Urban vegetation to adapt to climate change

Science topics June 2015 InnovationTransportEnergyCity

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  • Katia Chancibault and Fabrice Rodriguez
  • GERS Department/EE Laboratory

Locally, urbanisation leads to the creation of a heat island due to the use of anthropic materials which absorb and reflect solar radiation and the production of heat by heating systems and traffic. Urbanisation also has impacts on the water cycle as it uses impermeable materials which encourage flooding and interfere with groundwater recharge.

Climate change can accentuate these impacts by intensifying or increasing the frequency and/or the intensity of heatwaves and modifying rainfall patterns.


Vegetation helps regulate the water cycle and heat flowsIllustration Claude Joannis pour Ifsttar

Evapotranspiration, which includes both evaporation from the ground and transpiration from plants, contributes to water and heat flows. This physical phenomenon consumes energy in order to send a considerable proportion of precipitation back into the atmosphere in the form of water vapour. It is a process which cools down the air. Vegetation is therefore increasingly being seen as a way of mitigating the combined impacts of urbanisation and climate change. More precisely, it helps reduce the heat discomfort experienced by city dwellers and reduces energy consumption through a number of different processes: in addition to the « air conditioning » effect of evapotranspiration, the insulation of buildings is improved by vegetation on roofs and sometimes on frontages and shade from trees cools the street and the surrounding buildings, etc. Urban vegetation also improves the management of urban water by encouraging storage by the local seepage of rainwater (rain gardens, green roofs, ditches). Consequently, urban vegetation has become a feature of many projects, at the level of individual buildings, districts or urban areas, with somewhat empirical design rules or because of other benefits (biodiversity, landscape, etc.).

To enhance the sustainability of these projects, it is essential to improve our knowledge and develop tools that are able to evaluate the effectiveness of the different urban vegetation measures in terms of thermal comfort (both inside and outside buildings), energy consumption and water management.


Vegetation scenarios for sustainable urban projects

With this in mind, IFSTTAR’s Water and Environment Laboratory (Laboratoire Eau et Environnement -EE) took part in the ANR funded VegDUD project which was coordinated by the Urban Scientific and Technical Research Institute (IRSTV). The laboratory has developed hydrological and hydroclimatic models that have been used to evaluate a variety of vegetation scenarios for a district and a large area in the city of Nantes. These scenarios involve the installation of green roofs, ditches and trees in the streets. The results show the benefits of such vegetation for improving the thermal comfort of the urban population and urban water management.


Variation in the Urban Thermal Comfort Index (UTCI) according to the level of vegetation: on the right the level of vegetation is twice as high as on the left (25 June 2011 at 15UTC, in Nantes, between the Erdre and the Loire). Sources IFSTTAR