Sea ice
As sea ice is called ice frozen sea water, occasionally even the frozen waters of the polar oceans. Seasonal sea ice is also outside of the polar regions, among others, in the Baltic Sea, Scandinavian fjords, in the St. Lawrence Gulf or the Ochotskischem sea. The sea ice is part of the cryosphere our planet and covered in an average of about 6.5 percent ( equivalent to an area of 22.5 million km2 ) of the world's oceans. The polar sea ice and polar ice sheets of the mainland together form the polar ice. Sea ice plays a crucial role in the climate system of the earth due to the primarily caused by the ice - albedo feedback polar amplification.
- 5.1 History during the last glacial period
- 5.2 presence
Sea ice types
Sea ice has a great wealth of different forms, which are strongly influenced by the sea. On moving ocean surface is first formed Frazil - ice, which are fine, up to 2 cm ice needles or platelets, which condense to suppenartigem ice mud. With further growth then arises pancake ice, a layer usually circular, up to 3 m large pieces of ice with beaded edge. Without swell new ice can occur in the form of a closed ice cover ( nilas ). The ice is thicker especially by the freezing of water below the ice.
In general, the ice reaches to the end of the freezing period, a thickness of up to 2 m, and then forms one-year ice. From a thickness of about one meter isolated a sea ice covers the water under it so far that it no longer freezes. Sea ice thickness then decreases to mainly because that ice floes are pushed together. This is especially true for the most multi-year pack ice. By superimposing pushing the ice several meters high hummocks in the ice can occur. A lower percentage of perennial ice, as observed since the nineties has been accompanied by greater seasonal variations in sea ice cover.
Almost all of the sea ice of Antarctica is annual. It is located at a lower latitude and melts largely on prevailing milder water temperatures in the summer. However, parts of the Arctic sea ice are perennial in higher latitudes, they thaw in the Arctic summer is not complete but only when they transported the ice drift to lower latitudes.
Fast ice is on coastlines or on the seabed anchored ice so not free floating on the sea surface. Pack ice is often not anchored to the mainland and therefore may be subject to an ice drift. Drift ice consists of ice floes that have broken away from an ice sheet.
A natural ice-free area, but this is completely surrounded by sea ice, ie polynya. Artificial whipped into the ice trays and holes are called Wuhnen.
Not for the sea ice to the one created by glacial river ice shelf and icebergs.
Properties
The salt of the sea water ( about 35 parts per thousand salinity) lowers its freezing down to about -1.9 ° C. It is not incorporated into the crystal lattice at the ice growth of the ice, but remains partly in the surrounding water, it forms part of the brine bags in ice. The salinity of the sea ice is only about three to five per thousand. (See also: ice flower. )
At first missing convection ice formation thus leads to an increase in the salinity ( salt content ), and thereby also the density of the surrounding water. This can lead to destabilization of the density stratification and convection ( thermohaline circulation). The thermohaline circulation is essential for the formation of deep water and thus for the entire ocean circulation. The melting of sea ice, however, acts as a record of freshwater in the upper ocean, which stabilizes the stratification and counteracts convection.
The ice movement, driven by wind and ocean currents, is accompanied by a transport of freshwater and negative latent heat. Sea ice hampered the exchange of latent and sensible heat between the ocean and atmosphere. Even a thin ice cover prevents the heat flux almost completely. Where the ice cover is not completely closed, the heat output may take several hundred watts per square meter.
The mostly snow-covered sea ice is characterized by a very high reflectivity ( albedo) from for sunlight. From the ice-free ocean, much of the short-wave radiation is absorbed, reflected on the sea ice, however. This self-reinforcing feedback, the ice - albedo feedback, influenced very much the radiation balance of the polar regions and the planet as a whole.
Measurement
The remote sensing satellite sensors in the microwave range is the only way to obtain global information on the sea ice cover, and this is almost independent of light and cloud cover. Since 1979, the sea ice is measured by passive microwave sensors from satellites. It makes use of the property of the ice advantage to emit microwave radiation other than seawater.
