What is Polar Warming Amplification?

Earth has two polar regions—the Arctic in the north, and the Antarctic in the south. The Arctic is a vast ocean with large areas covered by sea ice, even in summer. The Arctic also contains a huge island covered by ice, called the Greenland ice sheet. Northern regions of North America, Europe, and Asia also belong to the Arctic—they are covered by a lot of snow in winter and have permafrost, which is the permanently frozen ground. Antarctica is a huge ice sheet, which is surrounded by the Southern Ocean. Although the two polar regions have different characteristics, they have one thing in common: they are both covered by snow and ice year-round, because they are very cold. However, due to the recent warming of our planet, polar regions are also warming (Figure 1A), and their snow and ice cover is shrinking (Figure 1B). What is more, the Arctic and part of Antarctica have warmed more rapidly and strongly than other regions of the Earth. This phenomenon is called polar amplification.

Why is Polar Amplification Important?

The consequences of warming in the polar regions can be devastating: animals like polar bears lose their habitats; people living in those regions cannot continue their traditional lifestyles; and land that was frozen for thousands of years collapses, destroying buildings or roads built on it. The melting of snow and ice in the polar regions also has consequences around the world. For example, the melting of the Greenland and Antarctic ice sheets causes large amounts of water to flow into the ocean, which causes global sea levels to rise and may flood many islands and low-lying coastal areas around the world. Climate change in the polar regions also influences the rest of the Earth’s weather. Thus, while someone may say that disappearing sea ice brings benefits like easier access to the Arctic sea routes (shown by the red line in Figure 1A), the bad consequences we listed above outweigh any economic benefits. Scientists are concerned about all these impacts, so they are investigating why the polar regions are warming so quickly, how much they will continue to warm in the near future, and what the local and global impacts of this warming might be.

Measuring Polar Amplification

How do we know that temperatures have been changing in the polar regions? Scientists can measure the temperatures of the air, ocean, snow, and ice in these regions (Figure 2). Some stations have been measuring for more than 70 years, so we know how temperatures have been changing every year, month, and day since then. These stations need people to be present to take the measurements; but recently, scientists have been using automatic weather stations that need maintenance only once a year and thus can be installed at many more locations [1]. Scientists also take a lot of measurements during expeditions, when they study changes in the ocean, ice, snow, and atmosphere simultaneously [2]. Since 1979, there have been scientific satellites measuring Earth from space and giving a lot of information about changes in the polar regions and across the globe (such as the CryoSat-2 satellite shown in Figure 1B).

Long-term measurements show that temperatures in the polar regions have increased at twice the global average. Since humans started adding lots of greenhouse gases to the atmosphere at the end of the 19th century, the Arctic and parts of the Antarctic Peninsula have warmed on average by more than 2°C, while the global average temperature increase has been 1°C. Large amounts of snow and ice have also melted, and the air has become more humid and cloudier, which has increased the amount of precipitation. It often rains instead of snowing in the Arctic now.

Why are the Polar Regions Warming Faster and Stronger? Local Feedback!

Several processes are responsible for making the warming in the polar regions stronger than in other areas of the Earth. These are known as feedback processes [3], and they are processes that increase or decrease the original change (Figure 3). The processes that make warming even stronger are called positive feedback. And “positive” does not mean they are good! We will explain three of these positive feedback processes in more detail.

Snow- and Ice-Albedo Feedback

The land and ocean in the polar regions can be covered in snow and ice. Snow- and ice-albedo feedback describes the relationship between the sun’s rays and the snow and ice covering a surface (Figure 3A) [4]. The balance between the incoming sun rays and those reflected from a surface is called albedo. This feedback only occurs during daylight, so it cannot occur during the polar night—in winter, when polar regions do not receive any sunlight. If the sun’s rays hit a bright surface like snow or ice, most of the rays are reflected back to the sky and are not taken up by the surface. This is called high albedo. Snow- and ice-free surfaces in the polar regions are darker and reflect fewer of the sun’s rays. Instead, these rays are stored by the surface, which is known as low albedo.

So, as the polar air warms, snow and ice melt. Large parts of the ocean and land surfaces that were once covered by snow and/or ice become exposed and take up more sun rays. This warms the land surface and ocean even more and leads to further melting of snow and ice. This process will continue until all the snow and ice are gone from a surface.

