Climatic changes were most clearly expressed in periodically occurring ice ages, which had a significant impact on the transformation of the land surface located under the body of the glacier, water bodies and biological objects found in the zone of influence of the glacier.
According to the latest scientific data, the duration of glacial eras on Earth is at least a third of the total time of its evolution over the past 2.5 billion years. And if we take into account the long initial phases of the origin of glaciation and its gradual degradation, then the eras of glaciation will take almost as much time as warm, ice-free conditions. The last of the ice ages began almost a million years ago, in Quaternary time, and was marked by the extensive spread of glaciers - the Great Glaciation of the Earth. The northern part of the North American continent, a significant part of Europe, and possibly also Siberia were under thick covers of ice. In the Southern Hemisphere, the entire Antarctic continent was under ice, as it is now.
The main causes of glaciations are:
space;
astronomical;
geographical.
Space groups of reasons:
change in the amount of heat on Earth due to the passage of the Solar system 1 time/186 million years through the cold zones of the Galaxy;
change in the amount of heat received by the Earth due to a decrease in solar activity.
Astronomical groups of reasons:
change in pole position;
the inclination of the earth's axis to the ecliptic plane;
change in the eccentricity of the Earth's orbit.
Geological and geographical groups of reasons:
climate change and the amount of carbon dioxide in the atmosphere (increase in carbon dioxide - warming; decrease - cooling);
changes in the directions of ocean and air currents;
intensive process of mountain building.
The conditions for the manifestation of glaciation on Earth include:
snowfall in the form of precipitation under low temperature conditions with its accumulation as material for glacier growth;
negative temperatures in areas where there is no glaciation;
periods of intense volcanism due to the huge amount of ash emitted by volcanoes, which leads to a sharp decrease in the flow of heat (sun rays) to the earth's surface and causes a global decrease in temperatures by 1.5-2ºC.
The most ancient glaciation is the Proterozoic (2300-2000 million years ago) in South Africa, North America, and Western Australia. In Canada, 12 km of sedimentary rocks were deposited, in which three thick strata of glacial origin are distinguished.
Established ancient glaciations (Fig. 23):
at the Cambrian-Proterozoic boundary (about 600 million years ago);
Late Ordovician (about 400 million years ago);
Permian and Carboniferous periods (about 300 million years ago).
The duration of ice ages is tens to hundreds of thousands of years.
Rice. 23. Geochronological scale of geological epochs and ancient glaciations
During the period of maximum expansion of the Quaternary glaciation, glaciers covered over 40 million km 2 - about a quarter of the entire surface of the continents. The largest in the Northern Hemisphere was the North American ice sheet, reaching a thickness of 3.5 km. All of northern Europe was under an ice sheet up to 2.5 km thick. Having reached their greatest development 250 thousand years ago, the Quaternary glaciers of the Northern Hemisphere began to gradually shrink.
Before the Neogene period, the entire Earth had an even, warm climate; in the area of the islands of Spitsbergen and Franz Josef Land (according to paleobotanical finds of subtropical plants), there were subtropics at that time.
Reasons for climate change:
the formation of mountain ranges (Cordillera, Andes), which isolated the Arctic region from warm currents and winds (mountain rise by 1 km - cooling by 6ºС);
creation of a cold microclimate in the Arctic region;
cessation of heat flow into the Arctic region from warm equatorial regions.
By the end of the Neogene period, North and South America connected, which created obstacles to the free flow of ocean waters, as a result of which:
equatorial waters turned the current to the north;
the warm waters of the Gulf Stream, cooling sharply in the northern waters, created a steam effect;
large amounts of precipitation in the form of rain and snow increased sharply;
a decrease in temperature by 5-6ºС led to glaciation of vast territories (North America, Europe);
a new period of glaciation began, lasting about 300 thousand years (the periodicity of glaciers-interglacial periods from the end of the Neogene to the Anthropocene (4 glaciations) is 100 thousand years).
Glaciation was not continuous throughout the Quaternary period. There is geological, paleobotanical and other evidence that during this time glaciers completely disappeared at least three times, giving way to interglacial eras when the climate was warmer than today. However, these warm eras were replaced by cold snaps, and the glaciers spread again. Currently, the Earth is at the end of the fourth epoch of Quaternary glaciation, and, according to geological forecasts, our descendants in a few hundred to thousand years will again find themselves in ice age conditions, not warming.
