Ice age
Ice Age, also short ice ages are periods of Earth's history, in which at least one pole of the earth is glaciated. Another definition is from an ice age only to talk when there are major glaciations in both hemispheres of the Earth.
After the first definition, the earth for about 30 million years ago in the current Cenozoic Ice Age; since that time, Antarctica is glaciated. According to the second definition of the current ice age began only about 2.7 million years ago when the Arctic glaciated. Thus it corresponds to the geological period Quaternary.
- 3.2.1 Earth's orbital geometry
- 3.2.2 Activity cycles of the sun
- 4.1 glaciations
- 4.2 The sea
- 4.3 Climate and atmosphere
- 4.4 lifeworld
Use of the words Ice Age and Ice Age
The term ice age is used in everyday language often used synonymously for glaciation ( glacial ). It was introduced in this sense in 1837 by Karl Friedrich Schimper and later used synonymously with Ice Age. While the term ice age is now used as a synonym for cold period, an ice age includes both the cold periods and the intervening warm periods ( interglacials ). The most recent, up to the present -reaching Cenozoic ice age covers the entire Quaternary, during the last glacial period, which is also named as Würm glaciation, in Northern Europe as Weichsel glaciation in the Alpine region, was completed in about 11,700 years. Geologically this latest ice age has not ended since the Holocene is just another interglacial period within the Cenozoic ice age, which is defined by continuous polar ice caps. In the geological terminology, the Pleistocene but is often used as a synonym for the most recent ice age, during the Holocene is referred to as " post-glacial ".
" Normal climate " ( Earth's poles unglaciated )
Interglacial
Interstadial
Glacial
Stadial
Ice age in Earth's history
Ice Age occurred in the course of the Earth on several occasions. The earth is, you look at their entire history, a largely ice-free planet on which, however, there were at intervals cold phases. During these phases, it came to the ice cover at the poles and in the mountains, as well as in Europe and North America during the last Ice Age glacier advances up to in medium widths.
The earliest phases of glaciation there was in the older Precambrian before about 2.2 billion years ago, and at its end before about 750 to 600 million years ago ( Cryogenium ). After the " Snowball Earth " hypothesis, the earth was several times almost completely covered by ice during the younger of the two. The evolution of life has been greatly compromised by the large-scale glaciation. But may have been the evolution of multicellular organisms, which began shortly after the end of the glaciation, much faster. Climate models of the early Earth support this hypothesis; However, it remains controversial.
More glaciations are known from the Ordovician and Silurian and Permo - Carboniferous to the Ice Age. Especially the southern continents ( Gondwana ) were overprinted by the ice advances of this era. Central Europe was at that time near the equator and had a tropical climate. The distributions of the deposits of this glaciation on different continents was one of the arguments for Alfred Wegener's continental drift hypothesis. A cold period in Jurassic and Cretaceous, which, inter alia, from temperature reconstructions results ( light blue bar in the figure), may have not led to an ice age because of the configuration of the continents.
Causes
The causes of the Ice Age the Earth's history have not yet been fully investigated. Especially in the older glaciation periods in the Precambrian and Paleozoic few direct observations are possible because of the comparable sparse tradition. However, it is likely that the hereinafter described with reference to the well- known and intensively studied evidence of the current ice age processes and procedures are to be applied to a similar extent to the other ice age the earth's history.
The causes of the general decline since the Paleogene are even discussed, while the short-term climate variability can be explained by periodic changes in the Earth's orbital parameters or with periodic variations in solar activity on the best currently mainly changes on Earth.
The search for the causes of the cyclically occurring cold and warm periods is still one of the challenges for paleoclimatology today. It is closely connected with the name James Croll, Milutin Milankovitch and. Both had ideas of the Frenchman Joseph- Alphonse Adhemar developed, according to which changes in the Earth's orbital geometry are responsible for recurrent cold periods.
Generally, it must adhere to that in the current state of research a single cause can not be held responsible for the changing and almost regular occurrence of glacial and Interglazialstadien in the Pleistocene. Proven by Milankovitch cycles are indeed responsible for temperature depression, but require this only a decrease of a few tenths of a degree, capture only one hemisphere and were beyond even in the Precambrian present where it now no to such a striking change from warm and cold periods came. It is also ultimately unclear why the change between warm and cold periods until about a million years to the rhythm of 41 ka ( obliquity ), then to the so-called Mid- Pleistocene Revolution but all 100 ka (apparently Eccentricity ) was performed. It is believed that the gains of the Milankovitch cycles are responsible by processes within the Earth's climate for it. Therefore, several causes of the Pleistocene exchange must be held responsible, are both exogenous and endogenous origin. The interaction of tectonic, astronomical, climatic and oceanic processes has to be strictly observed, since each process is not able to cause global ice ages.
Earthly causes
Main drive for general cooling in the Paleogene and Neogene were processes of plate tectonics, ie the displacements of the continental plates.
