Climate change and biodiversity loss are severe and intertwined global threats. Land-based efforts to address both require an understanding of the spatial relationships between carbon storage and biodiversity. Here, we present a systematic review and meta-analysis of the strength of these spatial relationships across the literature. We synthesize the estimated spatial correlations and infer how different factors (spatial scale, metrics, biome, human pressure) impact these strengths using linear mixed-effect models. Our results show that spatial scale is a significant factor, and the combination of metrics used to express carbon storage and biodiversity plays a more important role. While relationships are moderately positive across all conditions, the strength of the relationships decreases significantly from global to local scales. We find large variations in the strength for different metrics, across different biomes, and in the presence or absence of human pressure. We find a stronger relationship in natural rather than human-dominated landscapes for temperate forests, grasslands and deserts, but the opposite for tropical and subtropical forests. Ecosystem-level biodiversity proxies (habitat quality) show strong relationships to the total carbon pool, while taxonomic metrics (species richness) show a weaker relationship. The largest negative relationship is between total carbon and flora & fauna species richness. Our results suggest different synergies for different dimensions of carbon storage and biodiversity and shed light on where further effort is needed.