heat rejection, while gradually increasing our physical adaptability to rising temperatures? For less vulnerable populations, could we consider limiting the use of air conditioning altogether and instead prioritizing lower-energy solutions such as fans combined with passive cooling strategies?
Deploying architectural strategies
Although electromechanical systems are becoming increasingly efficient, they still account for a large share of building energy consumption, along with the associated greenhouse gas emissions. To counter rising temperatures and the intensification of urban heat islands, it is critical to design buildings that achieve the highest possible performance before relying on mechanical systems.
Bioclimatic design approaches that consider the path of the sun to prevent overheating, as well as prevailing summer wind directions to encourage natural ventilation, can deliver substantial gains in occupant comfort during heat waves.
When combined with high-albedo roofing and envelope materials (albedo being a value between 0 and 100 that measures a surface’s ability to reflect solar radiation, where 0 corresponds to a surface that absorbs all radiation), enhanced insulation and envelope performance strategies also help slow the transfer of heat from outside to inside buildings. This delay can extend occupant comfort by several days during extreme heat events. Inspired by traditional stone houses, some studies even suggest adding thermal mass to building envelopes to further delay heat migration and preserve indoor coolness for longer periods.
Finally, many passive strategies rooted in ancestral knowledge can be leveraged: deep roof overhangs, exterior shutters instead of interior curtains that trap heat indoors, damp curtains placed in open windows during the evening—every strategy counts when it comes to preventing overheating.
Demineralizing and re-vegetating
As climate change leads to declining air quality and biodiversity loss, replacing impermeable surfaces with generous landscaped areas emerges as a particularly effective solution to counter the urban heat island effect. In addition to absorbing CO₂ from the atmosphere through photosynthesis, cooling the air through plant transpiration, and supporting biodiversity, vegetated spaces absorb far less solar energy than mineral surfaces with lower albedo values.
For example, a study conducted in Lyon measured perceived temperature reductions of 0.5°C on grassed surfaces, 3°C beneath large trees and shrubs, 5°C at the surface of a water basin, and up to 8°C in the shade of trees adjacent to the basin (Les grands reportages, Pas de clim pour le climat). At night, while mineral surfaces release the heat accumulated during the day, cooler temperatures can also be observed on vegetated surfaces.
Increasing vegetation and urban tree canopy coverage must therefore be central to our efforts to prepare cities for the mega heat waves to come. Every opportunity should be seized to provide shade and cooling through vegetation. This includes reducing building footprints, prioritizing permeable surfaces, protecting mineral surfaces and sun-exposed façades through tree planting, creating parks and shaded respite areas, replacing parking spaces with landscaped zones, adding climbing plants on the east and west façades of buildings, and integrating these measures with the inclusion of water features.
Aware of the cooling potential offered by urban greening, an increasing number of cities are moving in this direction by adopting policies and planning guidelines. In Québec City, for example, the municipality aimed to reach a 35% canopy cover across its territory this year (City of Québec, Vision de l’arbre 2015–2025). To achieve this goal, the city implemented various strategies, including the protection of existing natural environments, the addition of plantings in large-scale projects, the creation of ecological corridors, and support for sustainable practices such as urban agriculture.
STGM is proud to have contributed to the funding and signage of the Croque ton quartier edible forest in Beauport, planted in 2022.
The integration of green roofs also helps protect buildings from solar gains by significantly reducing surface temperatures and heat radiation toward the interior. In addition to the intensive and extensive systems developed over recent decades, a new technology is emerging in Europe’s temperate climates: a vegetated moss roofing system installed on a draining textile membrane and secured to steel roofs using magnets. This ancient plant—found in tropical forests, deserts, and northern regions alike—can absorb up to 16 times its own weight in moisture, allowing it to withstand the drought conditions associated with extreme heat waves.
Ultimately, at their core, major climate challenges and the intensity of urban heat islands are largely driven by greenhouse gas emissions, which are responsible for global warming. We must therefore, collectively, begin by reducing our emissions in order to make our urban environments more viable.
In the meantime, it is important to remember that urban heat islands force buildings to rely more heavily on air conditioning and ventilation to maintain comfort, which in turn increases energy demand and, consequently, emissions.