The Quaternary glaciation of Antarctica developed along a different path. It arose many millions of years before glaciers appeared in North America and Europe. In addition to the climatic conditions, this was facilitated by the high continent that had existed here for a long time. Unlike the ancient ice sheets of the Northern Hemisphere, which disappeared and then reappeared, the Antarctic ice sheet changed little in its size. The maximum glaciation of Antarctica was only one and a half times larger in volume than the modern one and not much larger in area.
The culmination of the last ice age on Earth was 21-17 thousand years ago (Fig. 24), when the volume of ice increased to approximately 100 million km 3. In Antarctica, glaciation at this time covered the entire continental shelf. The volume of ice in the ice sheet apparently reached 40 million km 3, that is, it was approximately 40% more than its modern volume. The pack ice boundary shifted northward by approximately 10°. In the Northern Hemisphere, 20 thousand years ago, a gigantic Pan-Arctic ancient ice sheet formed, uniting the Eurasian, Greenland, Laurentian and a number of smaller shields, as well as extensive floating ice shelves. The total volume of the shield exceeded 50 million km 3, and the level of the World Ocean dropped by no less than 125 m.
The degradation of the Panarctic cover began 17 thousand years ago with the destruction of the ice shelves that were part of it. After this, the “sea” parts of the Eurasian and North American ice sheets, which had lost stability, began to collapse catastrophically. The collapse of glaciation occurred in just a few thousand years (Fig. 25).
At that time, huge masses of water flowed from the edge of the ice sheets, giant dammed lakes arose, and their breakthroughs were many times larger than today. Natural processes dominated in nature, immeasurably more active than now. This led to a significant renewal of the natural environment, a partial change in the animal and plant world, and the beginning of human domination on Earth.
The last retreat of glaciers, which began over 14 thousand years ago, remains in human memory. Apparently, it is the process of melting glaciers and rising water levels in the ocean with extensive flooding of territories that is described in the Bible as a global flood.
12 thousand years ago, the Holocene began - the modern geological era. Air temperature in temperate latitudes increased by 6° compared to the cold late Pleistocene. Glaciation has taken on modern proportions.
In the historical era - for about 3 thousand years - the advance of glaciers occurred in separate centuries with lower air temperatures and increased humidity and were called little ice ages. The same conditions developed in the last centuries of the last era and in the middle of the last millennium. About 2.5 thousand years ago, a significant cooling of the climate began. The Arctic islands were covered with glaciers; in the Mediterranean and Black Sea countries, on the verge of a new era, the climate was colder and wetter than it is now. In the Alps in the 1st millennium BC. e. glaciers moved to lower levels, blocked mountain passes with ice and destroyed some high-lying villages. This era saw a major advance of the Caucasian glaciers.
The climate was completely different at the turn of the 1st and 2nd millennia AD. Warmer conditions and the absence of ice in the northern seas allowed northern European sailors to penetrate far to the north. In 870, the colonization of Iceland began, where there were fewer glaciers at that time than now.
In the 10th century, the Normans, led by Eirik the Red, discovered the southern tip of a huge island, the shores of which were overgrown with thick grass and tall bushes, they founded the first European colony here, and this land was called Greenland, or “green land” (which is by no means now talk about the harsh lands of modern Greenland).
By the end of the 1st millennium, mountain glaciers in the Alps, the Caucasus, Scandinavia and Iceland had also retreated significantly.
The climate began to change seriously again in the 14th century. Glaciers began to advance in Greenland, summer thawing of soil became increasingly short-lived, and by the end of the century permafrost was firmly established here. The ice cover of the northern seas increased, and attempts made in subsequent centuries to reach Greenland by the usual route ended in failure.
Since the end of the 15th century, the advance of glaciers began in many mountainous countries and polar regions. After the relatively warm 16th century, harsh centuries began, called the Little Ice Age. In the south of Europe, severe and long winters often recurred; in 1621 and 1669, the Bosphorus Strait froze, and in 1709, the Adriatic Sea froze along the shores.
In the second half of the 19th century, the Little Ice Age ended and a relatively warm era began, which continues to this day.
Rice. 24. Boundaries of the last glaciation
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Rice. 25. Scheme of glacier formation and melting (along the profile of the Arctic Ocean - Kola Peninsula - Russian Platform)
The last ice age ended 12,000 years ago. During the most severe period, glaciation threatened man with extinction. However, after the glacier disappeared, he not only survived, but also created a civilization.
Glaciers in the history of the Earth
The last glacial era in the history of the Earth is the Cenozoic. It began 65 million years ago and continues to this day. Modern man is lucky: he lives in an interglacial period, one of the warmest periods in the life of the planet. The most severe glacial era - the Late Proterozoic - is far behind.
