The Harmonizing Global Biodiversity Modelling (HarmBio) initiative, designated as COST Action CA15206, addresses the urgent need for coordinated efforts in biodiversity modeling. With global biodiversity declining rapidly due to human activities, effective policy and adaptive management strategies require accurate projections of future changes. Unlike the well-organized climate modeling community, biodiversity modeling has been fragmented and lacks standardized metrics and datasets for model calibration and validation. HarmBio aims to harmonize existing models and datasets across terrestrial, freshwater, and marine ecosystems to enhance the reliability of biodiversity change projections. This cross-disciplinary initiative seeks to develop transparent and scientifically robust models through processes such as validation, calibration, and intercomparison. By fostering collaboration among researchers and standardizing methodologies, HarmBio strives to provide policymakers with state-of-the-art projections, enabling informed environmental decision-making under various policy scenarios.