The Early Warning System of the City of Medellín and the Aburrá Valley − SIATA, Colombia: a + 20-year prolific journey

• An operational EWS was successfully established in Colombia. • Over 4 million inhabitants can benefit from the information provided by the EWS. • SIATA combines technical, scientific and social expertise. • SIATA is a success story, a benchmark for innovation, and an example for many other cities. There is an urgent need to implement adaptation measures for weather, climate, and compound extreme events. Here, we present a multi-, inter-, and trans -disciplinary case study on the establishment of a local, operational, integrated, and people-centered Early Warning System − EWS, which aims to increase readiness in at-risk communities in the Aburrá Valley, Colombia. The goal has been to provide information and knowledge to slightly over 4 million inhabitants, who account for about 8.4% of the current total Colombian population. Over the past two decades, the project developed new scientific knowledge, accomplished significant technological advances in its real-time monitoring networks, and managed to enhance surveillance and integrated communication systems. Through a collaborative effort with risk managers and via sustained successful social processes, the project also established highly committed community-based EWSs distributed throughout the metropolitan area. The + 20-year prolific journey results from three-tiered factors of success, namely the collective and participatory approach, the close dialogue and interaction between the academia and the local government, and good governance and decision-making processes. Despite historical challenges, the EWS is being transformed once again into a real-time Risk Management System for Timely and Effective Decision Making. The project has been building the adaptive capacity the society needs to cope better with normal climate variability, ongoing global climate change, future air quality critical episodes, and other natural hazards, in a rapidly evolving and transforming region. The project is a success story, a benchmark for innovation, and an example for many other cities in developing nations.