The United Nations Framework Convention on Climate Change (UNFCCC 1992) calls for stabilization of atmospheric greenhouse gas (GHG) concentrations at a level that would prevent dangerous anthropogenic  interference with the climate system. We use three global energy system models to investigate the technological and economic attainability of meeting CO2 concentration targets below current levels. Our  scenario studies reveal that while energy portfolios from a broad range of energy technologies are neded to attain low concentrations, negative emission technologies-e.g., biomass energy with carbon capture and  storage (BECCS)-significantly enhances the possibility to meet low concentration targets (at around 30 ppm CO2).