The Arctic is Warming Four Times Faster than the Global Average
The underlying cause of the overall accelerated warming of the Arctic is well understood. Sea ice has a very high albedo, meaning it reflects a lot of the Sun’s radiation, whereas the underlying seawater has a low albedo, meaning it absorbs that energy. So as that ice melts, the albedo of the Arctic decreases, raising temperatures and melting more ice in a positive feedback loop.
The albedo effect causes increased melting of sea ice in the summer and early autumn, favouring increased evaporation of opened ocean water. This increase in water vapor in the lower atmosphere in turn facilitates the formation of low-level clouds which act to increase the greenhouse effect and trap more of the sun’s radiation at the Earth’s surface. Although the sun does not shine at all during the depths of winter above the Arctic Circle, warmer summers and autumns cause the coldest months to get hotter. All the extra warmth of summer is also being trapped in the Arctic Ocean, then released throughout the winter, causing the greatest warming in the Arctic to occur in winter, even though the greatest sea ice melting occurs in summertime.
It has been widely thought that the Arctic has been warming twice as rapidly as the rest of the world in recent decades, a phenomenon called Arctic Amplification (AA). However, that figure, found in scientific studies, advocacy reports, the popular press, and even the 2021 UN. Climate Assessment, has been updated in a series of studies published over the past year (Chylek et al., 2022; Jacobs et al., 2021; Rantanen et al., 2022; Voosen, 2022). These researchers conclude that the Arctic is warming four times faster than the global average, approaching an increase in temperature of nearly 1 degree per decade (as opposed to a recent global average warming of 0.2 degree per decade; Figure 1).
Rantanen et al. (2022) compared the observed AA ratio with that simulated by state-of-the-art CMIP5 and CMIP6 climate models (Figure 1). They found that the observed four-fold warming ratio over 1979–2021 is an extremely rare event in the climate model simulations. They conclude that their results indicate that the recent four-fold Arctic warming ratio is either an extremely unlikely event, or that the climate models systematically tend to underestimate the amplification. We prefer an interpretation in which this change is unlikely to be sustained through time, and that it will probably decrease due to internal variability rather than being sustained by internal forcing.
Temporal Variations in Warming Rate
Chylek et al. (2022) state that over the last few decades, the Arctic has not warmed at a consistent, predictable rate. The authors propose with very little evidence that changes occurred mainly in two discrete steps: one around 1985 and the other around 2000. It is only since this last increase in 2000 that they estimate an AA of about 4.5, contrasting with values of 2-3 pre-2000s. Regardless of timing, there is clear evidence of a recent acceleration of AA, suggesting that the scientific community and policymakers have been referring to figures that are far too low.
The causes of these sudden changes in AA are not yet clear. Chylek et al. (2022) hypothesise that the first spike in the 1980s was likely due to the sheer increasing concentrations of greenhouse gases in the atmosphere, but that the second one, around the turn of the century, may have been caused by natural climate variability responses such as changing ocean currents or atmospheric circulation patterns, which current models cannot capture accurately within the time scale of the physical phenomena.
Jacobs et al. (2021) suggest that Arctic warming has been underestimated by climate models for several reasons. First, climate scientists have tended to divide each hemisphere into thirds and label the area above 60°N as the Arctic. This contrasts with the “true” definition of the Arctic as defined by Earth’s tilt, with the Arctic Circle being a small circle at 66.6°N. Second, they have chosen different time periods over which the warming rate is calculated. Jacobs et al. (2021) focused on the past 30 years, when a linear warming trend emerged for the Arctic. Analyses that looked at longer term trends see less divergence between the Arctic and the world. That is because before 1990, the Arctic’s temperatures fluctuated, and even cooled for decades because of air pollution, including light-blocking sulfate aerosols that swept in from the northern midlatitudes, a scenario that is unlikely to occur again.
The underestimation of AA may also have been partly due to sociological effects, such as time lags in translating research into knowledge, information obsolescence, and a bias towards overly conservative descriptions of climate change.
Current and future changes in the Arctic have profound implications for the physical climate system, human populations and ecosystems, and geopolitical decision-making for commerce and global security. The consequences are already far-reaching. More melting, particularly in Greenland, causes sea levels to rise. Also, warmer waters undergo thermal expansion, further raising sea levels. Warming temperatures are thawing frozen soil (permafrost), causing major damage to infrastructure, roads and buildings in the Arctic. Increasing temperatures are also greening the Arctic landscape. As shrub species advance north, the vegetation traps more snow against the ground. This prevents the cold of winter from penetrating the ground, accelerating the thaw of permafrost. The extra vegetation also has a lower albedo, absorbing more of the sun’s radiation.
Temperatures in the Arctic have reached historically unprecedented levels of 38 degrees C. This kind of variability makes it difficult for models to describe how the Arctic is changing, and to predict how those changes will proceed to influence the larger climate system. One reason for concern is the potential for the climate system to reach a tipping point, in which warming triggers rapid and irreversible change (Risk Frontiers, BN 468, 2022). If the Arctic warms enough, for example, melting in Greenland might quickly accelerate. If such tipping points exist, it is not precisely known what level of warming could trigger such rapid changes. The scientific community urgently needs to identify such tipping points because the global community urgently needs to avoid them, as mitigation is unlikely to be an option should they be exceeded.
Figure 1: [Rantanen et al., 2022]
Chylek, P., Folland, C., Klett, J. D., Wang, M., Hengartner, N., Lesins, G., & Dubey, M. K. (2022). Annual mean Arctic Amplification 1970–2020: Observed and simulated by CMIP6 climate models. Geophysical Research Letters, 49, e2022GL099371.
Jacobs, P., N. Lenssen, G Schmidt and R Rohde (2021). A13E-02 – The Arctic Is Now Warming Four Times As Fast As the Rest of the Globe. AGU, Monday 13 December 2021. New Orleans.
Rantanen, M., Karpechko, A.Y., Lipponen, A. et al. The Arctic has warmed nearly four times faster than the globe since 1979. Commun Earth Environ 3, 168 (2022). https://doi.org/10.1038/s43247-022-00498-3
Risk Frontiers (2022). Potential Impact of Slowing of the Atlantic Meridional Overturning Circulation on La Niña and Weather in Eastern Australia. Briefing Note 468.
Voosen, Paul (2021). The Arctic is warming four times faster than the rest of the world: An important climatic indicator has been misreported by a factor of two. 14 DEC 2021. https://www.science.org/content/article/arctic-warming-four-times-faster-rest-world