" But the record from Lake El’gygytgyn of a very warm Arctic when atmospheric CO2 levels were last at about 400 parts per million (ppm) indicates the opposite, according to Brigham-Grette.“My feeling is we have underestimated the sensitivity, unless there are some feedbacks we don’t yet understand or we don’t get right in the models.”

Actually it is a lot simpler than defects in the model. The model is designed for a short distance view of the future. In other words, it reacts to the relatively 'prompt' warming response. But some factors can take a lot longer to reach full effect. This is why the forecast for a doubling of CO2 is about 3C within the century and up to 6C over a millennium or more. The arctic will warm a bit more than the planetary average.