Traffic Heat and Urban Cooling: How Cars Are Heating Toulouse and Manchester

Original publisher: Nature Communications. A modelling study shows that traffic heat can measurably raise average urban temperatures, demonstrated in Toulouse and Manchester, and discusses how reducing petrol and diesel traffic could mitigate heat by future city planning and vehicle choices.

• Traffic heat contributes to urban heat islands, with Toulouse ~0.4°C and Manchester ~0.25°C average annual warming.
• Conventional petrol and diesel vehicles emit more waste heat than electric vehicles, making rush hours and vehicle mix important for local heat exposure.
• Traffic-driven warming varies by time of day and season, with morning build-up in Toulouse and overnight warming in Manchester, and stronger effects in winter in both cities.
• Policy implications point to electrification, road design, congestion reduction, and urban greening as parts of a broader cooling strategy during heatwaves.

Overview: Traffic heat as a component of urban heat islands

The article summarises a recent study that adds a dedicated traffic-heat module to a widely used climate model to quantify how motorised traffic contributes to urban warming. It emphasizes that in many cities, traffic is a non-trivial source of waste heat, alongside buildings and other urban surfaces. The study focuses on two major European cities, Toulouse and Manchester, to illustrate how heat from traffic varies with urban form, traffic patterns, vehicle mix, and weather conditions. The authors note that while the numbers may seem small in annual averages, even modest increases in air temperature can intensify heat-discomfort during heatwaves, raising health risks and cooling demand in cities. They also point to historical projections showing that urban heatwaves are likely to become more intense, longer, and more frequent by mid-century, and suggest that curbing petrol and diesel traffic could partly mitigate these increases.

Methodology: adding traffic heat to climate projections

The authors describe adapting the Community Earth System Model, a widely used open-source framework for simulating land-atmosphere-climate interactions, by integrating a new traffic-heat module. This module estimates heat generation from traffic volume, vehicle type, road characteristics, and weather. The model also accounts for diurnal patterns and local weather, meaning the heating effect can differ by time of day and season and across different city districts. The approach yields spatially resolved estimates of heat emissions from traffic that interact with atmospheric flows and urban morphology to modulate local temperatures.

Key findings: Toulouse and Manchester compared

The study finds that traffic heat increases average annual air temperatures by about 0.4°C in Toulouse and around 0.25°C in Manchester. Although these figures appear modest, they are significant in urban climate terms, especially during heatwaves when even small temperature gains can worsen thermal comfort and health risks and raise cooling demand. The results also reveal important temporal dynamics: in Toulouse, morning traffic heat builds during the day and persists into the night, whereas in Manchester, evening rush-hour activity drives stronger overnight warming with peak air temperature increases around 3 a.m. on average. In both cities, the traffic-induced warming is greater in winter than in summer, with Toulouse showing about 0.5°C warming in winter versus 0.3°C in summer, and Manchester about 0.35°C in winter versus 0.16°C in summer.

Broader implications: heatwaves, health, and planning

The article connects these city-scale heat contributions to policy and planning. It notes that awareness of urban heat risk is rising, but traffic’s role is often underappreciated in climate adaptation and transport planning. The authors propose that reducing urban petrol and diesel traffic could partially mitigate heatwave-related health risks and energy demand for cooling, complementing other strategies such as urban trees, parks, and cool-roof designs. They also highlight questions for future research, including how electrification of vehicles would quantitatively reduce heat, how changes in road design and congestion patterns would affect local heat exposure, and what mix of transport strategies could most effectively limit heatwave impacts. The broader aim is to move toward more realistic simulations of future cities and inform decisions that integrate mobility, energy use, and climate resilience.

Conclusion: traffic as a lever for cooler cities

Ultimately, the study frames traffic not merely as a source of air pollution and carbon emissions but as a tangible heat source that can be managed through policy and planning. The authors argue that a more complete representation of traffic heat in urban climate models could improve projections of heatwave risks and help evaluate the cooling potential of various transport and urban-design interventions. The work underscores the need for integrated approaches that address traffic emissions, road design, and urban greening in tandem to create cooler, healthier, and more resilient cities.