sea ice and climate

sea ice and climate Ice which forms from the freezing of sea water is called sea ice. The presence of salt in water has the effect of reducing the freezing point below that of fresh water, and there is a standard empirical formula which tells us that the freezing point will be lowered by approximately 0.05 °C for every 1 PSU (practical salinity unit) of salt in the water. The freezing point also decreases with increasing pressure. For the typical surface salinities in the world ocean the freezing point is approximately –1.9 °C. The presence of salt also increases the density of the water, and for salinities greater than 24.7 PSU (which is much lower than typical ocean surface salinities) the density of sea water increases until it reaches the freezing point. This means that as the water is cooled it will undergo thermal convection and descend, forcing deeper warmer water to the surface. Therefore, unless the water column is stratified, to form ice, the entire water column has to be cooled to the freezing point. With a salinity stratification only the surface waters have to be cooled. This is the situation found in the Arctic Ocean where there is a surface layer of lower salinity above what is termed the Arctic halocline. This means that just the upper 30–50 m of the water column has to be cooled for ice to form.

The initial form of sea ice is a slurry of ice crystals called frazil ice, which under various conditions grow together to form many different and complex ice forms. As the frazil ice crystals coalesce, some salt from the sea water is trapped within the crystal matrix. This makes the ice slightly saline (up to 8 PSU), but most of the salt is rejected into the surface layers of the ocean. This salt rejection causes a local density increase in the surface water, which can lead to local convection and under certain conditions to deep convection. There is a standard but complex set of descriptive terms for types of sea ice which has been developed by the World Meteorological Organization, but by far the most common distinction is between first-year and multi-year ice. First-year ice is generally considered to be new ice up to approximately 750 mm thick, which can be reached in one year's growth. Multi-year ice is first-year ice which has survived one or more summers, increasing in thickness each winter up to 5 m thick. The quantity of salt trapped in the crystal matrix of multi-year ice is less than that of first-year ice (3–5 PSU), since the surface of the ice can melt in summer, flushing salt from the crystal matrix.

The different geography of the Arctic and Antarctic has a considerable effect on the type of ice within each region. The central Arctic Ocean is an enclosed basin and retains an extensive covering of permanent multi-year ice throughout the year. At its minimum the ice covers an area in September of approximately 9 × 10⁶ km², rising to a maximum of approximately 16 × 10⁶ km² in March, as the ice expands from the Arctic Basin to cover the Hudson Bay, Baffin Bay, the Canadian Archipelago, and the Kara Sea. Large portions of other peripheral seas are also covered, such as the Greenland Sea, Barents Sea, and smaller areas of the Labrador Sea and the Sea of Japan. In contrast, Antarctica is surrounded by open ocean and the permanent sea ice cover is restricted to small regions of the Weddell Sea, the Bellingshausen Sea, and the Amundsen Sea. Because of the smaller regions of permanent ice cover, Antarctic sea ice has a much smaller percentage of multi-year ice. At its minimum in February the ice covers approximately 3.8 × 10⁶ km² but the comparative increase in area in winter is much greater with the ice expanding from the continent to cover an area of almost 19 × 10⁶ km². The global sea ice coverage can best be observed by satellites with passive microwave sensors such as NASA's Nimbus 7 and Nimbus 5. These sensors can show the interannual variation of the area covered by Arctic and Antarctic sea ice.

To calculate the volume of water stored in sea ice we must know its thickness. This is much more difficult to measure than the extent of the sea ice. Most thickness measurements from the Arctic ocean have been obtained from nuclear submarines using upward-looking sonar. The current measurements of thickness of the Antarctic sea ice have all been made by drilling holes. In the early 1990s submarine-derived ice thicknesses showed that sea ice north of Greenland was thinning drastically, but this was shown to be the product of meteorological effects rather than a genuine reduction of ice. The water stored in sea ice is negligible in comparison with the volume of water stored in the world's glaciers and the ice sheets of Greenland and Antarctica. However, sea ice covers a vast area of the world's surface. In the Arctic winter, sea ice can occupy almost 5 per cent of the entire northern hemisphere and in the austral winter the Antarctic sea ice can occupy as much as 8 per cent of the southern hemisphere. On a global scale approximately two-thirds of the ice-covered areas of the Earth are covered by sea ice, which exerts a strong effect on climate.

