The distribution of dryland trees and their density, cover, size, mass and carbon content are not well known at sub-continental to continental scales 1-14. This information is important for ecological protection, carbon accounting, climate mitigation and restoration efforts of dryland ecosystems 15-18. We assessed more than 9.9 billion trees derived from more than 300,000 satellite images, covering semi-arid sub-Saharan Africa north of the Equator. We attributed wood, foliage and root carbon to every tree in the 0-1,000 mm year -1 rainfall zone by coupling field data 19, machine learning 20-22, satellite data and high-performance computing. Average carbon stocks of individual trees ranged from 0.54 Mg C ha -1 and 63 kg C tree -1 in the arid zone to 3.7 Mg C ha -1 and 98 kg tree -1 in the sub-humid zone. Overall, we estimated the total carbon for our study area to be 0.84 (±19.8%) Pg C. Comparisons with 14 previous TRENDY numerical simulation studies 23 for our area found that the density and carbon stocks of scattered trees have been underestimated by three models and overestimated by 11 models, respectively. This benchmarking can help understand the carbon cycle and address concerns about land degradation 24-29. We make available a linked database of wood mass, foliage mass, root mass and carbon stock of each tree for scientists, policymakers, dryland-restoration practitioners and farmers, who can use it to estimate farmland tree carbon stocks from tablets or laptops.