In this section, we’ll answer fundamental physics questions on sea-ice formation. We’ll show you why a briny ocean freezes differently than a freshwater lake. We’ll explain how tiny ice crystals first form “frazil ice”, which then, depending on weather conditions, transforms into pancake ice or nilas, and ultimately into pressure ridges several metres thick. We’ll show you how sea ice “ages” and how it can lose salinity with time. In this section, we’ll also show how the sea ice interacts with the ocean and atmosphere, together with its pivotal role in the Earth’s climate system. Despite how far from us the polar regions are, they are of tremendous importance for our entire planet. The sea-ice cover in summer determines how much solar radiation is absorbed by the ocean or reflected back into space by the ice. This aspect, referred to as ice-albedo feedback, helps to shape how temperatures develop on the Earth.
Differences Between the Arctic and Antarctic
The Arctic and Antarctic, the two extremes: both polar regions are home to extreme climatic conditions characterised by cold, ice and snow. Despite their many similarities, however, there are also clear differences.
The Arctic isn’t a continent; it’s a sea surrounded by continents (North America and Eurasia). Atop the Arctic Ocean, which extends to depths of up to 5,500 metres, lies several-metre-thick ice cover. Though much of the ice is permanent, some parts are seasonal – they form in winter and melt again in summer. Arctic sea ice has been subject to this seasonal cycle since the Pleistocene (ca. one million years ago).
One exception to the image of the Arctic as an ice-covered ocean can be found in Greenland, where an ice sheet with an area of 1.7 million square kilometres and up to three kilometres thick covers the island (volume: 2.9 million cubic kilometres) (Marshall, 2012).
The outer geographic border of the Arctic – the northern polar circle – predominantly lies on land and includes not only forests and tundra, but also communities and industry on the North American and Eurasian continents. Today there are nearly four million people living in the Arctic (Arktis-Klima-Report, 2005).
The Antarctic consists of a central landmass (the continent of Antarctica) surrounded by an ocean (Southern Ocean). Here, it is far colder than in the Arctic and the sea-ice extent shows more seasonal variation. Accordingly, most of the ice in the Southern Ocean is first-year ice. The southern polar circle mostly lies on water. The ecosystems on land are relatively species-poor because the ocean geographically separates them from the rest of the world. Larger plants like trees and shrubs are nowhere to be seen (Zachos et al., 2001).
35 to 40 million years ago, tectonic activity opened two new seaways. From this time on, the Drake Passage separated South America from Antarctica, while the Tasmanian Passage emerged between Australia and Antarctica. This is when the ring-shaped ocean we know today – in which a gigantic, cold ocean current flows about the southern continent – first formed. This Antarctic Circumpolar Current (ACC) directly connects the Indian, Atlantic and Pacific Oceans, while climatically isolating the Antarctic continent from the rest of the world (Zachos et al., 2001). The ACC is powered by the continuously blowing winds and storms of the westerlies in the Southern Hemisphere (Rintoul, 2001; Denny, 2008).
The extent and thickness of the sea ice vary with the seasons and are markedly different in the Arctic and Antarctic. In the High North, the minimum extent is reached in September and the maximum in March, at the end of winter. In terms of the long-term average (1979 – 2019), the Arctic sea-ice extent varies between ca. 15 and 6 million km2.
In the Antarctic, the minimum is reached in February (summer in the Southern Hemisphere) and the maximum in September (winter). Here, the sea-ice extent is characterised by far more substantial seasonal variation, ranging from ca. 18.5 to 3 million km2 in terms of the long-term average.
Another major difference between north and south can be seen in the shelf ice. It is composed of frozen freshwater – that is, of snow that has been compressed into ice in the course of centuries and millennia. From the kilometre-thick ice sheets on Greenland and Antarctica, glaciers flow down to the coast until they hit the ocean. Here they form shelf ice, which floats on the water but remains connected to the land ice. As such, shelf ice is not considered to be sea ice, even though it covers parts of the ocean.
In terms of area, the shelf regions in the Antarctic account for roughly one third of its waters. The Antarctic is home to fifteen major shelf regions. Its largest ice shelves include the Ross Ice Shelf (472,960 km²), Filchner-Ronne Ice Shelf (422,420 km²), Amery Ice Shelf (62,620 km²) and Larsen C Ice Shelf (48,600 km²). In contrast, the Arctic contains very little shelf ice: less than 1,000 square kilometres (Turner et al., 2009).
Ice growth and ice melting are shaped by the same energy flows in both polar regions, although many influencing factors differ considerably between the two. One key factor: heat transfer. Due to pronounced layering in the Arctic Ocean, the heat transfer from the ocean to the atmosphere is estimated at 2 W/m², whereas in many parts of the Southern Ocean (with less pronounced layering) it can surpass 30 W/m².
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