Despite global warming, scientists predict the onset of a new ice age. And if the real one will come only after millennia, then the Little Ice Age, which will reduce annual temperatures by 2-3 degrees, may come quite soon.
The glacier became a real test for man, forcing him to invent means for his survival.
Last Ice Age
The Würm or Vistula glaciation began approximately 110,000 years ago and ended in the tenth millennium BC. The peak of cold weather occurred 26-20 thousand years ago, the final stage of the Stone Age, when the glacier was at its largest.
Little Ice Ages
Even after the glaciers melted, history has known periods of noticeable cooling and warming. Or, in another way - climate pessimums And optimums. Pessimums are sometimes called Little Ice Ages. In the XIV-XIX centuries, for example, the Little Ice Age began, and during the Great Migration of Nations there was an early medieval pessimum.
Hunting and meat food
There is an opinion according to which the human ancestor was more of a scavenger, since he could not spontaneously occupy a higher ecological niche. And all known tools were used to cut up the remains of animals that were taken from predators. However, the question of when and why people began to hunt is still a matter of debate.
In any case, thanks to hunting and meat food, ancient man received a large supply of energy, which allowed him to better endure the cold. The skins of killed animals were used as clothing, shoes and walls of the home, which increased the chances of survival in the harsh climate.
Upright walking
Upright walking appeared millions of years ago, and its role was much more important than in the life of a modern office worker. Having freed his hands, a person could engage in intensive housing construction, clothing production, processing of tools, production and preservation of fire. The upright ancestors moved freely in open areas, and their life no longer depended on collecting the fruits of tropical trees. Already millions of years ago, they moved freely over long distances and obtained food in river drains.
Upright walking played an insidious role, but it still became more of an advantage. Yes, man himself came to cold regions and adapted to life in them, but at the same time he could find both artificial and natural shelters from the glacier.
Fire
Fire in the life of ancient man was initially an unpleasant surprise, not a blessing. Despite this, the human ancestor first learned to “extinguish” it, and only later use it for his own purposes. Traces of the use of fire are found in sites that are 1.5 million years old. This made it possible to improve nutrition by preparing protein foods, as well as to remain active at night. This further increased the time to create survival conditions.
Climate
The Cenozoic Ice Age was not a continuous glaciation. Every 40 thousand years, the ancestors of people had the right to a “respite” - temporary thaws. At this time, the glacier was retreating and the climate became milder. During periods of harsh climate, natural shelters were caves or regions rich in flora and fauna. For example, the south of France and the Iberian Peninsula were home to many early cultures.
The Persian Gulf 20,000 years ago was a river valley rich in forests and grassy vegetation, a truly “antediluvian” landscape. Wide rivers flowed here, one and a half times larger in size than the Tigris and Euphrates. The Sahara in certain periods became a wet savannah. The last time this happened was 9,000 years ago. This can be confirmed by rock paintings that depict an abundance of animals.
Fauna
Huge glacial mammals, such as bison, woolly rhinoceros and mammoth, became an important and unique source of food for ancient people. Hunting such large animals required a lot of coordination and brought people together noticeably. The effectiveness of “teamwork” has proven itself more than once in the construction of parking lots and the manufacture of clothing. Deer and wild horses enjoyed no less “honor” among ancient people.
Language and communication
Language was perhaps the main life hack of ancient man. It was thanks to speech that important technologies for processing tools, making and maintaining fire, as well as various human adaptations for everyday survival were preserved and passed on from generation to generation. Perhaps the details of hunting large animals and migration directions were discussed in Paleolithic language.
Allörd warming
Scientists are still arguing whether the extinction of mammoths and other glacial animals was the work of man or caused by natural causes - the Allerd warming and the disappearance of food plants. As a result of the extermination of a large number of animal species, people in harsh conditions faced death from lack of food. There are known cases of the death of entire cultures simultaneously with the extinction of mammoths (for example, the Clovis culture in North America). However, warming became an important factor in the migration of people to regions whose climate became suitable for the emergence of agriculture.
Many of us believe that the Ice Age ended a long time ago and no traces of it remain. But geologists say we are only approaching the end of the Ice Age. And the people of Greenland are still living in the Ice Age.
Approximately 25 thousand years ago, the peoples who inhabited the central part of North America saw ice and snow all year round. A huge wall of ice stretched from the Pacific to the Atlantic Ocean, and north to the Pole. This was during the final stages of the Ice Age, when all of Canada, most of the United States and northwestern Europe were covered in a layer of ice more than one kilometer thick.