Using the satellite data, sea ice and sea ice extent are calculated, which serve to estimate the actual sea ice cover. To calculate the sea ice rink is the percentage for each unit area in the satellite data, usually 25 km2 ( the sea ice concentration) estimated and calculated from it and from the Flächeinheit the entire sea ice. The extent of sea ice, however, is the sum of the surface units in which the estimated sea ice share a threshold, usually exceeds 15%. Since surface changes, such as snow cover or meltwater puddles that make it difficult to estimate exactly how sea ice concentration, sea ice extent is the consistent level.
Other important dimensions are the thickness of sea ice, which one. Using satellite missions such as ICESat or CryoSat -2, determined at random from the surface or undersea with sonar, and the sea ice volume, calculated from sea ice concentration and thickness
To verify the satellite data sheets of ice can also be measured directly. Used for this purpose by the ship or hand towed sensors, ice cores, and a ruler. With a few parameters can be an ice floe describe in detail ( ice thickness, draft, freeboard, depth of meltwater ponds, thickness of snow cover ).
Sea ice in the Arctic
History
Geological data suggest that the history of Arctic sea ice is closely linked to climatic changes caused by changes in the concentrations of greenhouse gases, the Earth's orbit and the tilt of Earth's axis. Large seasonal ice sheets formed in the Arctic after the Paleocene / Eocene Thermal Maximum about 47 million years ago. First multi-year ice was about 13-14 million years ago. With the beginning of the Quaternary period, before about 3 million years ago ( see also Känozoisches Ice Age ), took to the ice cover, where it could have possibly given in interglacials seasonally ice-free periods. At the beginning of the Holocene, about 10,000 years ago, there was a period comparatively low ice cover. Since then, there have been no large, comprehensive the entire Arctic changes to the late 19th century into it.
Presence
The sea ice cover in the north varies in timing between about 15 million square kilometers in April and approximately 3.5 million square kilometers in September. Every summer dwindle or about ⅔ to ¾ of all the ice. In winter, this area then freezes again, which is a re-icing of around 10 million square kilometers. ( This corresponds to about 28 times the area of the Federal Republic of Germany. ) The ice front moves in the summer melt by several thousand kilometers north- ward and in the winter ice as far south - see illustration.
Since the beginning of satellite measurements, the extent of sea ice in the Arctic is declining. Declining trends are observed in all regions and all months, the months with the strongest decline in September, July and August, the regions with the strongest decline in the Barents and Kara Sea resources. The proportion of perennial, thick ice is plummeting. Since 1996 predominates in the time of maximum sea-ice extent, the proportion of one-year ice. The current loss of ice seems, at least compared to the last few thousand years, to be exceptional and can not be explained by natural causes of past changes. In addition to long-range oscillations such as the North Atlantic Oscillation, global warming is a cause.
See also consequences of global warming in the Arctic, Arctic ice cap.
Sea ice in the northern seas
For individual regions are also observations that go back before the satellite measurements. In the Barents Sea the average sea ice extent in April, which is the month with maximum extension, over a period of 1850-2001 with the help of Norwegian and Soviet ice charts, Norwegian and American reconnaissance and satellite data was observed from 1966. Here, a continuous decline over this period has been found.
In the northern European Arctic Ocean from 75 ° N in the last 150 years, the ice has declined by almost 30 % on average. It play both ocean currents and sea surface temperature and atmospheric effects play a role in changes in ice extent. The North Atlantic Oscillation determined very much the change in ice extent. The Norwegian Sea is frequently influenced by mostly northeastward pulling low pressure areas, while the Labrador Sea is rather influenced by north winds.
For the years 1920-1998 there were for the given area here a decrease in ice extent of about 10 % for April and about 40 % for August. At the same time the temperature rose to Spitsbergen by about 3 ° C in the spring and about 1 ° C in winter. There is an overall decline in sea ice of about 30 % of the area since 1850. In the Barents Sea the sea ice disappears almost entirely in the summer.
