Land cover data are commonly used to model the terrestrial carbon (C) sink, yet these data have wide margins of error that signi!cantly alter estimates of global C storage. Here we demonstrate this data vulnerability in grasslands, which are critical to C cycling but whose estimated distribution has varied by >50"million"km2 (3.5–42% of the Earth’s terrestrial surface). Comparing multiple high-resolution land cover products with expertly annotated grassland data from six continents, we show sources of mapping error and discuss C implications based on 2023 United Nations (UN) FAO estimates. Past misidenti!cation arose from inconsistent de!nitions on grassland identity and classi!cation #aws especially relating to woody plant cover. Correcting these errors adjusted grassland coverage to 22.8% of the terrestrial land base (30.1"million"km2), elevating UN projections of soil C stocks to 155.02"Pg (0–30"cm depth). These !ndings underscore the challenges of biome mapping for ecosystem accounting and policy, when lacking !eld-validated remotely sensed data.