But this does not mean that it was always very cold. In the northern part of the United States, temperatures were only 5 degrees lower than today. The cold summer months caused an ice age. At this time, the heat was not enough to melt the ice and snow. It accumulated and eventually covered the entire northern part of these areas.
The Ice Age consisted of four stages. At the beginning of each of them, ice formed moving south, then melted and retreated to the North Pole. This happened, it is believed, four times. Cold periods are called “glaciations”, warm periods are called “interglacial” periods. The first stage in North America is thought to have begun about two million years ago, the second about 1,250,000 years ago, the third about 500,000 years ago, and the last about 100,000 years ago.
The rate of ice melting during the last stage of the Ice Age was different in different areas. For example, in the area where the modern state of Wisconsin is located in the USA, the melting of ice began approximately 40,000 years ago. The ice that covered the New England region of the United States disappeared about 28,000 years ago. And the territory of the modern state of Minnesota was freed by ice only 15,000 years ago!
In Europe, Germany became ice-free 17,000 years ago, and Sweden only 13,000 years ago.
Why do glaciers still exist today?
The huge mass of ice that began the Ice Age in North America was called the “continental glacier”: in the very center its thickness reached 4.5 km. This glacier may have formed and melted four times during the entire Ice Age.
The glacier that covered other parts of the world did not melt in some places! For example, the huge island of Greenland is still covered by a continental glacier, except for a narrow coastal strip. In its middle part, the glacier sometimes reaches a thickness of more than three kilometers. Antarctica is also covered by an extensive continental glacier, with ice up to 4 kilometers thick in some places!
Therefore, the reason why there are glaciers in some areas of the globe is because they have not melted since the Ice Age. But the bulk of the glaciers found today were formed recently. They are mainly located in mountain valleys.
They originate in wide, gentle, amphitheatrically shaped valleys. Snow gets here from the slopes as a result of landslides and avalanches. Such snow does not melt in the summer, becoming deeper every year. Gradually, pressure from above, some thawing, and refreezing remove air from the bottom of this snow mass, turning it into solid ice. The impact of the weight of the entire mass of ice and snow compresses the entire mass and causes it to move down the valley. This moving tongue of ice is a mountain glacier.
In Europe, more than 1,200 such glaciers are known in the Alps! They also exist in the Pyrenees, the Carpathians, the Caucasus, and also in the mountains of southern Asia. There are tens of thousands of similar glaciers in southern Alaska, some 50 to 100 km long!
The last ice age led to the appearance of the woolly mammoth and a huge increase in the area of glaciers. But it was only one of many that cooled the Earth throughout its 4.5 billion years of history.
So, how often does the planet experience ice ages and when should we expect the next one?
Major periods of glaciation in the history of the planet
The answer to the first question depends on whether you are talking about large glaciations or small ones that occur during these long periods. Throughout history, the Earth has experienced five major periods of glaciation, some of which lasted for hundreds of millions of years. In fact, even now the Earth is experiencing a large period of glaciation, and this explains why it has polar ice caps.
The five main ice ages are the Huronian (2.4–2.1 billion years ago), the Cryogenian glaciation (720–635 million years ago), the Andean-Saharan glaciation (450–420 million years ago), and the Late Paleozoic glaciation (335–260 million years ago). million years ago) and Quaternary (2.7 million years ago to the present). 
These major periods of glaciation may alternate between smaller ice ages and warm periods (interglacials). At the beginning of the Quaternary glaciation (2.7-1 million years ago), these cold ice ages occurred every 41 thousand years. However, significant ice ages have occurred less frequently over the past 800,000 years—about every 100,000 years.
How does the 100,000 year cycle work?
The ice sheets grow for about 90 thousand years and then begin to melt during the 10 thousand year warm period. Then the process is repeated.
Given that the last ice age ended about 11,700 years ago, perhaps it's time for another one to begin?
Scientists believe we should be experiencing another ice age right now. However, there are two factors associated with the Earth's orbit that influence the formation of warm and cold periods. Considering also how much carbon dioxide we emit into the atmosphere, the next ice age won't start for at least 100,000 years.
What causes an ice age?
The hypothesis put forward by Serbian astronomer Milutin Milanković explains why cycles of glacial and interglacial periods exist on Earth.
As a planet orbits the Sun, the amount of light it receives from it is affected by three factors: its inclination (which ranges from 24.5 to 22.1 degrees on a 41,000-year cycle), its eccentricity (the change in the shape of its orbit around of the Sun, which fluctuates from a near circle to an oval shape) and its wobble (one complete wobble occurs every 19-23 thousand years).
