The Arctic is Warming Four Times Faster than the Global Average
- Briefing Note 469
The underlying cause of the overall warming of the Arctic is well understood. Sea ice has a very high albedo, meaning it reflects a lot of the sun’s radiation, whareas 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.
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. However, that figure, found in scientific studies, advocacy reports, the popular press, and even the 2021 U.N. climate assessment, has been found to be incorrect 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 Arctic is warming four times faster than the global average.
Rantanen et al. (2022) compared the observed Arctic amplification ratio with the ratio simulated by state-of-the-art climate models CMP5, CMP6, MPI-GE, and CanESM5. They found that the observed four-fold warming ratio over 1979–2021 is an extremely rare occasion 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.
Temporal Variations in Warming Rate
Chylek et al. (2022) show that over the last few decades, the Arctic has not warmed at a consistent, predictable rate. The changes occurred mainly in two discrete steps: one around 1985 and the other around 2000. After this last increase in the 2000, Arctic amplification has been about 4.5 compared to the two or three times as fast as it was before. This means that the scientific community and policymakers have been referring to figures that are far too low.
The cause of these sudden changes in Arctic temperatures not yet clear. Chylek et al. (2022) think that the first one in the 1980s was probably due to increasing concentrations of carbon dioxide in the atmosphere, and that the second one around the turn of the century may have been due more to variability in the climate, for example changing ocean currents, 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 for several reasons. First, climate scientists have tended to divide each hemisphere into thirds and label the area above 60°N as the Arctic, although the true definition of the Arctic is defined by Earth’s tilt, the Arctic Circle is a line starting 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 is unlikely to occur again.
The underestimation of Arctic amplification 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.
Timing of the Albedo Effect
The albedo effect causes melting of sea ice in the summer and early autumn. Because of this evaporation of water, and because a larger area of water is open at that time, this water vapor enters the atmosphere and forms low-level clouds. These clouds reflect some of the sun’s radiation back into space, but they also absorb some of it, and remain through the winter, trapping heat against the 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.
Simultaneously, storms have been transporting moisture from lower latitudes into the Arctic, further encouraging the development of clouds. Also, injections of warmer water from the south, brought north by ocean currents, further melt sea ice. As it melts, water evaporates and increases atmospheric humidity, which causes an increase in cloudiness in winter, with the clouds generating infrared radiation down to the surface. This feedback loop that can cause increased Arctic temperature, and is thought to be one of the causes of the increase in temperature around 2000.
There has been a lag in how areas at high latitudes have responded to greenhouse gases compared to the rest of the planet. It has taken time for sea ice to melt, but now that it is melting, the heat feedback loop in the Arctic has strengthened, and the rate of change has become much more prominent. The tropics warmed faster first, but now the poles are catching up.
Impacts
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 are advancing 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. 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.
References
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
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