At the beginning of the 20th century saw a decline in ice extent, while an increase of 3 ° C and on Svalbard was even observed around 9 ° C for the Arctic winter average temperature. From 1949 until the mid- 1960s, the extent of sea ice in the Arctic Ocean grew temporarily, since the mid- 1960s, she goes back. The North Atlantic current brings warm water masses in the north. Overall, the sea surface temperature has risen by about 1 ° C over the period 1860 to 2000 ~. The ice expansion is dependent on heating of the sea water to a great extent. If atmospheric and oceanic effects simultaneously, which leads to an even larger decline or increase in ice extent. 1970 former is in connection with the global warming of the case.
Sea ice in Antarctica
History during the last glacial period
During the last glacial period (about 20,000 years ago) sea ice in Antarctica was further extended by 70 to 100% than it is today. Thus, the oceanic circulation and temperature gradients in the Southern Ocean had a different expression.
Methods
Diatoms ( diatoms ), radiolarians ( radiolarians ) and planktonic foraminifera ( chamber compacts) are one-celled animals. From the distribution of their sedimented silica or calcareous deposits on the ocean floor, the summer surface temperature and the winter and summer sea-ice extent can be determined. For this purpose, sediment cores are evaluated from the ocean floor, a time series to return to the maximum of the last glacial period ( 23000-19000 years ago) and also enable. The age of individual sediment layers in the sediment cores is determined by means of radiocarbon dating ( 14C dating) and oxygen isotope ratios.
By determining the density of the sedimented now radiolarian and diatom silicon residues in the core, the sea surface temperature can be determined. From Diatomeenverbreitung in different sediment cores in the meridional direction, the sea ice extent is determined. Below the sea ice diatoms live less, so the Diatomeenhäufigkeit is lower in sea ice-covered areas. Some species (eg Frgilariopsis obliquecostata ) occur only in very cold water temperatures ( less than -1 ° C) and their occurrence thus marks the minimum summer sea ice extent.
Compared to today
In the Antarctic winter sea ice extent ( about 39 x 106 km ²) to the maximum of the last ice age was about 70 to 100% greater than today (19 x 106 km ²). Likewise, the Antarctic Circumpolar Current was shifted by about 5-7 ° of latitude to the north, so he stretched himself up to the present Polar Front zone. Among other things, this resulted in that the summer sea surface temperature was in the Antarctic zone in the Atlantic sector of less than 1 ° C and in the Indian and Pacific sectors below 2 ° C. These values are about 3-4 ° C below the current values.
However, since the southern subtropical front had migrated northwards in the ocean only a little, this resulted in an increased thermal gradient in the Southern Ocean. Thus, the zonal water transport was faster in comparison to today, and atmospheric circulation patterns, such as the westerlies were shifted to the north.
Furthermore, the shift of the Antarctic Circumpolar North led to a weakening of the cold-water transport through the Drake Passage between South America and the Antarctic Peninsula in the Atlantic. A part of the cold water was diverted on the west coast of South America to the north. The import of warm, salty water from the Indian to the Atlantic Ocean south of Africa, however, was not blocked, but weakened. These two opposing effects of blocked cold water import and little modified hot water import in the southern Atlantic Ocean, the southern subtropical eddy compared to today was cooled only slightly. This resulted in a strong temperature gradient between subtropics in the southern Atlantic and southern polar regions.
Sea ice blocked the exchange of carbon dioxide between the atmosphere and ocean. The increased winter sea ice could be doing, along with the colder surface water temperatures, have played an important role in the decrease in atmospheric CO2 concentration of the last ice age.
The summer extent of sea ice during the Last Glacial Maximum performance is not safe reconstructions. You could have south of the Atlantic and the western Indian Ocean occasionally passed in the Southern Ocean in the field according to recent studies until the present winter sea-ice extent. However, the low incidence of Eisindikators diatoms ( Frgilariopsis obliquecostata ) could also be a summer sea-ice extent, which is not significantly greater than the currently prevailing. There are indications that (about 29000-23000 ) the summer expansion was in the millennia before but much larger. Overall, the difference in summer sea ice extent ( 5-6 x 106 km ² ) compared to today (3 x 106 km ²) is lower than in the winter sea-ice extent. This suggests an enhanced seasonality during the last ice age.
