The Earth's climate, marked by long-term shifts and punctuated events, shapes terrestrial and marine ecosystems. Despite the present interglacial period's warmth and stability compared to preceding glaciations, the Holocene has witnessed significant cooling events with worldwide consequences. Leveraging marine records from the Nordic Seas, we provide the first detailed account of a cooling event centered around 6.8 ka BP. Utilizing paleoceanographic proxies and advanced modelling, we unveil a distinct subsurface water cooling, associated with a stepwise increase in sea-ice cover in the eastern Fram Strait. Our findings emphasize the role of Greenland Sea deep convection onset and the subsequent westward shift in Atlantic Water flow, enabling sea-ice advection from the Barents Sea. The heightened sea-ice cover weakens Atlantic Water advection, perturbing overturning circulation in the eastern Nordic Seas. These perturbations propagate worldwide, affecting North Atlantic deep-water circulation, inducing widespread hemispheric cooling, shifting the Intertropical Convergence Zone southward, and weakening the East Asian monsoon. Incorporating rigorous modelling supports and augments proxy-based paleoreconstructions, underscoring sea-ice dynamics and ocean circulation's critical influence. This study highlights the potential for localized cooling events within ostensibly stable climatic intervals, underscoring the need to comprehend their mechanisms for precise climate predictions and informed policymaking toward a sustainable future.