Vegetation change plays a critical role in the Earth’s carbon (C) budget and its associated radiative forcing in response to anthropogenic and natural climate change^{1, 2, 3, 4}. Existing global estimates of aboveground biomass carbon (ABC) based on field survey data provide brief snapshots that are mainly limited to forest ecosystems^{5, 6, 7, 8}. Here we use an entirely new remote sensing approach to derive global ABC estimates for both forest and non-forest biomes during the past two decades from satellite passive microwave observations. We estimate a global average ABC of 362 PgC over the period 1998–2002, of which 65% is in forests and 17% in savannahs. Over the period 1993–2012, an estimated −0.07 PgC yr⁻¹ ABC was lost globally, mostly resulting from the loss of tropical forests (−0.26 PgC yr⁻¹) and net gains in mixed forests over boreal and temperate regions (+0.13 PgC yr⁻¹) and tropical savannahs and shrublands (+0.05 PgC yr⁻¹). Interannual ABC patterns are greatly influenced by the strong response of water-limited ecosystems to rainfall variability, particularly savannahs. From 2003 onwards, forest in Russia and China expanded and tropical deforestation declined. Increased ABC associated with wetter conditions in the savannahs of northern Australia and southern Africa reversed global ABC loss, leading to an overall gain, consistent with trends in the global carbon sink reported in recent studies^1, 3, 9.