The Urban ReLeaf Toolkit: Citizen-powered data ecosystems for inclusive and green urban transitions

Across Europe and beyond, cities are facing growing environmental challenges. Heatwaves are more frequent and intense. Air pollution continues to affect people’s health and wellbeing. Access to quality greenspaces remains uneven, often reflecting wider social inequalities. At the same time, local governments are being asked to make faster and more informed decisions, often with limited resources and incomplete data. Yet some of the most valuable knowledge about these challenges already exists within our communities. Residents experience heat stress in their neighbourhoods, notice changes in local air quality, and understand how parks, trees, and public spaces shape daily life. When this local knowledge is combined with scientific methods and public decision-making, it can become a powerful driver of positive change. This vision sits at the heart of Urban ReLeaf. The project was built on a simple but ambitious idea: that cities become stronger when public authorities, researchers, civil society organisations, and communities work together to better understand and improve the urban environment. Through citizen science, people are not only contributors of data, but active partners in identifying problems, generating evidence, and shaping solutions. The experiences gathered through Urban ReLeaf show that citizen science can do more than produce information. It can strengthen trust, create new partnerships, and help ensure that policies reflect the realities of the people they are intended to serve. It can also help cities reach groups whose voices are too often missing from planning and environmental decision-making. This commitment to inclusivity, evidence-based innovation, and transformative impact runs throughout this toolkit. This toolkit brings together lessons from six European cities, real-world experiences, and guidance for local authorities that want to explore or expand the use of citizen science. Whether your city is taking its first steps or building on existing initiatives, this toolkit offers ideas, methods and inspiration that can help turn local knowledge into meaningful action. This toolkit is not the final destination. It is part of a wider journey. As you work through the chapters ahead, we encourage you to consult our repository of tools, ranging from mobile apps to participatory methods, to help you get started. Every city holds the roots of change. The role of this toolkit – and yours – is simple: to help them grow!