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Tackling urban heat through data, paying attention to our citizens’ perception

Tackling the effects of rising urban heat goes beyond objective physical measurements. For sure, technical services are in need of blended indicators about the effects of local policies: under which conditions tree planting, fountains, urban design contribute to reduce island heats. The project has created such tools, but is also willing to take seriously into account the subjectivity, the perceived heat, the sensations of our citizens, for they contribute hugely to our collective resilience.

Innovation Summary

Innovation Overview

The urban area of Bordeaux is exposed to the consequences of global warming in a very visible way. The temperatures recorded in Bordeaux are often the highest in France, whatever the season. Summer heat peaks are reaching new heights. Public authorities have put in place initiatives to mitigate the effects of the heat, in particular a plan to plant one million trees. The project presented here aims at creating tools to get reliable indicators not only on the objective effects of public policies but also on the sensation felt, on the subjective thermal comfort of the inhabitants.

The model implemented is multifactorial. It makes it possible to establish a map of comfort today, but it also makes it possible to predict the potential effects (objective and subjective) of future policies for the creation of islands of urban freshness, thanks to trees, fountains and other solutions. More specifically, it must allow to:

  • Know the intensity and frequency of periods of overheating
  • Know the impact of existing and future green spaces
  • Feed a predictive simulation model of the thermal environment according to future developments, and in particular re-vegetation policies, taking into account the feelings and perceptions of our inhabitants.

We want to create this knowledge at three scales: the entire urban area, at the level of a neighborhood, at the level of the tree itself. To carry out the project, the prerequisite was a collection of data. This was made possible, for the objective data, by the installation of sensors in 8 cities of the urban area, in sites with various topographical characteristics (public squares, alleys, more or less dense areas, wooded areas ). To analyse the feelings of our inhabitants and their perception of the heat level, surveys on the ground, interviews of people were implemented. Analysis of social networks are planned to get a broader set of data.

The project established typologies of urban spaces to better measure temperature differences. It took as a reference a very mineral place, without trees, and is now comparing the temperatures with various other archetypes: with a mineral place with trees, with a street with or without trees, with a park, with an urban forest. More specific factors are introduced: color of the ground and facades (more or less dark), presence of grass, presence of small vegetation. The model also takes into account the maintenance of trees (it thus proves that the watering of trees strongly determines their ability to cool an area).

First results: temperature differences measured between the most exposed areas and the areas most cooled by vegetation are significant: up to 8 degrees Celsius. The qualitative analysis of the feelings of the inhabitants is just as important. It starts from the analysis of many statements collected in the urban space (e.g.: "There is no shade, the trees are very small and it is very hot there compared to Palmer Park which is just aside.” “As soon as we get too hot, we leave…”). This qualitative study is an originality of the project. It is important because it allows us to understand how our inhabitants will react in the long term in a future where summers will be warmer, and how to better create conditions of resilience. It leads to specific pragmatic short term recommendations and conclusions, of which the following are three examples.

  • Parks with trees (large canopy) seem to provide much more perceived freshness than spaces with short vegetation or small trees (limited canopy) even if all layers of vegetation are important
  • The coolness felt is limited to the perimeters of the developed areas.
  • Calm and tranquility are highly sought-after criteria in urban areas in addition to freshness, and are often mentally linked to it

The next stage of the project is to make it possible to carry out simulations and predictions before the urban work of planting trees in an area. For example, the simulation of the impact of the refreshing potential of future trees on a square in the town of Pessac, near Bordeaux.

Of course, the issue of heat in summer arises in many cities, and increasingly. We believe that the objective measurement of heat but also the measurement of the sensations of our inhabitants is of interest to all cities, so we wish to set up cooperation projects and the exchange of tools and good practices on this subject.

Innovation Description

What Makes Your Project Innovative?

The innovation of the project was made possible by the trust give by public decision-makers, and their desire not to launch a major tree planting project (one million trees over 5 years) without having, at the beginning and then throughout the project, precise and useful data. The goal is to maximize the physical impact of the trees, but also to create the social, psychological and simply human conditions so that each plantation provides maximum actual and perceived comfort and collective resilience in the face of urban heat.

