Can Anything be Done to Make Cities Safer During Heat Waves?

Three actions that landscape architects can take to reduce the danger of overheated cities

Google Earth image of a typical area in Paris, France

From August 4 to 18, 2003 France sweated through their hottest weather in more than 500 years.  Almost 15,000 people died as a result of the heat wave, many of them due to dehydration, hyperthermia, and heat stroke (Poumadère et al 2005). Areas of Europe that have long been considered to have a temperate climate are now threatened by an increasing frequency and intensity of heat waves (Meehl and Tebaldi 2004) and every segment of the population is at risk (Fouillet et al 2006).

All of Canada and much of northern USA are also currently considered temperate climates and in general we don’t worry too much about heat waves, but that seems to be changing.  Not only are heat waves becoming more common, they’re also increasing in severity (e.g. Meehl and Tebaldi 2004).

But do all parts of Canada experience the same number and intensity of heat waves?  Some regions of Canada are more affected by heat waves than others.  For example the Prairies, Southern Ontario, and the St. Lawrence River Valley typically have the most frequent and intense heat waves, and the effects are “profound and far-reaching” (Smoyer-Tomic et al 2003). 

Not all areas within a region are equally affected by heat waves.  Urban heat islands intensify heat waves in cities compared to the surrounding countryside increasing the health risks for urban dwellers. Considering that two thirds of Canadians live in urban areas it’s important to understand the characteristics of urban landscapes that have the biggest impact on human health during heat waves. 

One of my MLA advisees, Drew Graham, explored the relationship between physical characteristics of Toronto neighbourhoods and the incidence of heat-related morbidity during extreme heat events (Graham 2012).  If you attended the 2014 CSLA Congress in Ottawa you will have heard Drew talk about his research.  But in case you missed it I’ll review his results here.

He looked at four extreme heat events from three years for the city of Toronto.  He acquired emergency medical response (EMR) call data for the periods leading up to, during, and after each of the heat waves.  Heat-related ambulance calls were 12.3% higher during the heat events than in the preceding or the following week. This result was similar to other studies that have found that there are more medical emergencies during heat waves and confirms that heat waves are dangerous to human health. 

He then compared the number of EMR calls per Census Tract.  He found something potentially more interesting – something that will provide terrific evidence for landscape architects who are working in urban areas.

The number of heat-related ambulance calls was negatively correlated to canopy cover. That means, the more canopy cover, in general, the fewer emergency medical response calls during heat waves.  Have a look at this graph:


Toronto Census Tracts with less than 10% canopy cover (on the left-hand side of the graph) had approximately twice as many heat-related calls during heat waves as those with more tree cover, and nearly four times as many heat-related calls as Census Tracts with a high canopy cover (>70%).  These results have important implications for human health during heat events, particularly in the context of global climate change and urban heat island intensification, both of which are trending toward hotter urban environments in future.

Action #1 – Take every opportunity to increase the canopy cover in urban areas, particularly areas that currently have few trees.

A Google Earth image of an area in Toronto, Canada.

Drew took his study a step further in an attempt to quantify the human health value of increased canopy cover.  He selected two Census Tracts that were in the lowest canopy cover category and estimated the effect of increasing the canopy cover to 12% and 25% respectively.  The simulations indicated that the increased canopy cover would reduce the number of heat-related EMS call frequencies by 40 to 50% as compared to existing conditions.

He then looked at the micro-scale and investigated whether it mattered where the trees were planted within an area.  He used a human energy budget model called COMFA (Brown and Gillespie 1986) to estimate the effects that various landscape treatments had on the heat input to individual people.  The simulations suggested two actions that had beneficial effect in cooling people during heat waves.

Action #2 - Plant deciduous trees to the south and west of areas where people are likely to be.

Providing shade for people in outdoor areas reduces the risk of them becoming overheated from a large input of solar radiation.


Action #3 - Plant deciduous trees to the south and west of buildings, parking lots, and streets.

Unshaded hard dry surfaces heat up when they absorb solar radiation.  Hot surfaces emit large amounts of terrestrial radiation that will be absorbed by people in the landscape, heating them up.  Reducing the solar radiation falling on hard surfaces will keep them cooler so they will emit less terrestrial radiation onto people. 

These actions might already be quite familiar to you, but the results from Drew’s study provide strong evidence that should convince even the most skeptical client. There are many ways to make cities safer during heat waves.  These three actions are a good start.


Brown, R.D. and T.J. Gillespie. 1986. Estimating outdoor thermal comfort using a cylindrical radiation thermometer and an energy budget model. International Journal of Biometeorology. 30:43-52.

Fouillet, A., G. Rey, F. Laurent, G. Pavillon, S. Bellec, C. Guihenneuc-Jouyaux, J. Clavel, E. Jougla, Denis Hémon. 2006. Excess mortality related to the August 2003 heat wave in France.  International Archives of Occupational and Environmental Health. October 2006, Volume 80, Issue 1, pp 16-24

Graham, Drew, A. 2012. Census Tract-Level Outdoor Human Thermal Comfort Modelling and Heat-Related Morbidity Analysis During Extreme Heat Events in Toronto: The Impact of Design Modifications to the Urban Landscape.  Master of Landscape Architecture thesis, University of Guelph, Canada. (You can download a PDF of Drew’s thesis from the Atrium of the University of Guelph Library.)

Poumadère, M., Mays, C., Le Mer, S. and Blong, R. 2005. The 2003 Heat Wave in France: Dangerous Climate Change Here and Now. Risk Analysis, 25: 1483–1494. doi: 10.1111/j.1539-6924.2005.00694.x   

Meehl, G.A., and C. Tebaldi. 2004.  More Intense, More Frequent, and Longer Lasting Heat Waves in the 21st Century. Science 13 August 2004:Vol. 305 no. 5686 pp. 994-997. DOI: 10.1126/science.1098704

Smoyer-Tomic K.E., R. Kuhn, A. Hudson. 2003. Heat Wave Hazards: An Overview of Heat Wave Impacts in Canada. Natural Hazards. March 2003, Volume 28, Issue 2-3, pp 465-486