Opening and closure of straits
By closing or opening of the sea routes to changing critical ocean currents (and thus the heat transport ) on the earth. Thus the drift of Australia and later South America from Antarctica opened in Oligocene two straits ( Tasmanian Passage and Drake Passage ). Thus, a flow system around Antarctica has established itself. This cold current zirkumantarktische isolated Antarctica from warm surface waters completely. Antarctica cooled and the formation of an ice cap over the continent at the South Pole was started around 35 million years ago. Previously, the ocean currents around Antarctica were strongly deflected towards the equator, so that warmer water masses could reach and heat the continent.
The formation of a land bridge between North and South America prior to 4.2 to 2.4 million years ago caused the diversion of warm sea currents to the north, and thus also for the emergence of the Gulf Stream. Although the transport of warm water into the north caused first a warming of the northern hemisphere, but also provided the necessary moisture willing to give rise in Greenland, North America and Northern Europe with further cooling glacier.
Formation of high mountains
Due to the collision of plates it came from the younger Tertiary ( Neogene ) to increased orogeny. The uplift of the mainland in significant heights changed primarily large - and small-scale air currents. The formation of fold mountains such as the Alps, the Rocky Mountains or the Himalayas, which has led to the change in circulation patterns in the atmosphere, also brought the necessary moisture to the continents, which further contributed parts of the northern hemisphere to glaciation. At the same time the high mountains also preferred areas of glacier origin.
One theory sees the highlands of Tibet while in a central position, because it assumes a nearly complete glaciation of the highlands. The significant increase in the albedo ( whiteness ) in the region of the Tibetan Plateau ( 30-40 ° N ) resulted due to the radiation- location to a cooling of the atmosphere and thus to a global temperature drop of about 5 ° C. This has favored the formation of lowland glaciers in Scandinavia and North American regions, which leads to a self- reinforcing effect, whereby Tibet ice continued to grow and his Auslassgletscherzungen are flowed down through the mountain range of the plateau through to about 1000-2000 m above sea level ( glacial ). Due to the Milankovitch cycle, the temperature increased and led to an increase of the snow line of nearly 500 m - this and the glazialisostatische sinking of the plateau led by Matthias Kuhle a the melting of the lowland glaciers and Auslassgletscherzungen the Tibet ice and caused a reduction in the global albedo and thus reheating ( interglacial ). However, the closed glaciation of Tibet is partially rejected because, as an author thinks that the glacial erosion and accumulation forms extend only to higher areas of Tibet. Changes in albedo changes in the duration of snow cover in the highlands, however, are undisputed.
More earthly causes
In the Cretaceous and Paleogene there were significantly stronger than in the Neogene and Quaternary volcanism. Since an extensive release of carbon dioxide associated with each volcanic eruption, the CO2 content of the atmosphere at that time was higher. Accordingly stronger worked in the Cretaceous and Paleogene and the natural greenhouse effect of the earth.
Astronomical causes
Earth's orbital geometry
The change in the Earth's orbit geometry caused the sun, planets, moon in the system by mutual gravitational forces. To change the shape of the elliptical Earth orbit (eccentricity ) around the Sun with a period of about 100,000 years, the inclination of the Earth to orbit with a period of about 40,000 years ( obliquity of the ecliptic ), while the day - and -night match on the elliptical orbit by about 20,930 years back to the same position on the ellipse occupies ( Tropical Apsidendrehung ). By this so-called Milanković cycles, the distribution of solar energy on the earth is periodically changed.
Inspired by the German meteorologist Wladimir Peter Köppen Milutin Milankovitch formulated in 1941 in his work The Canon of Erdbestrahlung and its application to the ice age problem, the hypothesis that a cold period always occurs when the summer insolation at high northern latitudes is minimal. Cool summer are according to the Köppen ice build crucial than cold winters. So Milankovitch studied there for the causes of ice ages, where they are most evident in the high northern latitudes.
The variations in the Earth's orbital parameters ( Milankovitch cycles) were the trigger and appropriate boundary conditions, the effect was brought forward and reinforced by other factors. To be accepted as a cause for the onset of both the Antarctic as the Northern Hemisphere glaciation tectonic processes and their influence on the oceanic circulation. In addition, the CO2 content of the atmosphere has played a major role, which has a tight coupling with the temperature fluctuations, as various studies of ice cores of Antarctica and Greenland over the last 800,000 years show. Thereafter, the decrease in concentration of the greenhouse gas carbon dioxide is (along with methane and nitrous oxide ) are available for about a third of the change in temperature between hot and cold period, according to a recent publication even for half. Other positive feedback processes such as the ice - albedo feedback, the vegetation cover and the variability of the water vapor content in the atmosphere played an additional role. For the variability of the cold periods between so-called stadials and interstadials feedback effects are assumed in connection with the thermohaline circulation.
Activity cycles of the sun
In the last ice age ( Weichsel glacial period ), there were two dozen significant climate swings, which rise within just one decade, the air temperature over the North Atlantic by up to twelve degrees Celsius. This Dansgaard - Oeschger events usually occurred every 1470 years. This periodicity is trying to explain of 87 and 210 years, with an overlay of two known activity cycles of the sun. After 1470 years, the 210cc - cycle seven times, and the 86.5 he cycle is seventeen expired. In the present interglacial period ( the Holocene ) these Dansgaard - Oeschger events did not recur because the weak sun fluctuations could no longer disturb the stable Atlantic currents of the last 10,000 years.