Cloud Feedback

As the atmosphere becomes warmer, many processes happen that involve clouds—some lead to cooling and some to more warming. It is very complicated, and there are still many things we do not understand about these processes. Scientists measure clouds to understand their behavior, and they work with computer models to create virtual clouds, to study clouds’ role in the climate system. One process that is being studied is called cloud feedback [2].

Clouds are made from little water droplets and/or ice crystals. Like ice and snow on the ground, clouds also have a high albedo and therefore reflect a large part of the sun’s energy. However, in the polar regions, the sun does not shine during the polar night. During this time, the clouds block the energy emitted by the Earth’s surface and send it back down, trapping it between the surface and the clouds. This energy warms the air (Figure 3B).

Warmer air contains more water vapor due to evaporation from the ocean, so it helps to form even more clouds…which increase the temperature even more. You can see that this is a positive feedback loop.

Ocean Feedback

Ocean feedback means that the ocean increases the warming of the air near the ocean surface [5], and this also interacts with other types of feedback (Figure 3C). First, because of increased ice/snow melting during the summer, the Arctic Ocean and land absorb more solar energy due to the snow- and ice-albedo feedback. Clouds warm the surface further, especially during the long polar winter, melting the ice and snow even more via cloud feedback. Scientists also found that global warming is causing increased inflow of warmer Atlantic water into the Arctic. All these processes warm the Arctic Ocean and melt sea ice, making it thinner and less abundant in winter. This is when ocean feedback starts to play a role: in winter, the Arctic Ocean is much warmer than the air above it, especially as the ocean continues to warm due to global warming. When there is less ice or thinner ice, the ocean can give more heat to the air, warming it even more. This is another positive feedback loop!

Summary

During the past few decades, the entire Arctic region and parts of the Antarctic have warmed more than the rest of the world has. We know this thanks to long-term measurements and expeditions. This phenomenon is known as polar amplification, and it occurs because of various feedback processes, such as snow- and ice-albedo feedback, cloud feedback, and ocean feedback. Computer models of the climate system show that warming will continue in the future, with Arctic sea ice disappearing completely in summer over the next 20–30 years. Disappearing Arctic sea ice in summer harms indigenous people, as well as animals depending on ice and fragile ecosystems, while melting ice sheets raise global sea level. Although the current situation is dire, if all the countries of the world do their part by reducing greenhouse gas emissions, we can still slow down this warming and possibly even bring Arctic sea ice back!

Glossary

Sea Ice: ↑ Ice that originates from freezing of ocean water.

Ice Sheet: ↑ A large body of ice that originated from compacted snowfall accumulating on rocky land over many years (currently there are two ice sheets—Greenland and Antarctic).

Permafrost: ↑ Ground that stays frozen throughout the year, even in summer.

Polar Amplification: ↑ Greater warming near the poles compared to the rest of the world in response to global climate change.

Feedback: ↑ A process that makes an initial change (such as a temperature change) stronger (positive feedback) or weaker (negative feedback).

Albedo: ↑ The amount of the sun’s rays that are reflected (and thus not absorbed) by a surface (ex. 10% albedo means 10% of the rays are reflected and 90% are absorbed).

Polar Night: ↑ The winter period in the Arctic or Antarctic, when the sun does not rise and the night lasts several months.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

IG thanks FCT/MCTES for financial support to CESAM (grant nos. UIDP/50017/2020, UIDB/50017/2020, and LA/P/0094/2020) and the FCT Project ATLACE - ATLantic interactions via atmospheric water cycle: exploiting a unique dataset from the Antarctic Circumnavigation Expedition for better understanding of clouds and precipitation (CIRCNA/CAC/0273/2019) through national funds. ML and TK acknowledge the funding by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) - Project number 268020496 - TRR 172, within the Transregional Collaborative Research Center-ArctiC Amplification: Climate Relevant Atmospheric and SurfaCe Processes, and Feedback Mechanisms (AC)3. Written informed consent was obtained from the individual(s) for the publication of any identifiable images or data included in this article. The authors thank Clara Burgard, Chris Jones, and Robin Matthews for their helpful comments on the text and also thank our young reviewers for their interest and helpful feedback.