In 1976, a landmark paper in the journal Science presented evidence that these three orbital parameters explained the planet's glacial cycles.
Milankovitch's theory is that orbital cycles are predictable and very consistent in the history of the planet. If the Earth is experiencing an ice age, it will be covered with more or less ice, depending on these orbital cycles. But if the Earth is too warm, no change will occur, at least in terms of increasing amounts of ice.
What can affect the warming of the planet?
The first gas that comes to mind is carbon dioxide. Over the past 800 thousand years, carbon dioxide levels have ranged from 170 to 280 parts per million (meaning that out of 1 million air molecules, 280 are carbon dioxide molecules). A seemingly insignificant difference of 100 parts per million results in glacial and interglacial periods. But carbon dioxide levels are significantly higher today than in past periods of fluctuation. In May 2016, carbon dioxide levels over Antarctica reached 400 parts per million.
The Earth has warmed up this much before. For example, during the time of dinosaurs the air temperature was even higher than it is now. But the problem is that in the modern world it is growing at a record pace because we have released too much carbon dioxide into the atmosphere in a short time. Moreover, given that the rate of emissions is not currently decreasing, we can conclude that the situation is unlikely to change in the near future.
Consequences of warming
The warming caused by this carbon dioxide will have big consequences because even a small increase in the Earth's average temperature can lead to dramatic changes. For example, the Earth was on average only 5 degrees Celsius colder during the last ice age than it is today, but this led to a significant change in regional temperatures, the disappearance of huge parts of flora and fauna, and the emergence of new species.
If global warming causes all the ice sheets of Greenland and Antarctica to melt, sea levels will rise by 60 meters compared to today's levels.
What causes major ice ages?
The factors that caused long periods of glaciation, such as the Quaternary, are not as well understood by scientists. But one idea is that a massive drop in carbon dioxide levels could lead to colder temperatures. 
For example, according to the uplift and weathering hypothesis, when plate tectonics causes mountain ranges to grow, new exposed rock appears on the surface. It easily weathers and disintegrates when it ends up in the oceans. Marine organisms use these rocks to create their shells. Over time, stones and shells take carbon dioxide from the atmosphere and its level drops significantly, which leads to a period of glaciation.
The oldest glacial deposits known today are about 2.3 billion years old, which corresponds to the lower Proterozoic geochronological scale.
They are represented by fossilized mafic moraines of the Gowganda Formation in the southeastern Canadian Shield. The presence in them of typical iron-shaped and teardrop-shaped boulders with polishing, as well as the occurrence on a bed covered with hatching, indicates their glacial origin. If the main moraine in English-language literature is denoted by the term till, then more ancient glacial deposits that have passed the stage lithification(petrification), usually called tillites. The sediments of the Bruce and Ramsay Lake formations, also of Lower Proterozoic age and developed on the Canadian Shield, also have the appearance of tillites. This powerful and complex complex of alternating glacial and interglacial deposits is conventionally assigned to one glacial era, called the Huronian.
Deposits of the Bijawar series in India, the Transvaal and Witwatersrand series in South Africa, and the Whitewater series in Australia are correlated with the Huronian tillites. Consequently, there is reason to talk about the planetary scale of the Lower Proterozoic glaciation.
As the Earth further developed, it experienced several equally large ice ages, and the closer to modern times they took place, the greater the amount of data we have about their features. After the Huronian era, the Gneissian (about 950 million years ago), Sturtian (700, perhaps 800 million years ago), Varangian, or, according to other authors, Vendian, Laplandian (680-650 million years ago), then Ordovician are distinguished (450-430 million years ago) and, finally, the most widely known Late Paleozoic Gondwanan (330-250 million years ago) glacial eras. Standing somewhat apart from this list is the Late Cenozoic glacial stage, which began 20-25 million years ago, with the appearance of the Antarctic ice sheet and, strictly speaking, continues to this day.
According to the Soviet geologist N.M. Chumakov, traces of the Vendian (Lapland) glaciation were found in Africa, Kazakhstan, China and Europe. For example, in the basin of the middle and upper Dnieper, drilling wells uncovered layers of tillites several meters thick dating back to this time. Based on the direction of ice movement reconstructed for the Vendian era, it can be assumed that the center of the European ice sheet at that time was located somewhere in the Baltic Shield region.
