Bioenergy with carbon capture and storage (BECCS) can act as a negative emission technology and is considered crucial in many climate change mitigation pathways that limit global warming to 1.5–2 °C; however, the negative emission potential of BECCS has not been rigorously assessed. Here we perform a global spatially explicit analysis of life-cycle GHG emissions for lignocellulosic crop-based BECCS. We show that negative emissions greatly depend on biomass cultivation location, treatment of original vegetation, the final energy carrier produced and the evaluation period considered. We find a global potential of 28 EJ per year for electricity with negative emissions, sequestering 2.5 GtCO2 per year when accounting emissions over 30 years, which increases to 220 EJ per year and 40 GtCO2 per year over 80 years. We show that BECCS sequestration projected in IPCC SR1.5 °C pathways can be approached biophysically; however, considering its potentially very large land requirements, we suggest substantially limited and earlier deployment.