Sea Ice

Sea ice can occur seasonally – i.e., be formed and disappear again as seasons change – or be more permanent, surviving for two or more years. Both types can be found in the polar regions. Smaller areas of seasonal ice also form in other regions, like the Baltic Sea and Sea of Okhotsk, which stretches from Russia to the coast of Japan.

If temperatures are low enough, ice crystals, which look like small platelets or needles, form in the ocean’s upper mixed layer. These crystals rise to the water’s surface and gradually create a soupy mixture – known as “frazil ice”. When the weather is calm and there is no wave action, this eventually turns into a seamless, thin crust of ice known as “nilas”. Initially, the layer is so thin that it appears to be black, because the dark ocean water below shows through. Over time, the ice becomes thicker and paler, while its surface remains smooth and even. Conversely, in the presence of wind and waves, the frazil ice forms smaller, individual sheets of ice resembling pancakes (“pancake ice”). These sheets collide with one another, become stacked, and ultimately freeze together. This also produces a thick layer of ice, though its surface is rough and uneven.

Because sea ice can conduct heat, it has an insulating effect on underlying water layers. As a result, the thicker the ice grows, the lower the freezing rate becomes. From a thickness of three metres, the melting processes are just as fast as the formation processes – the ice is effectively in “equilibrium” and no longer gains in thickness. That being said, far thicker ice can be formed by dynamic processes. The sheets of ice, measuring a few metres thick, are constantly moved by the wind and ocean currents. Due to this motion – and to the tides and swell – they break up, resulting in floes separated by channels of open water. The floes are then transported across the ocean by the wind and currents (drift).

Off the coasts of Greenland and Canada, for example, ice drift can produce virtual “traffic jams”. Because the drift patterns focus at certain points (convergence), the ice is pressed together. This can result in ice more than six metres thick. Similar phenomena can be observed – though far less frequently – in the Antarctic, e.g. in the Weddell Sea, where the ice drifts in a clockwise pattern and is pressed against the Antarctic Peninsula. When the pressure exerted on the floes by wind and water is particularly high, the sheets become stacked one atop the other and form pressure ridges, which can be up to 50 metres thick in the Arctic.

The Arctic and Antarctic are both polar regions, but differ in terms of their geographic positions, which also influence the formation and longevity of their sea ice. The Arctic Ocean is surrounded by landmasses and is only connected to the Atlantic and Pacific by comparatively narrow passages. Consequently, it’s impossible for the sea ice formed there to spread unhindered in any given direction, allowing parts of it to survive several Arctic summers. But the situation is just the opposite in the Antarctic, where the continent is surrounded by the Southern Ocean. The sea ice here can be found at lower latitudes, between 55 and 75 degrees South, whereas in the Arctic, it is concentrated right at the pole. Due to the open water and the powerful winds whipping around the southern continent, Antarctic sea ice can spread farther and reaches a higher winter extent. However, the majority of this ice melts in the summer months, which is why the Antarctic is home to less multiyear ice.

Another reason for this difference – more multiyear ice in the Arctic and less in the Antarctic – is the snow. In the Arctic, there is little snowfall in winter, which means there is no insulating layer atop the ice. Accordingly, the ice is directly exposed to the frigid air, causing it to grow thicker and thicker. In the Antarctic there is much more snowfall, forming a thick protective layer. The ice cools less, doesn’t become as thick, and melts more rapidly in summer.

The Arctic sea ice reaches its maximum extent at the end of winter, in late February or early March. At this point it covers an area of nearly 15 million square kilometres, which is nearly 1.5 times the size of the USA. In summer there is extensive melting, especially in the Arctic’s southern reaches, until the minimum extent is reached in September. But even then, the sea ice covers between 4 and 5 million square kilometres. In the Antarctic, the sea ice reaches its maximum extent at the end of the southern winter, in September, by which time it covers ca. 18 million square kilometres. Viewed from a global perspective, at this point the sea ice covers roughly 10 percent of the world’s oceans. Then the ice begins melting again. In February, at the end of the Antarctic summer, the sea ice reaches its lowest extent at ca. 2.5 million square kilometres, an area roughly seven times the size of Germany. Accordingly, at circa 15.5 million square kilometres, seasonal variation in sea-ice extent is far more pronounced in the Antarctic than in the Arctic (ca. 10.5 million square kilometres).

