Forests are a major terrestrial carbon sink, but anthropogenic changes in land use and climate have greatly reduced the extent of this system. Remote sensing estimates for quantifying carbon losses from global forests are subject to significant uncertainty, and we lack a comprehensive ground-based assessment to compare these estimates with. Here, we combine several ground-based and satellite-based approaches to assess the extent of the global carbon potential of forests outside agricultural and urban areas. Despite regional variations, the predictions show remarkable consistency at the global scale, with only a 12% difference between ground-based and satellite estimates. Currently, global forest carbon storage is well below natural potential, with an overall deficit of 226 Gt (model range = 151–363 Gt) in areas with a low human footprint. Most (61 %, 139 Gt C) of this potential is located in areas with existing forests, where ecosystem protection can allow forests to recover to maturity. (Lidong Mo, Constantin M. Zohner, Thomas W. Crowther)