As global warming increases the frequency and intensity of heat waves, knowing where heat islands are in our cities — and who lives in them — becomes a crucial public health issue. We used satellite imagery to pinpoint the areas with the hottest land surface temperatures across 17 Canadian metropolitan areas. Then, we overlayed the temperatures with the Canadian census data, showing that immigrants and people with low income are most likely to live in the hottest urban areas. As one expert told us, “this is evidence that there is injustice in who will be more vulnerable to climate change.”
Altogether, this series garnered more than 700,000 page views. In ‘Here’s who lives in your city’s worst heat islands’, almost every reader typed in their postal code to get information about their neighbourhood. They also interacted with the chart arrows three times on average. User engagement was well above average for all pieces in the series. On top of the stories, we also published all of our analysis data and code, allowing anyone to review our work and reuse it. Shiab was contacted by the University of Calgary (Canada), McMaster University (Canada), Laval University (Canada), UQAM (Canada), the University of Waterloo (Canada), the Bureau of Investigative Journalism (UK), and Data journalisme Paris (France) to talk about the project.
First, we had to answer the question: Where are Canada’s urban heat islands and who lives in them?
To do this, senior data producer Naël Shiab and data scientist Isabelle Bouchard used satellite imagery to estimate land surface temperature in Canada’s biggest metropolitan areas. Land surface temperatures in metropolitan areas were estimated using Landsat 8 imagery. Three cloud-free images were selected for each metropolitan area from the summer months. The land surface temperature of each pixel was then calculated and averaged.
The project was looking at urban heat. So anything that didn’t fit that definition needed to be excluded. Natural Resources Canada’s 2015 land cover data was used to crop out lakes, rivers, fields and other non-urban areas.
Various factors (highly reflective surface material, residual cloud, measurement errors, etc.) can create very high or very low land surface temperature. These outliers were also excluded for each metropolitan area.
The final stage involved overlaying Canadian census information. Because the 2021 census was not yet out, the analysis was based on 2016 data. Dissemination areas were used to ensure the smallest geographic unit of aggregation for variables like income and immigration status publicly available.
These steps were done with the R programming language.
For the dataviz, we used ThreeJS and custom shaders to create a high-performance visualization with more than 28,000 animated dots.
Context about the project:
Reporter Jaela Bernstien also worked with the HealthyDesign.City program out of the University of Toronto’s Dalla Lana School of Public Health to access then-unpublished maps that used census data and information about the way cities are built to illustrate where the most vulnerable populations live.
Using this data allowed us to expand the project to look at access to shade and pinpoint neighbourhoods that would best tell the stories of those living in extreme heat.
The series was published in July 2022 in English and French. It combined data-driven storytelling, user personalization, illustrative and photojournalism techniques to engage audiences on topics from the national implications of urban heat to stories of those in sweltering apartments without AC.
The data also enhanced our ability to engage readers. In ‘Here’s who lives in your city’s worst heat islands,’ users can see how their neighbourhood compares to overall trends, before jumping into city and then national findings.
In ‘Sweltering cities’ and ‘Cooling canopy,’ mapping from HeathyDesign.City allowed us to explore areas of Vancouver and Montreal and visualize the difference between neighbourhood tree cover and, with it, access to cooling shade.
What can other journalists learn from this project?
CBC/Radio-Canada wanted to show Canadians the growing dangers and existing inequalities when it comes to urban heat. We wanted to answer the question: Where are Canada’s urban heat islands and who lives in them?
By using satellite imagery, any journalist on the planet can answer the same questions for their own country. We published our code and methodology, and we hope it will be useful to other newsrooms worldwide.