MacDougall, ASVanzant, BSulik, JBagchi, SNaidu, DMuraina, TOSeabloom, EWBorer, ETWilfahrt, PSlette, IHierro, JLPearson, DEAbedi, MAkasaka, MAlberti, JAleksanyan, AAmisu, AAAnderson, TMArnillas, CAAyer, MBakker, JDBasant, SBasto, SBiederman, LBloodworth, KJBoscutti, FBoughton, EHBruschetti, CMBuckley, HLBuckley, YMBugalho, MNCaldeira, MCCampetella, GCannone, NCarbognani, MCarbutt, CCarniello, MACervellini, MChaudhary, TChen, QClark, ATCousins, SDalle Fratte, MDay, NJDeák, BDietrich, JDixon, AEisenhauer, NElgersma, KJEren, OEskelinen, AEstrada, CFay, PAFayvush, GFlynn, KCGarcía Meza, DGargano, DGherardi, LGirkin, NTGonzález, LGraff, PHagenberg, LWCHalbritter, AHHavrilchak, NAHerdoiza, NHersch-Green, EHopping, KJentsch, AJimoh, SOKerby, JKirkman, KKnops, JMHKoerner, SEKoltz, AKomatsu, KJKoura, BIKruse, SLaanisto, LLannes, LSLi, WLiang, MLkhagva, ALópez-Olmedo, LLorenzo, PLortie, CJLoydi, ALuo, WMacek, PMalfasi, FMariotte, PMartina, JPMartínez-Blancas, AMartinson, HMartorell, CMeave, JAMedina-Villar, SMganga, KZMonsimet, JNerlekar, ANNiu, SOhlert, TOliveras Menor, IOñatibia, GROrtega, YKOsborne, BPalpurina, SPascual, JPennings, SCPérez-García, EPeri, PLPetit Bon, MPetraglia, APijcke, FProber, SMQuiroga, RERamirez, JIReed, SRosado, BHPRoscher, CRowley, DWSereda, ISmall, DMSmith, NGSong, YStevens, CSuarez Jimenez, LETe Beest, MTedder, MTerry, RSThornton, KSTian, DTitcomb, GValkó, O'Ciska' Veen, GFVirtanen, RWelti, EARWheeler, GRWolf, AAWolff, PYoung, ALYoung, HSZeglin, LHZhu, KZong, SSiewert, MB2026-04-172026-04-172026-01-27Nature Ecology & Evolution, ISSN: 2397-334X (Print); 2397-334X (Online), Nature Research, 10(2), 246-257. doi: 10.1038/s41559-025-02955-62397-334X2397-334Xhttp://hdl.handle.net/10292/20943Land cover data are commonly used to model the terrestrial carbon (C) sink, yet these data have wide margins of error that significantly 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 misidentification arose from inconsistent definitions on grassland identity and classification flaws 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 findings underscore the challenges of biome mapping for ecosystem accounting and policy, when lacking field-validated remotely sensed data.Open Access. This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.41 Environmental Sciences4104 Environmental Management15 Life on Land3103 Ecology3104 Evolutionary biology4104 Environmental managementGrasslandCarbon SequestrationEcosystemJournal ArticleOpenAccess10.1038/s41559-025-02955-6