Land use carbon fluxes are major uncertainties in the global carbon cycle. This is because carbon stocks, and the extent of deforestation, degradation and biomass growth remain poorly resolved, particularly in the densely populated savannas which dominate the tropics. Here we quantify changes in aboveground woody carbon stocks from 2007-2010 in the world's largest savanna-the southern African woodlands. Degradation is widespread, affecting 17.0% of the wooded area, and is the source of 55% of biomass loss (-0.075 PgC yr-1). Deforestation losses are lower (-0.038 PgC yr-1), despite deforestation rates being 5× greater than existing estimates. Gross carbon losses are therefore 3-6x higher than previously thought. Biomass gains occurred in 48% of the region and totalled +0.12 PgC yr-1. Region-wide stocks are therefore stable at ~5.5 PgC. We show that land cover in African woodlands is highly dynamic with globally high rates of degradation and deforestation, but also extensive regrowth.