Arctic Amplification (AA) – the amplified surface warming in the Arctic relative to the globe – is a robust feature of climate change. However, there is a considerable spread in the reported magnitude of AA. While earlier observations and model simulations suggested that the Arctic has been warming at a rate two to three times as the globe, a recent study reports an alarming amplification factor of four since 1979.

Here, we reconcile this discrepancy by revealing that natural variability boosted the degree of AA over recent decades. Based on three observational datasets and 34 models from CMIP6, we show that the observed temperature evolutions are distinct from the model-simulated forced responses and that the differences are correlated with modes of natural variability. Specifically, the Interdecadal Pacific Oscillation (IPO) decelerated global warming after 2000, while an Arctic internal mode (AM) amplified Arctic warming after 2005, both contributing positively to the recent increase of Arctic Amplification to fourfold. By estimating and removing the effect of natural variability on the observed temperature changes, we reveal that the externally forced AA has consistently remained close to three throughout the historical period.

The seasonal mean temperature presents similar decadal to multi-decadal variation as the annual mean temperature in both the global and Arctic mean. As a result, the degree of the seasonal mean AA presents similar variation as that of the annual mean AA, with natural variability (IPO and AM) modulating the winter and summer mean AA proportionally as AAwinter-1= 2.9(AAsummer-1).

Our results reassure previous briefs on the degree of AA and suggests that the CMIP6 model ensemble does not notably underestimate the externally forced AA and should be considered reliable for future projection. Recognizing the effect of IPO and AM on AA provides important insights into the decadal prediction of AA. In the coming decades, AM is likely to shift to a negative phase and IPO is likely to be positive, so we should expect a reduced degree of AA relative to the externally forced value, that is, AA is likely to be below three and potentially close to two as in 1970-2004.