The Gondwana Ice Age has attracted the attention of specialists for almost a century. At the end of the last century, geologists discovered in southern Africa, near the Boer settlement of Neutgedacht, in the river basin. Vaal, well-defined glacial pavements with traces of shading on the surface of gently convex “ram foreheads” composed of Precambrian rocks. This was a time of struggle between the theory of drift and the theory of sheet glaciation, and the main attention of researchers was focused not on the age, but on the signs of the glacial origin of these formations. The glacial scars of Neutgedacht, “curly rocks” and “ram’s foreheads” were so well defined that A. Wallace, a well-known like-minded person of Charles Darwin, who studied them in 1880, considered them to belong to the last ice age.
Somewhat later, the late Paleozoic age of glaciation was established. Glacial deposits were discovered underlying carbonaceous shales with plant remains from the Carboniferous and Permian periods. In the geological literature, this sequence is called the Dvaika series. At the beginning of this century, the famous German specialist on modern and ancient glaciation of the Alps A. Penck, who was personally convinced of the amazing similarity of these deposits with young Alpine moraines, managed to convince many of his colleagues of this. By the way, it was Penkom who proposed the term “tillite”.
Permocarbonaceous glacial deposits have been found on all continents of the Southern Hemisphere. These are the Talchir tillites, discovered in India back in 1859, Itarare in South America, Kuttung and Kamilaron in Australia. Traces of the Gondwanan glaciation have also been found on the sixth continent, in the Transantarctic Mountains and the Ellsworth Mountains. Traces of synchronous glaciation in all these territories (with the exception of the then unexplored Antarctica) served as an argument for the outstanding German scientist A. Wegener in putting forward the hypothesis of continental drift (1912-1915). His rather few predecessors pointed out the similarity of the outlines of the western coast of Africa and the eastern coast of South America, which resemble parts of a single whole, as if torn in two and distant from each other.
The similarity of the Late Paleozoic flora and fauna of these continents and the commonality of their geological structure have been repeatedly pointed out. But it was precisely the idea of the simultaneous and, probably, single glaciation of all the continents of the Southern Hemisphere that forced Wegener to put forward the concept of Pangea - a great proto-continent that split into parts, which then began to drift across the globe.
According to modern ideas, the southern part of Pangea, called Gondwana, split about 150-130 million years ago, in the Jurassic and early Cretaceous periods. The modern theory of global plate tectonics, which grew out of A. Wegener’s guess, allows us to successfully explain all the currently known facts about the Late Paleozoic glaciation of the Earth. Probably, the South Pole at that time was close to the middle of Gondwana and a significant part of it was covered with a huge ice shell. Detailed facies and textural studies of tillites suggest that its feeding area was in East Antarctica and possibly somewhere in the Madagascar region. It has been established, in particular, that when the contours of Africa and South America are combined, the direction of glacial striations on both continents coincides. Together with other lithological materials, this indicates the movement of Gondwanan ice from Africa to South America. Some other large glacial streams that existed during this glacial era have also been restored.
The glaciation of Gondwana ended in the Permian period, when the proto-continent still retained its integrity. This may have been due to the migration of the South Pole towards the Pacific Ocean. Subsequently, global temperatures continued to gradually increase.
The Triassic, Jurassic and Cretaceous periods of the Earth's geological history were characterized by fairly even and warm climatic conditions over most of the planet. But in the second half of the Cenozoic, about 20-25 million years ago, the ice again began its slow advance at the South Pole. By this time, Antarctica had occupied a position close to its modern one. The movement of the fragments of Gondwana led to the fact that there were no significant areas of land left near the southern polar continent. As a result, according to the American geologist J. Kennett, a cold circumpolar current arose in the ocean surrounding Antarctica, which further contributed to the isolation of this continent and the deterioration of its climatic conditions. Near the planet's South Pole, ice from the most ancient glaciation of the Earth that has survived to this day began to accumulate.
In the Northern Hemisphere, the first signs of the Late Cenozoic glaciation, according to various experts, are between 5 and 3 million years old. It is impossible to talk about any noticeable shifts in the position of the continents over such a short period of time by geological standards. Therefore, the cause of the new ice age should be sought in the global restructuring of the energy balance and climate of the planet.
The classic region, which has been used for decades to study the history of the ice ages of Europe and the entire Northern Hemisphere, is the Alps. The proximity to the Atlantic Ocean and the Mediterranean Sea ensured a good moisture supply for the Alpine glaciers, and they sensitively responded to climate change by a sharp increase in their volume. At the beginning of the 20th century. A. Penk, having studied the geomorphological structure of the Alpine foothills, came to the conclusion that there were four major glacial epochs experienced by the Alps in the recent geological past. These glaciations were given the following names (from oldest to youngest): Günz, Mindel, Riss and Würm. Their absolute ages remained unclear for a long time.