At the most simple level the ice shuts off the exchanges of momentum, heat, and mass between the atmosphere and the ocean. It thus tends to stabilize the climate system, reducing the heat loss from the ocean in winter. The ice cover also reflects a significant amount of the incident solar radiation (up to 55 per cent for bare sea ice and as much as 90 per cent for snow-covered sea ice) and so provides positive feedback in the climate system. If the ice extent increases, more solar radiation is reflected and ice extent can increase further. If the ice extent decreases, the opposite applies. The ice cover in the polar oceans is not continuous, nor is it complete. Sea ice is a complex and flexible medium and it is moved and broken and piled up by a combination of winds, ocean currents, and (at the edge of the ice pack) wave energy. These forces move the ice to create open water, even in winter, in the form of linear cracks called leads and large areas of persistently open water called polynyas. The winter open water, although small in area (up to 2 × 10⁶ km² in the Arctic and 4 × 10⁶ km² in the Antarctic), is of great importance to the heat balance of the polar oceans: about half the total heat exchange takes place through them. Polynyas have also been called ice factories: the open water creates considerable amounts of ice, and salt rejection results. In the 1970s there was a persistent area in the Antarctic where the Weddell Sea Polynya formed which was thought to be responsible for forming deep water. This polynya has, however, been absent since the late 1970s, although the area still shows a reduced ice concentration.

Oceanographers now believe that the world ocean circulation is linked through a series of strong currents driven by deep water formation in the polar seas and heating of water in the tropical seas of the central Pacific Ocean. This circulation is called the ‘ocean conveyor belt’ or ‘thermohaline circulation’. (‘Thermohaline’ because the circulation is forced by heat and salt.) The formation of sea ice and salt rejection are probably the principal force driving the deep water formation and the cold end of the conveyor belt, and are also thought to be important in the exchange and absorption of carbon dioxide from the atmosphere to the ocean. In the Greenland Sea the ocean loses heat to the atmosphere, rapidly forming a distinctive sea-ice feature in the central Greenland Sea called the ‘Odden’ (Norwegian for headland). The associated salt rejection from the Odden can drive deep convection. Over thousands of years this continued forcing circulates the cold dense waters of the Arctic Ocean to the Pacific Ocean via deep currents through the Atlantic and Indian Ocean, returning warm waters to the Arctic. There is much evidence from ice cores and pollen records to suggest that there have been rapid variations in the Earth's climate of several degrees Celsius in fifty or so years in the recent geological past, which have been linked to changes in the pathways of the conveyor belt or even in the modes of the conveyor-belt circulation. These changes could have been initiated by modifications in the sea-ice cover.

The ice extent in the global ocean would appear to be fairly consistent from year to year. There are, however, regional differences. The Arctic sea ice extent has decreased, whereas the Antarctic sea ice extent shows no statistically significant change. Because of the complex link between ocean currents, ice formation, and deep convection, the key issue in any reduction in ice extent is the location.

The Odden ice tongue has been known to exist in the Greenland Sea for at least the past two hundred years from whaling records and has been the subject of much recent research. Peter Wadhams of the Scott Polar Research Institute has alerted the scientific community to the reduction in size of the Odden ice tongue over the last 10 years and its total absence in 1994 and 1995. This reduction has in turn been linked to the depth of convection and resulting deep-water formation in the Greenland Sea; whereas convection once reached down to almost 4 km it now extends down only to 1 km, and there has been no formation of Greenland Sea Deep Water since at least 1980. The reduction of deep water formation may not necessarily mean that the conveyor belt has stopped at the polar end. What it is important to remember is that although we can measure sea ice extent and the changes relatively easily we cannot measure the volume; changes in sea ice extent could be linked to as yet undetermined, but more important, thickness changes in the global sea ice cover.

Mark A. Brandon