Cities house most of the global population and generate the majority of greenhouse gas emissions. Achieving a 1.5 °C pathway depends on how city data is created, governed, and trusted. Building on the updated Global Research and Action Agenda (GRAA) and insights from the 2021 and 2024 Innovate4Cities conferences, this chapter shows how digitalization tools such as AI, IoT, and digital twins can shift practice from compliance reporting to opportunity-focused, equity-centered action. We trace the evolution of city knowledge systems, identify technical, institutional, and ethical bottlenecks through the eight GRAA towers, and illustrate emerging solutions with three cases (ClimateView, CityCatalyst in Brazil’s CHAMP pilot, and DUCTExplorer). A systems perspective links open standards, participatory data, and ethics-by-design to finance-ready portfolios. We conclude with a 12-step action plan to turn fragmented data into a trustworthy foundation for faster, fairer urban climate action