The current ice age
Glaciations
During the cold periods of the current ice age, the inland ice and mountain glaciers spread out strong and eventually covered about 32 % of the solid surface. Today, only about 10 % of the land surface of glaciers are covered. Especially in the northern hemisphere large parts of Europe, Asia and North America were glaciated. The traces of glaciation (eg, U-shaped valleys, moraines, glacial striations, the glacial deposits ) are there everywhere today.
The change of the inland ice of Antarctica was not as dramatically during the ice age compared to the Arctic. On the one hand it is believed that this is due to the fact that the build up of ice on land and shallow Schelfen the northern hemisphere is more effective than in zirkumantarktischen ocean areas. On the other hand, Antarctica is almost completely glaciated today. An enlargement of the ice sheet was so there limited. An extension of the ice sheet is mainly attributed to the lowering of the sea level.
During the current post-glacial ( Holocene ) the extent of the glaciers fluctuated widely. After many glaciers had advanced during a cold phase towards the end of the Younger Dryas, occurred in the early Holocene to a reduction, some glaciers disappeared. This applies to the period about 7000 years ago at the height of the post-glacial ( Holocene ) for many glaciers in Iceland and probably some of the Scandinavian Peninsula. In the Alps, most of the glaciers at the time were probably less than the late 20th century. The fact that the present-day glaciers in the Alps or Scandinavia are remnants of the last glaciation, it is frequently assumed, but shall accordingly not for many, they are more than 6,000 years old. Many glaciers reached their maximum extent of several hundred years ago.
The sea level
The formation of continental ice sheets the seas has been massively deprived of water. During the height of the last Ice Age, the sea level was about 120 to 130 m lower than today. This resulted in numerous land bridges. Besides seas and shallow seas like the North Sea were partially or completely dry. Great importance was the land bridge across the Bering Strait today that connected Asia with North America. The exchange of numerous animal and plant species and the human settlement of the American continent was older theories about these land bridge.
Air and atmosphere
During the ice ages was, globally, due to lower temperatures significantly less rainfall than during warm periods. The changes in precipitation during the cold periods were regional and zonal however very different. While it was rather dry in the high and mid-latitudes, there were significant in the subtropics humid phases. The rand tropical deserts were extremely dry even at this time, while the area of the humid tropics was significantly lower at that time. The available water supply in the high and mid-latitudes, however, was during the ice ages partly higher than today, because the evaporation was significantly lower due to lower temperatures and therefore missing the forest.
The Last Glacial Maximum ( LGM ) was about 21,000 years ago. The average global temperature was about 5-6 K lower than today. Because of the gas inclusions in polar ice, we know that the atmospheric concentration of the greenhouse gases carbon dioxide (CO2), only 70% and methane ( CH4), only 50% of the pre-industrial value was (CO2 LGM: 200 ppmv pre-industrial 288 ppmv today (2011 ): 390 ppmv, CH4 in the LGM: 350 ppbv preindustrial: 750 ppbv, today: 1800 ppbv ).
During the final phases of the individual ice ages increased due to the natural increase of the sunlight out the global temperature, and this was followed, in response to this initial increase, the content of the greenhouse gases CO2 and methane. The time delay is a few hundred years. The same is true for cooling phases, where each cooling attracts a decrease in gas concentration by itself. The temperature development controls the concentrations in a clearly proportional dependence: the curves of CO2 and methane follow the temperature curve with the said time offset almost congruent. This congruence of curves over time is unique and has no discontinuities or tipping situations, so that in the period the related: Sun - Earth's temperature appears to be dominant.
However, it is also discussed that deviates from this context theory: The release of greenhouse gases caused by feedback processes to an acceleration of warming and a further release of greenhouse gases, until finally einstellten equilibrium states and both the climate and the greenhouse gas concentrations in the warm periods relatively stable remained. This mechanism of a natural warming could play a role in the current global warming, since an increase in the content of greenhouse gases due to human activity may be enhanced by this effect and the global temperature continues to rise. The radiation from the sun plays many scientists believe in the current warming of only a subordinate role.
Lifeworld
The climatic fluctuations of the Ice Age had a significant impact on the flora and fauna of their time. With the cool downs and re- warming the air to the corresponding adapted creatures were forced to shift their habitats. Numerous plant and animal species could therefore large rooms not re- colonize or died completely. This effect was in Africa and Europe, where the Mediterranean Sea and extends from east to west mountain ranges barriers to the migration of the species represented, significantly greater than in North America and East Asia.
Characteristic of the Ice Age were animals like mammoths, mastodons, saiga, saber-toothed cats, cave lions, cave bears, and other forms. Also lived Homo heidelbergensis, which grew out of it and the Neanderthals around 40,000 years ago immigrated from Africa modern humans (Homo sapiens ) during cold periods in Europe.