This global experimental study investigates how the comprehensibility of IPCC climate graphs can be improved. A total of 690 climate change decision-makers from research, governance, civil society, and the public sector in over 50 countries participated in two studies testing four theoretically promising principles from cognitive science for redesigning IPCC graphs. The study experimentally assessed how graph redesign affects both graph comprehension and participants’ awareness of their understanding. The principles tested included: (1) matching spatial and conventional features, (2) representing a single main message per graph, (3) guiding viewers to make correct inferences, and (4) aligning visual information with expectations. Findings indicate that matching spatial and conventional features and focusing on a single main message per graph significantly improved comprehension. Additionally, emphasizing a central message increased participants’ awareness of their understanding. No clear effects were observed for guiding inferences or matching visual information to expectations. These results provide an unprecedented evidence base for designing comprehensible climate graphs and offer concrete recommendations for future IPCC reports. By improving climate science communication, the findings support enhanced evidence-based decision-making in policy, helping decision-makers worldwide interpret and act on critical climate data.