Around the same time, information began to arrive from various sources that the lowland territories of Europe had repeatedly experienced the advance of ice. As actual position material accumulates polyglacialism(the concept of multiple glaciations) became increasingly stronger. By the 60s. century, the scheme of quadruple glaciation of the European plains, close to the Alpine scheme of A. Penck and his co-author E. Brückner, was widely recognized in our country and abroad.
Naturally, the deposits of the last ice sheet, comparable to the Würm glaciation of the Alps, turned out to be the most well studied. In the USSR it was called Valdai, in Central Europe - Vistula, in England - Devensian, in the USA - Wisconsin. The Valdai glaciation was preceded by an interglacial period, whose climatic parameters were close to modern conditions or slightly more favorable. Based on the name of the reference size in which the deposits of this interglacial were exposed (the village of Mikulino, Smolensk region) in the USSR, it was called Mikulinsky. According to the Alpine scheme, this period of time is called the Riess-Würm interglacial.
Before the beginning of the Mikulino interglacial age, the Russian Plain was covered with ice from the Moscow glaciation, which, in turn, was preceded by the Roslavl interglacial. The next step down was the Dnieper glaciation. It is considered to be the largest in size and is traditionally associated with the Rissian Ice Age of the Alps. Before the Dnieper Ice Age, the warm and humid conditions of the Likhvin interglacial existed in Europe and America. The deposits of the Likhvin era are underlain by rather poorly preserved sediments of the Oka (Mindel in the Alpine scheme) glaciation. The Dook Warm Time is considered by some researchers to be no longer an interglacial, but a pre-glacial era. But in the last 10-15 years, more and more reports have appeared about new, more ancient glacial deposits uncovered in various points of the Northern Hemisphere.
Synchronizing and linking the stages of the development of nature, reconstructed from various initial data and in different geographical locations of the globe, is a very serious problem.
Few researchers today doubt the fact of the natural alternation of glacial and interglacial eras in the past. But the reasons for this alternation have not yet been fully elucidated. The solution to this problem is hampered, first of all, by the lack of strictly reliable data on the rhythm of natural events: the stratigraphic scale of the Ice Age itself causes a large number of critical comments and so far there is no reliably verified version of it.
Only the history of the last glacial-interglacial cycle, which began after the degradation of the ice of the Ris glaciation, can be considered relatively reliably established.
The age of the Ris Ice Age is estimated at 250-150 thousand years. The Mikulin (Riess-Würm) interglacial that followed reached its optimum about 100 thousand years ago. Approximately 80-70 thousand years ago, a sharp deterioration in climatic conditions was recorded throughout the globe, marking the transition to the Würm glacial cycle. During this period, broad-leaved forests degrade in Eurasia and North America, giving way to the landscape of cold steppe and forest-steppe, and a rapid change of faunal complexes occurs: the leading place in them is occupied by cold-tolerant species - mammoth, hairy rhinoceros, giant deer, arctic fox, lemming. At high latitudes, old ice caps increase in volume and new ones grow. The water needed for their formation is draining from the ocean. Accordingly, its level begins to decrease, which is recorded along the ladder of marine terraces on the now flooded areas of the shelf and on the islands of the tropical zone. The cooling of ocean waters is reflected in the restructuring of the complexes of marine microorganisms - for example, they die out foraminifera Globorotalia menardii flexuosa. The question of how far continental ice advanced at this time remains debatable.
Between 50 and 25 thousand years ago, the natural situation on the planet again improved somewhat - the relatively warm Middle Würmian interval began. I. I. Krasnov, A. I. Moskvitin, L. R. Serebryanny, A. V. Raukas and some other Soviet researchers, although the details of their construction differ quite significantly from each other, they are still inclined to compare this period of time with an independent interglacial.
This approach, however, is contradicted by the data of V.P. Grichuk, L.N. Voznyachuk, N.S. Chebotareva, who, based on an analysis of the history of the development of vegetation in Europe, deny the existence of a large cover glacier in the early Würm and, therefore, do not see grounds for identifying the Middle Wurm interglacial epoch. From their point of view, the early and middle Wurm corresponds to a time-extended period of transition from the Mikulino interglacial to the Valdai (Late Wurm) glaciation.
In all likelihood, this controversial issue will be resolved in the near future thanks to the increasing use of radiocarbon dating methods.
