European cities face escalating environmental pressures, including air pollution, rising heat stress, and uneven access to safe and high-quality green spaces. These pressures affect health and well-being and can deepen inequalities, as some neighbourhoods have fewer trees and less cooling from nature. Many cities want to expand nature-based solutions, but they often lack fine-grained, locally trusted evidence on where action is most needed and how people experience heat and green space in daily life.
Urban ReLeaf uses citizen science to strengthen inclusive urban greening. It brings residents and public authorities together to improve the evidence base for decision-making. The project combines citizen observations with low-cost sensing and Earth Observation to produce information that cities can use for planning, monitoring and policy. Participation is designed to reach diverse groups and neighbourhoods, including vulnerable or marginalised communities.
Urban ReLeaf works with six pilot cities that test policy-oriented campaigns. These include participatory tree registries, thermal comfort mapping that identifies hot spots and comfort zones, and community air quality monitoring in traffic-affected areas. The pilots also link sensor data with perceptions of heat and park quality, and explore integration into municipal systems so results can support day-to-day workflows.
The project starts from city needs and co-designs campaigns with local stakeholders. It runs repeated cycles and delivers quality-assured, reusable datasets and tools that can support decisions on tree management, greenspace improvements, heat risk reduction and air quality actions. Over time, this approach can strengthen partnerships between residents and public authorities and enable more equitable, data-driven urban planning.
Social sciences and humanities are integrated through participatory methods that capture perceptions, lived experience and barriers to participation, supporting trust and long-term uptake in governance.
