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Arctic Ocean

Climate Change & Cryosphere – Why is the Arctic sea-ice cover retreating?

Climate Change & Cryosphere – Why is the Arctic sea-ice cover retreating?

The Arctic Ocean surface is darkening as its sea-ice cover is shrinking. The exact processes driving the ongoing sea-ice loss are far from being totally understood. In this post, we will investigate the different causes of the recent retreat of the Arctic sea-ice cover, using the most updated literature…


Arctic sea ice is disappearing

Due to its geographical position centered around the North Pole, the Arctic Ocean is relatively cold compared to other world oceans. This means that, each winter, ocean temperatures fall below the freezing point, and sea ice forms on top of the ocean surface.

The Arctic sea-ice extent reaches about 15 million km2 in March, at its maximum (see left panel in Fig. 1 and Fig. 2). In spring, the ice starts to melt and reaches its minimum extent in September, which is about three times smaller than its maximum extent (see right panel in Fig. 1 and Fig. 2).

Figure 1: Maps of mean Arctic sea-ice concentration (percentage of sea ice in a given grid cell) in March (left) and September (right), averaged over 1979-2015, from satellite observations. The red line (right panel) shows the sea-ice edge in September 2012 (record minimum) [Credit: Ocean and Sea Ice Satellite Application Facility (OSI SAF)].

Satellite observations clearly show that the Arctic sea-ice cover has been shrinking since the beginning of the satellite record in 1979 (see this post and this post for more information about sea-ice satellite observations). The sea-ice loss is about 13% per decade in September and 3% per decade in March (see this post, this post and this post for further information on recent Arctic sea-ice changes). A recent study using data from a series of different observations (ship reports, airplane surveys, analyses by national services, etc.) shows that the recent Arctic sea-ice loss, as measured by satellites, is unprecedented as far back as 1850.

Figure 2: Seasonal cycle of Arctic sea-ice extent from satellite observations. The solid blue and dashed red lines show the 2019 (ongoing) and 2012 (record minimum) values. The dark gray curve shows the average over the period 1981-2010 with the corresponding uncertainty range in light gray (+/- 2 standard deviations) [Credit: National Snow and Ice Data Center].

The year 2012 was particularly exceptional in the sense that it featured the record minimum in September since the beginning of satellite measurements (dashed red curve in Fig. 2). 2019 was on a ‘good path’ to break this record, but the sea-ice loss rate started to lower from mid-August (blue curve in Fig. 2, see also here).

 

What are the drivers of the Arctic sea-ice loss?

The recent changes in Arctic sea ice have been caused by three main factors:

  1. External forcing: the variability caused by external factors, which can either be human (e.g. anthropogenic greenhouse gas emissions) or natural (e.g. changes in solar activity, volcanic eruptions).
  2. Internal variability: the variability caused by the chaotic nature of processes at work in the climate system. It is internal variability that prevents us to make accurate weather forecasts beyond a few days.
  3. Positive feedbacks: the processes by which a change in the climate can amplify, e.g. the ice-albedo feedback. These feedbacks are described in more detail in this post.

 

External forcing

Several studies have analyzed the links between the changes in external forcing and the recent changes in Arctic sea ice in both observations and models. It has been found that the anthropogenic global warming, caused by increased greenhouse gas concentration in the atmosphere, is the main driver of the long-term sea-ice loss in the Arctic. In particular, Notz and Stroeve (2016) found that for each ton of CO2 released into the atmosphere, the Arctic loses about 3 m2 of sea ice in September, as shown in Fig. 3 below (see this post).

Figure 3: September Arctic sea-ice area against cumulative CO2 emissions since 1850 for the period 1953-2015. Grey circles and diamonds show the results from in-situ (1953-1978) and satellite (1979-2015) observations, respectively. The thick red line shows the 30-year running mean and the dotted red line represents the trend of 3 m2 in sea-ice area loss per ton of CO2 emitted [Credit: D. Notz, based on Notz & Stroeve (2016)].

Other changes in external forcing have had a more limited impact on the recent changes in Arctic sea ice. For example, the volcanic eruptions of El Chichón in 1982 and Mount Pinatubo in 1991 caused a small increase in Arctic sea-ice extent (see this study), but their impact cannot be clearly identified in individual climate models (see here). Similarly, the impact of the solar activity on the recent Arctic sea-ice changes is very small.

 

Internal variability

The sea-ice evolution is also strongly subject to internal variability. A good explanation of the concept of internal variability can be found here and in this study.

Internal variability is often estimated using climate models. Running the same model with exactly the same parameters and external forcing, but with slightly different initial conditions (for example a different sea surface temperature), is a common method to get an idea of the internal variability of the climate system. Figure 4 below shows the evolution of the Arctic sea-ice extent using the Community Earth System Model (CESM) run 40 times with different initial conditions. The spread of the ensemble represents the range of the effect of internal variability.

Figure 4: Evolution of Arctic September sea-ice extent using the Community Earth System Model Large Ensemble (CESM-LE) in the less optimistic scenario (RCP8.5). The blue curves show all the 40 model members. The red curve shows the NSIDC satellite observations [Credit: Fig. 1a of Jahn et al. (2016)].

Variations in heat transport from the Atlantic Ocean due to internal variability have caused strong reductions in sea-ice area in the Barents Sea (see this study and this study), and probably other seas located in the Atlantic sector of the Arctic Ocean (as shown here). Changes in large-scale atmosphere circulation, also associated with internal variability, have contributed to sea-ice reductions as well (see this study).

However, several studies found that internal variability was not the key cause of the recent Arctic sea-ice loss over the past 40 years (e.g. this study). Instead, internal variability acts as an amplifier of the external forcing (see this study), so it only explains a small part of the recent Arctic changes. In the climate models used in the IPCC AR5 report, the impact of internal variability is of maximum 1 million km2 (see this study).