Presence
As the Arctic sea ice is also subject to the Antarctic seasonal fluctuations. Its maximum extent has it in September, at the end of the Antarctic winter, its minimum in February. Unlike in the Arctic sea ice melts in the Antarctic summer almost completely, as it is at lower latitudes. Accordingly, there is mainly from one-year ice.
In the period 1979-2006, ie since satellite measurements are available for the area of the Antarctic generally had a slightly increasing trend in sea ice extent. However, this is regional and seasonal mixed. Currently it is only in the Amundsen and Bellingshausen Sea decreasing, increasingly in other areas, if also slowed in part the increase. In the area south of the Indian Ocean, there was a reversal of the trend, with a decrease to an increase in ice cover.
The reasons for the different and sometimes increasing trend, despite increasing air and water temperatures are not finally resolved. Firstly, the decrease in the ozone layer over Antarctica (ozone hole ) circumpolar winds could have strengthened. Thus, the ice is distributed over a larger area and larger areas open sea can freeze. On the other could be a cause, according to model calculations, the decreasing due to reinforced rainfall and meltwater entry salinity of the Southern Ocean.
See also consequences of global warming in Antarctica
Compare Arctic - Antarctica
The difference in geographical location of the sea ice in the Arctic and Antarctica due to significant differences between the two regions. Arctic sea ice is located in a semi- enclosed by continents and ocean in much higher latitudes than the Antarctic sea ice that surrounds the Antarctic continent and thus geographically almost forms a contrast. Antarctic sea ice moves more freely, with higher drift velocities and has a much higher variability, as it is not surrounded by land masses. Almost the entire Antarctic sea ice may therefore have always drifting in the summer in warmer latitudes and melt. During one year, thin ice in Antarctic sea ice dominated for a long time, it predominates in the Arctic only in the past few years due to the sharp decline multi-year ice, and is doing more recently to increased variability.
During the ice during the last years and decades has declined in the Arctic, they grew in Antarctica. However, the decline of Arctic sea ice amount outweighed the growth of Antarctic sea ice significantly. Thus, the minimum sea ice extent in the Arctic was 13.0% per decade back ( absolutely almost 3.5 million km ² compared with the average from 1979 to 2010 ), during which grew by 3.2% per decade in Antarctica (absolute about 0, 68 million km ² compared with the average since 1979). In both polar regions, there is a decrease in the average sea ice thickness and sea ice volume, which, however, in 2012, 72 % below the average since 1979 is much stronger than in the Antarctic fails in the Arctic. The latter recorded for the summer months even a slight increase in ice volume, but at 160 km ³ per year increase in less than a hundredth of the volume loss accounts in the Arctic.
In 2009, said John Turner of the British Antarctic Survey, that the reason for the increasing in the Antarctic ice is to be found in the ozone hole, which made there in recent years for a cool down. He expected that this effect will last up to a decade and then there is a decrease in the amounts of ice will be observable. In addition, increased the effect of the ozone hole Antarctic storm vortex, which warmed the Antarctic Peninsula, the Ross Sea but allowed to cool. In the Arctic peninsula therefore decreased ice cover, while it increased in the Ross Sea. Several studies support the influence of changed winds on the formation of sea ice in Antarctica; this resulted in the ice cream was spread over a larger area, but also deflected to heat flow.
Arctic sea ice cover in comparison to the simulated curve as it presents itself in standard climate models. The actual melt rate exceeds the modeled melt rate significantly.
Development of the expansion of the Antarctic sea ice over the last 30 years
Ecology
The hostile at first glance, sea ice is a habitat for numerous mostly small plant and animal species, with forms of plankton dominate. It refers to those species that live in the ice or connected with the ice, as sympagisch. In sea ice, both autochthonous, ie occurring only there and there only temporarily living species occur. Sea ice provides habitats on the ice, there to press ice, the snow cover melting or water puddles, also in its interior, in brine channels, and at its bottom, where colonizing algae are food of krill. From krill hang out in the food chain directly or indirectly many animals of the arctic fauna on how crabs, fish, whales or seals. Polynyas, where seals appear to breathe, are an important hunting ground for polar bears.