June 19, 2024
Research Highlight

Natural Variability Increased the Observed Arctic Amplification to Fourfold in Recent Decades

Modes of natural variability substantially modulate the degree of observed Arctic Amplification


An alarming fourfold increase in Arctic Amplification in recent decades challenges previous briefs and is rarely reproduced by climate models.

(Photo by Cassie Matias via Unsplash)

The Science

Arctic Amplification (AA)the amplified surface warming in the Arctic relative to the global meanis 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 that of the globe, recent studies reported an alarming amplification factor of four since 1979. This alarming fourfold AA challenges previous beliefs and is rarely reproduced by model realizations, indicating either a rare event contributed by natural variability or systematic model underestimation of the externally forced AA. If the former is true, climate scientists should expect a weaker AA than the fourfold warming in the coming decades. If the latter is true, scientists need to update AA estimates and prepare for more severe consequences from AA. Given such distinct implications, it is critical to correctly interpret the observed AA.

The Impact

Researchers provide clear evidence to show that the fourfold AA over recent decades (e.g., 19802020) is an anomaly caused by dominant modes of natural variability. Specifically, the fourfold AA is contributed by both the negative phase change of the Interdecadal Pacific Oscillation (IPO) around 2000 that decelerated global warming and the positive phase change of the Arctic Mode (AM) around 2005 that accelerated Arctic warming. By revealing the role of IPO and AM, researchers provide a process-level understanding of the effect of natural variability on AA. Furthermore, by excluding the effects of IPO and AM from the observed temperature trends, researchers reveal that the externally forced AA remains close to three throughout the historical period. This reassures researchers’ previous beliefs on the degree of AA and suggests that the ensemble of climate models does not notably underestimate the externally forced AA and should be considered reliable for future projections. 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 researchers 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 during the period between 1970 and 2004.


The alarming fourfold AA in recent decades challenges previous briefs and is rarely reproduced by climate models. It remains elusive whether the observed AA reflects a temporary anomaly due to natural variability or a forced state of Arctic warming systematically underestimated by models. Here, researchers show that the recent fourfold Arctic warming is an anomaly contributed by modes of natural variability and the externally forced AA has a consistent degree of ~3 throughout the historical period. Researchers explain the role of natural variability by showing that the differences between the observed temperature evolutions and the model-simulated forced responses are highly correlated with the phases of internal modes. Specifically, the periods of accelerated/decelerated global warming in observations relative to models are well explained by the IPO, while the periods of accelerated/decelerated Arctic warming are well explained by a dominant mode in the Arctic. The recent fourfold AA is contributed by both the decelerated global warming from IPO and the accelerated Arctic warming from the AM. By estimating and eliminating the effect of natural variability on the observed temperature evolutions, researchers uncover a consistent degree of the externally forced AA close to three throughout the historical period (19672020).

PNNL Contact

L. Ruby Leung, Pacific Northwest National Laboratory, ruby.leung@pnnl.gov


This research was supported by the Department of Energy, Office of Science, as part of research in the Regional and Global Model Analysis program area, Earth and Environmental System Modeling Program.

Published: June 19, 2024

W. Zhou, L. R. Leung, and J. Lu. 2024. “Steady threefold Arctic Amplification of externally forced warming masked by natural variability,” Nature Geoscience, 1-8. https://doi.org/10.1038/s41561-024-01441-1