About 25 thousand years ago (according to some scientists, somewhat earlier), the last continental glaciation of the Northern Hemisphere began. According to A. A. Velichko, this was the time of the most severe climatic conditions during the entire Ice Age. An interesting paradox: the coldest climate cycle, the thermal minimum of the late Cenozoic, was accompanied by the smallest area of glaciation. Moreover, this glaciation was very short in duration: having reached the maximum limits of its distribution 20-17 thousand years ago, it disappeared after 10 thousand years. More precisely, according to data summarized by the French scientist P. Bellaire, the last fragments of the European ice sheet broke up in Scandinavia between 8 and 9 thousand years ago, and the American ice sheet completely melted only about 6 thousand years ago.
The peculiar nature of the last continental glaciation was determined by nothing more than excessively cold climatic conditions. According to paleofloristic analysis data summarized by the Dutch researcher Van der Hammen and co-authors, average July temperatures in Europe (Holland) at this time did not exceed 5°C. Average annual temperatures in temperate latitudes decreased by about 10°C compared to modern conditions.
Oddly enough, excessive cold prevented the development of glaciation. Firstly, it increased the rigidity of the ice and, therefore, made it more difficult for it to spread. Secondly, and this is the main thing, the cold shackled the surface of the oceans, forming an ice cover on them that descended from the pole almost to the subtropics. According to A. A. Velichko, in the Northern Hemisphere its area was more than 2 times greater than the area of modern sea ice. As a result, evaporation from the surface of the World Ocean and, accordingly, the moisture supply of glaciers on land sharply decreased. At the same time, the reflectivity of the planet as a whole increased, which further contributed to its cooling.
The European ice sheet had a particularly poor diet. The glaciation of America, which received its nourishment from the unfrozen parts of the Pacific and Atlantic oceans, was in much more favorable conditions. This was the reason for its significantly larger area. In Europe, glaciers of this era reached 52° N. latitude, while on the American continent they descended 12° to the south.
An analysis of the history of the Late Cenozoic glaciations of the Earth’s Northern Hemisphere allowed specialists to draw two important conclusions:
1. Ice ages have occurred many times in the recent geological past. Over the past 1.5-2 million years, the Earth has experienced at least 6-8 major glaciations. This indicates the rhythmic nature of climate fluctuations in the past.
2. Along with rhythmic and oscillatory climate changes, a tendency towards directional cooling is clearly visible. In other words, each subsequent interglacial turns out to be cooler than the previous one, and the glacial eras become more severe.
These conclusions relate only to natural patterns and do not take into account the significant anthropogenic impact on the environment.
Naturally, the question arises about what prospects such a development of events promises for humanity. Mechanical extrapolation of the curve of natural processes into the future leads us to expect the beginning of a new ice age within the next few thousand years. It is possible that such a deliberately simplified approach to forecasting will turn out to be correct. In fact, the rhythm of climate fluctuations is becoming shorter and shorter and the modern interglacial era should soon end. This is also confirmed by the fact that the climatic optimum (the most favorable climatic conditions) of the post-glacial period has long passed. In Europe, optimal natural conditions occurred 5-6 thousand years ago, in Asia, according to the Soviet paleogeographer N.A. Khotinsky, even earlier. At first glance, there is every reason to believe that the climate curve is descending towards a new glaciation.
However, it is far from so simple. In order to seriously judge the future state of nature, it is not enough to know the main stages of its development in the past. It is necessary to find out the mechanism that determines the alternation and change of these stages. The temperature change curve itself cannot serve as an argument in this case. Where is the guarantee that starting tomorrow the spiral will not begin to unwind in the opposite direction? And in general, can we be sure that the alternation of glaciations and interglacials reflects some single pattern of natural development? Perhaps each glaciation separately had its own independent cause, and, therefore, there is no basis at all for extrapolating the generalizing curve into the future... This assumption looks unlikely, but it also has to be kept in mind.
The question of the causes of glaciations arose almost simultaneously with the glacial theory itself. But if the factual and empirical part of this direction of science has achieved enormous progress over the past 100 years, then the theoretical understanding of the results obtained, unfortunately, went mainly in the direction of quantitatively adding ideas that explain this development of nature. Therefore, at present there is no generally accepted scientific theory of this process. Accordingly, there is no single point of view on the principles of compiling a long-term geographical forecast. In the scientific literature one can find several descriptions of hypothetical mechanisms that determine the course of global climate fluctuations. As new material about the Earth's glacial past accumulates, a significant part of the assumptions about the causes of glaciations are discarded and only the most acceptable options remain. Probably, the final solution to the problem should be sought among them. Paleogeographical and paleoglaciological studies, although they do not provide a direct answer to the questions that interest us, nevertheless serve as practically the only key to understanding natural processes on a global scale. This is their enduring scientific significance.
