Five global climate models used in the Arctic Climate Impact Assessment are utilized to estimate the local enhancement of Arctic warming attributable to sea ice retreat in 21st century B2-scenario greenhouse gas (GHG) simulations. The models show a wide range of ice retreat, resulting in a corresponding range in the enhancement of warming. The enhancement is highly seasonal, varying locally from essentially zero in the summer to several degrees (°C) in the late autumn and early winter. The composite climate model response to GHG forcing manifests as a nonlinear amplification of seasonally modulated warming enhancement. The magnitude of the warming enhancement increases with the threshold decline in ice concentration used to define retreat because higher thresholds better isolate the warming enhancement signal over ice retreat areas. A threshold of 20% ensures that all models in this study have enough ice retreat area to sample the enhancement because all start with ice concentrations at least that high over substantial northern hemisphere areas. All estimates are lower bounds because they do not account for advective effects.