The Arctic and Antarctic sea ice cover constitutes an important habitat. For example, its freezing and melting determine from which point and for how long polar bears can hunt seals in the Arctic. During the same time, amphipods and copepods can eat their fill of ice algae on its underside. In turn, these well-fed crustaceans are prey for Arctic cod, which are themselves fed upon by seals, seabirds and whales. When the ice disappears in summer, the underside-dwelling species’ pantry goes with it. At the same time, more sunlight reaches the topmost water layer. Free-water algae grow, die, and sink to seafloor, providing nutrients for the fauna of the deep sea. Whereas the polar bear is the most well-known victim of sea-ice retreat in the Arctic, in the Antarctic it’s the krill. These tiny shrimp-like crustaceans, measuring up to six centimetres long, are the main food source for all larger animals. But especially in their first year of life, the krill need sufficiently extensive and thick pack-ice cover to survive the Antarctic winter, which is more than nine months long. The sea ice affords them protection and represents an important basis for polar food webs – as such, when the ice disappears in summer, much of this unique ecosystem is endangered.

Only the ice that forms directly from freezing seawater is referred to as sea ice. But the ocean can also be covered by what are known as ice shelves – the outliers of the Greenland and Antarctic ice sheets and glaciers, which extend over the ocean. Ice shelves lie atop the ocean but remain connected to the glacier. These ice masses form from snow, which means they exclusively consist of freshwater. Just like ice shelves, the icebergs that calve from glaciers and ice sheets and drift out to sea are not sea ice, either.

Sea ice that is only a few years old has virtually no influence on the sea level, as it is formed on the ocean, floats atop it, and displaces roughly the same amount of water that is released when it melts. However, the same is not true for inland ice: when this ice, which in some cases has been connected to the land for several hundred thousand years, melts, it increases the amount of water in the ocean, causing the sea level to rise.

Over the past 30 years, the sea-ice cover in the Arctic has changed significantly. Especially in September, a substantial decline in sea-ice extent can be observed. Due to the warming of the Arctic Ocean and the atmosphere, the ice is melting earlier in spring and freezing later in autumn. In September 2012, the Arctic sea-ice extent reached its all-time minimum to date at 3.6 million square kilometres. The results of model-based climate simulations indicate that these changes are evidence of a global climate warming trend that, in all likelihood, will intensify in the future. In addition, sea-ice thickness has decreased significantly: in the 1960s, the majority of the sea ice was still ca. 3.0 metres thick in summer. By the 1990s the number had shrunk to over 2.0 metres, and in recent years, to only 0.9 metre.

The sea-ice extent in the Antarctic differs greatly from region to region. Whereas the amount of sea-ice cover is declining markedly in the Amundsen and Bellingshausen Seas of the western Antarctic, it is increasing in e.g. the Ross Sea. All told, the Antarctic sea-ice extent has increased slightly over the past few decades. However, extremely low sea-ice extents have recently been observed in the Antarctic. In February 2023, the lowest-ever sea-ice extent in the region was reported. Just why the extent was so low in that particular month is something no-one can say with certainty. The prevalent assumption is that it could have been due to severe natural variability together with changes in the wind. Increased meltwater input from the land to the oceans, where the water freezes again, may have also been a contributing factor. The various causes are still being intensively discussed in the scientific community, which is anxious to see how things develop in the years to come.

As the sea ice retreats, the Arctic Ocean will become increasingly interesting from an economic perspective. If parts of the Northeast and Northwest Passages linking the Pacific and Atlantic north of the continents remain ice-free longer, it could reduce the distances between global markets. Moreover, intra-Arctic transport is becoming more and more important in connection with natural resource exploration in the region. But increased ship traffic in the Arctic could also lead to environmental pollution with unforeseeable consequences. Further, increased shipping could interfere with the migratory movements of countless marine mammals and birds. However, the most discussed environmental risks are the introduction of invasive species, risks in connection with oil pollution, regular emissions of wastewater and greywater, additional greenhouse-gas emissions from ships, and noise pollution.

In order to determine the extent to which sea ice in the Arctic and Antarctic is dwindling in response to climate change, researchers calculate its volume. To do so, they need information on the ice extent and thickness that is as precise as possible. Whereas the sea-ice extent and thickness can be accurately measured by satellite in the Arctic, measuring the thickness is far more difficult in the Antarctic, due to the differences in snow cover. Consequently, to produce forecasts that are as accurate as possible, researchers most often combine a number of complementary methods: for example, the EM-Bird is a specially designed device for measuring ice thickness. Here, “EM” is an abbreviation of the unwieldy designation “aerogeophysical electromagnetic (EM) induction method in frequency range”. The Bird is lowered below a plane or helicopter on a cable and hauled over the ice at a low altitude. Taking advantage of the fact that saltwater is more conductive than sea ice, it can measure the distance between the ice’s surface and underside. There are also ground-based EM readings, gathered with a sledge towed over the ice, and an “under-the-ice” sonar system – an acoustic transmitter and receiver, moored to the ocean floor, that measures the distance between it and the sea ice. When the ice grows or melts, its thickness changes, and with it, the distance between it and the sonar. Lastly, during expeditions on the sea ice, researchers still use the “classic” method – collecting ice cores – to measure ice thickness. The cores offer a valuable benchmark for assessing the precision of other methods.