Quantifying Contributions of External Forcing and Internal Variability to Arctic Warming During 1900-2021

Arctic long-term warming trend is modulated by decadal-to-multidecadal variations. Improved understanding of how different external forcings and internal variability affect Arctic surface air temperature (SAT) is crucial for explaining and predicting Arctic climate changes. We analyze multiple observational datasets and large ensembles of climate model simulations to quantify the contributions of specific external forcings and various modes of internal variability to Arctic SAT changes during 1900-2021. We find that the long-term trend and total variance in Arctic-mean SAT since 1900 are largely forced responses, including warming due to greenhouse gases and natural forcings and cooling due to anthropogenic aerosols. In contrast, internal variability dominates the early 20th century Arctic warming and mid-20th century Arctic cooling and accounts for ~40% of the recent Arctic warming from 1979-2021. Unforced changes in Arctic SAT can largely be attributed to two leading modes. The first is pan-Arctic warming with stronger loading over the Eurasian sector, accounting for 70% of the unforced variance and closely related to the positive phase of the unforced Atlantic Multidecadal Oscillation (AMO). The second mode exhibits relatively weak warming averaged over the entire Arctic with warming over the North American-Pacific sector and cooling over the Atlantic sector, explaining 10% of the unforced variance and likely caused by the positive phase of the unforced Interdecadal Pacific Oscillation (IPO). The AMO-related changes dominate the unforced Arctic warming since 1979, while the IPO-related changes contribute to the decadal SAT changes over the North American-Pacific Arctic.