Until now permafrost carbon feedback modeling has focused on gradual thaw of near-surface permafrost in terrestrial environments, which leads to enhanced carbon dioxide (CO2) and methane (CH4) emissions that accelerate global climate warming. The state-of-the-art land models do not simulate emissions from deeper permafrost thaw beneath thermokarst lakes or other abrupt-thaw processes, and so have not quantified the impact of abrupt thaw on the permafrost carbon feedback. We reanalyzed output from the Community Land Model (CLM4.5BGC), to quantify carbon emissions originating from gradual permafrost thaw in the terrestrial environment, and added to this box-model-projected permafrost carbon emissions from abrupt thaw beneath thermokarst lakes. Simulations spanned 2010 to 2100 under moderate and high Representative Concentration Pathways (RCP4.5 and RCP8.5). Supported by field observations, radiocarbon dating, and remote sensing, this re-analysis of model data leads to four striking conclusions. First, accounting for abrupt permafrost thaw beneath lakes more than doubles the radiative effect of circumpolar permafrost carbon release in the 21st century beyond that of gradual thaw alone. Second, permafrost carbon emissions from lakes are similar under RCP4.5 and RCP8.5, but their contribution to the circumpolar permafrost carbon radiative effect (CPCRE) is much larger under the moderate warming scenario. Third, CH4, not CO2, is the dominant driver of the CPCRE, responsible for up to ~70% of circumpolar permafrost-carbon radiative forcing this century. Finally, including abrupt thaw beneath lakes, a process that accelerates mobilization of ancient, deeply frozen carbon, increases old permafrost soil carbon (C-CO2e) emissions by ~125% to 190% compared to gradual thaw alone. Since abrupt thaw has not been considered in earth system models, these findings have important implications for climate change scientists and policy makers, who will now need to account for a >100% larger radiative effect from permafrost carbon loss this century.