 

Feedbacks

A last cause for the recent changes in Arctic sea ice is the presence of positive feedbacks, which can amplify ongoing changes. One of the main feedbacks acting in the Arctic is the ice-albedo feedback (see this post and this study). Since ice reflects more sunlight than water, if the sea-ice cover decreases, more heat is trapped by the surface of the Arctic Ocean, leading to more ice melting.

However, as we have seen above, there is a clear linear relationship between Arctic sea-ice extent and cumulative CO2 emissions (Fig. 3). If the ice-albedo feedback was important in explaining the recent loss of Arctic sea ice, this linear relationship would break after years of strong or weak ice loss, which is not the case.

In fact, the positive feedbacks (like the ice-albedo feedback) are partly compensated by negative feedbacks that stabilize the climate system (see this previous post for a description of feedbacks in polar regions). Thus, while these feedbacks play a key role in the short term, they cannot explain the bulk part of the sea-ice loss since 1979.

 

The future

In conclusion, the recent loss of Arctic sea ice is strongly linked to anthropogenic global warming, although the changes in atmosphere circulation and ocean heat transport, associated with internal variability, also influence the sea-ice evolution. Research continues on the topic in order to capture the exact contribution of the different causes to the Arctic sea-ice loss.

On the long term, Arctic sea ice will continue disappearing. Based on the current emission rates of greenhouse gas emissions into the atmosphere, it is probable that the Arctic Ocean will be ice free during summer before 2050 (see this post, this study and this study).

Due to its linear relationship with CO2 emissions, the Arctic sea-ice cover is a strong indicator for the pace of current climate change. Its rapid disappearance should be seen as a warning light for other impacts to come…

 

Further reading

Edited by Clara Burgard


David Docquier is a post-doctoral researcher at the Earth and Life Institute of Université catholique de Louvain (UCL) in Belgium. He works on the development of processed-based sea-ice metrics in order to improve the evaluation of global climate models (GCMs). His study is embedded within the EU Horizon 2020 PRIMAVERA project, which aims at developing a new generation of high-resolution GCMs to better represent the climate.

Image of the Week – Sea-ice dynamics for beginners

Image of the Week – Sea-ice dynamics for beginners

When I ask school children or people who only know about sea ice from remote references in the newspapers: ‘How thick do you think is the Arctic sea ice?’, I often get surprising answers: ’10 meters? No, it must be thicker – 100 meters!’. It seems like sea ice, often depicted as a uniform white cover around the North Pole and as a key element in accelerated warming of the Polar Regions, imposes a majestic image. Unfortunately, sea ice is much more fragile.


Growing in the current

Actually, sea ice is on average just about 2 m thick. It used to be thicker, up to 3 m, but such ice needs several winters to grow and is quite rare in the modern-day Arctic as winters are warmer than they used to be (see this previous post). Currently, more than half of the Arctic sea-ice area melts away completely during summer and grows back during the next winter. Such a thin layer of frozen water floating on the ocean is not strong enough to resist the forces of the wind, which pushes it around in ocean surface currents. In order for the ice to move, it has to deform and breaks into ice floes (read more in this previous post). Some ice floes move apart (divergence) in leads and polynyas (see this previous post), while others are pressed together (convergence) in pressure ridges, where blocks of ice pile up against and on top of each other (see our Image of the Week).

Ice grows from the ocean surface layer by water freezing. This is called thermodynamical growth. Thermodynamical growth produces most of the ice forming in the time from freeze-up in fall until the ice becomes about 1 to 2 m thick in mid-winter. At that point, sea ice approaches equilibrium thickness, i.e. the sea ice is thick enough to insulate the cold atmosphere from the relatively warm ocean. But because sea ice deforms, it can continue growing during the rest of the winter too. Pressure ridges sails can stick several meters out of the icy landscape, while their much larger and bulkier keels are hidden below the surface. Ridges can store large volume of sea ice – about a third (Hansen et al, 2013)! At the same time, new ice can grow in leads where open water is exposed to the atmosphere.

The following video is a collection of movies showing consequences and acts of sea-ice deformation. The first part is taken from R/V Lance – the ice-strengthened research ship of Norwegian Polar Institute, while she is navigating along a lead in late winter. Observe how much space is taken by pressure ridges! The second part of the movie shows a pressure ridge growing. Listen to the sound of deforming ice!

Another positive feedback

In winter, temperatures are so low that all the fractures, leads and pressure ridges freeze back – they heal. In summer, however, these damages are the first to appear again. Dark water with low albedo (read more about the albedo feedback in this post) is exposed and the ice melts faster in such regions. Because the Arctic sea ice became relatively thin over the recent decades, it also became less resistant to the forces of the wind. Such thin ice breaks more easily (e.g. Itkin et al, 2017). This means that, as more of such damaged ice is present in summer, the ice cover melts faster. So, here is an additional positive feedback for the Arctic ice under climate change: thinner ice melts faster also because it has become weaker and therefore breaks up easier.

Further reading

Edited by Clara Burgard


Polona Itkin is a Post-doctoral Researcher at the Norwegian Polar Institute, Tromsø. She investigates the sea ice dynamics of the Arctic Ocean and its connection to the sea ice thickness. In her work she combines the information from in-site observations, remote sensing and numerical modeling. Polona is part of the social media project ‘oceanseaiceNPI’ – a group of scientists that communicates their knowledge through social media channels:

Instagram.com/OceanSeaIceNPI, Twitter.com/OceanSeaIceNPI, Facebook.com/OceanSeaIceNPI, contact Email: polona.itkin@npolar.no