This is the first dimension of innovation: innovation by the attention paid to the feelings and perceptions. But the project is also innovative in the data blend it has created, which paves the way for the use of artificial intelligence in the exploitation of the results in the future.

What is the current status of your innovation?

After 18 months of implementation of the project and first results, our goals for 2023 are to:

  • Expand the corpus of users' feedback by social networks posts analysis
  • Develop other use cases for diagnostic and simulation with the park and urbanism technicians on the field
  • Try a new protocol to collect feed backs from the users in situ, liked to their perceived sensation of temperatures in determined environments

The project is also entering in a dissemination phase, and is looking for ways to create experiments accross various cities in Europe, to share results, enrich the protocols, consolidate a collective vision of the best practices in this field.

Innovation Development

Collaborations & Partnerships

The project has required from early on the involvment of various departements inside Bordeaux Metropole :

  • The departement in charge of ecological transition
  • The various technical units on the field, in charge of urban and tree projects
  • The Digital Innovation Department, spearheading the project
  • The communication department, in close contact with the various municipalities where the project has been tested
  • A private partner, the Verdi consulting company

Users, Stakeholders & Beneficiaries

The main beneficiary group is our inhabitants. Having to deal with the consequences of long term warming of our urban area, they are now fully taken into account in the heat mitigation projects, not only from an objective standpoint, which is the inclination of our technical departments, but also in their comprehensive experience of our urban spaces. That's why the project is supported by our urban planners and council members, for they see in it an archetype of more user caring policies.

Innovation Reflections

Results, Outcomes & Impacts

The project has delivered the expected results:

  • Analysis of the first data gathered through sensors and users' surveys
  • Specification of diagnostic and planning tools
  • Modeling of a cartographic tool for urban planning
  • Modeling a large-scale survey tool
  • Sensor rotation (to gather information on different urban areas)
  • Compilation of the set of results and recommendations
  • Provision of data and data models.

The project has been able to prove (see presentation attached, slide 9) the strong variation in terms of heat depending on the urban and trees implementation. It has also proved the importance of perceived heat, by collecting users' feed backs in various areas.

Challenges and Failures

The main challenge has been to create a progressive paradigma twist among the community of technicians in charge of trees implementation. It has mostly consisted in creating a shared agreement on the difference between subjectivity and irrationality. The perception of the inhabitants is a key success in the use of the urban coolness island we pretend to create: bad image of a place, even if objectively cooler than the surroundings, means failure of the project from a user point of view. This collective enrichment of our vision has supposed to listen to everyone's initial grid of lecture, professional culture, and show, with proofs from the field, that taking into account the human factor is far from being a technical triviality, or a methodology impurity.

Conditions for Success

As mentioned in the challenges faced, the project couldn't have been carried out without a specific effort to take into account everyone's approach to a complex challenge, and giving room for both technical, material, financial, but also users and human centered approaches. The project has also benefited from the specific involvement of a catalyst project manager, who brought to a higher level her skills in terms of community management.


The council members of Bordeaux Metropole have supported the project from day one, and its scale of replication could now be the whole territory of the 28 cities of Bordeaux Metro area. The sustainability of the scale up is what is now taking most of the team's effort, to prioritize scenarios and areas where new experiments would mean replicable conclusions.

Frequent conversations with other European cities (from Belgium, Spain, Italy) have convinced us to create a joint initiative to design cooperation and best practices projects on this topic. By essence lightweight and replicable, it could easily result in a data set of comparable measurements of objective and subjective heat perception at a much broader scale.

Lessons Learned

The project has forced us to face questions that may seem usual to innovation promoters. They have allowed us to verify, in the context of the project, that innovation, indeed, requires to spend time to align the priorities, the values, the perception of the actors involved. We met also the positive need to regularly make sure that no discrepancy emerges along the way, allowing thereby the project to benefit from a vibrant collective energy.

Year: 2021
Level of Government: Local government


  • Evaluation - understanding whether the innovative initiative has delivered what was needed

Innovation provided by:

Date Published:

3 January 2023

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