Scientific issues

Cities are environments that concentrate a large part of the challenges of mitigating and adapting to climate change. While more than 50% of the world’s population (>70% in Europe) now lives in urban areas, cities account for more than 70% of global greenhouse gas emissions. To reduce the human carbon footprint, to make cities more resilient to future climates and to improve air quality, fundamental changes are needed in the construction of cities, human behavior and the management of rural territories.

Two major health risks are exposure to intense heat and air pollution. These risks affect well-being and productivity at work, and are responsible for a large number of excess deaths. Due to the urban heat island effect (UHI), whereby the air temperature in the built environment is higher than that of the rural environment, cities increase and prolong the heat stress of its inhabitants, with direct implications on human thermal comfort, mortality and energy consumption. As high temperatures are increasingly common in Europe, city dwellers are expected to experience amplified consequences of climate change.

Air pollution is among the top three global risk factors for disease and death and has adverse effects on ecosystems and building structures. This major environmental concern, with a significant socio-economic impact, is particularly pronounced in cities. Indeed, cities are responsible for significant local emissions of pollutants (e.g. traffic, building heating, kitchen emissions, use of solvents) which themselves interact with biogenic emissions in the city and in rural areas on the outskirts of the city. To make progress on health effects of pollution, more precise quantification of human exposure to high levels of air pollution in urban environments is necessary.

Flooding of urban environments during extreme precipitation events is topic that can have significant effects on urban populations.

A major obstacle precisely concerns heterogeneous systems which combine a wide range of sizes (from meters to several kilometres), significant temporal variability and very complex patterns of human activity. This means that it is very difficult to make representative measurements or to simulate processes with numerical models.

However, in order both to improve forecasting and crisis management in the event of extreme phenomena, and to support decision-making concerning the sustainable development of urban environments in the context of adaptation to climate change, it is necessary to better understand and represent the exchange processes between the surface (buildings, streets, vegetation, human activities) and the atmosphere, and their impacts on the transport of heat and pollution. ​For example, we need to improve our understanding of the impact of urban heat on biogenic emissions, the impact of atmospheric circulation in the boundary layer on the mixing between urban air and forest emissions in the urban plume, the implications of buoyancy on the volume of the mixed boundary layer for the dispersion of pollution as well as the vertical transport of moisture which feeds clouds and hence precipitation.

The projects implemented from the summer of 2022 will rely on recent advances in numerical simulations and measurement technologies to study processes in the complex urban environment with an unprecedented level of detail.