6. From Pleistocene to Holocene
|1. Introduction||2. The Pleistocene Ice Ages|
|3. The ice-free part of the World||4. Last Glacial Maximum|
|5. Temperature and CO2||6. Milankovic's Climate Theory|
|7. Interglacials||8. The Super volcano Toba|
Phanerozoic is the part of Earth's history, where there has been visible tangible life. It is divided into Paleozoic, Mesozoic and Cenozoic, which are popularly called the Earth's antiquity, medieval and present.
The Cenozoic is the age of the mammals. The part of the Cenozoic, where man has existed is called Quaternary, which means the fourth (age). Quaternary is composed of the periods Pleistocene and Holocene.
However, the Pleistocene lasted 2.6 million years, which is far, far more than the Holocene, which so far has only lasted about 11,000 years. Basically, the Holocene is just another inter-glacial, of which have already been many in the Pleistocene, but it is in this very special inter-glacial that human civilizations have evolved and all of the known history has taken place, and therefore we find this period in the geological and climatic history of Earth immensely important.
But, this article will deal with Pleistocene.
Pleistocene is the period in Earth's history that we commonly refer to as the Ice Age. Through much of this period, the Earth's northern and southern regions were covered by kilometer thick glaciers. It is important to recognize that the Pleistocene was a series of real ice ages, separated by relatively short interglacial periods. The Pleistocene started 2.6 million years ago and lasted until the termination of the Weichsel glaciation about 11,711 years ago.
Timeline of Earth's geological periods. Time progresses from right to left.
The glowing inferno just after Earth was formed is named Hadean. In Archean water condensed and an atmosphere of nitrogen and methane was formed together with the first rocks that we know about. In Proterozoic cyano bacteria produced oxygen, which oxidized iron and methane, at the end of the period life emerged on the seabed.
Phanerozoic represents the era in which there has been visible tangible life. It is divided in Paleozoic, Mesozoic and Cenozoic. Paleozoic was the period of early life. Mesozoic was the time of the dinosaurs, and Cenozoic is the era of mammals, which latter further is divided into Tertiary and Quaternary. Tertiary represents the age of mammals and means the third (time). Quaternary means the fourth (time) where not only mammals but also humans existed.
Pleistocene is the first and most part of Quaternary and the subject of this article.
Holocene is marked with red and is slightly visible to the left.
Timeline of Earth's past and present glacial periods. In this figure time progress from left to right. The known glacial periods in Earth's climate history are the Huronian Ice Age and the Cryogenian Ice Ages, which are Stuartian, Marinoan and Gaskiers ice ages. In Phanerozoic - which is the period of life on Earth - came first the Andean-Saharan ice age and later the Karoo ice age, the Pleistocene Ice Age is called "current".
In Earth's climate history huge glaciers had covered large parts of the northern and southern continents several times. Already at the beginning of Proterozoic occurred the prolonged Huronian Ice Age, at the end of Proterozoic occurred three ice ages, namely Stuartian, Marinoan (also known as the Varanger Ice Age after the Norwegian peninsula, where it was first detected) and Gaskiers. In Phanerozoic came first the Andean-Saharan Ice Age (also called the Hirnantian Ice Age) at the transition from Ordovician to Silurian and later the Karoo Ice Age at the transition from Carboniferous to Permian. Compared to previous glacial periods the Pleistocene Ice Age has until now only lasted a short time.
The Pleistocene ice ages was a natural continuation of the cooling of Earth that had taken place throughout Cenozoic. Time progresses from left to
The cold climate of the Pleistocene was a natural continuation of the last 55 million years falling temperatures. In particular two factors were essential for the formation of the great glaciers.
One was that the temperature dropped so much that the snow did not melt during the summer and thus could accumulate year after year.
Second was that Earth's continents were so positioned that warm ocean currents flowed against the north, and released their heat and moisture as precipitation in form of snow.
The entire Quaternary is often referred to as the ice age, because two large permanent glaciers continuously existed during the period, namely on Antarctica and Greenland. During the Pleistocene's coldest periods, which also are called ice ages, existed also enormous glaciers in Europe, North America and in Patagonia on the southern hemisphere. The shorter and warmer intervals between the recurrent Pleistocene glaciations are termed interglacials.
Over long periods of time 30% of Earth's land masses were thus covered by dazzling white ice and snow, and thereby Earth's albedo increased dramatically, and a very high proportion of solar radiation was reflected back to space, which intensified further cooling.
On geological congresses, it has been agreed that the Pleistocene ice age ended about 11,711 years ago, and we have defined that present is a brand new period, called the Holocene. But, however, we cannot define and decide us away from the fact that the present Holocene warm period is an interglacial, of which there have already been many. With high probability the glaciers will return to the northern and southern parts of the world's continents, only we do not know when.
Glaciers came and disappeared again during the Pleistocene. The period was really a series of glacials interrupted by short warming periods. There were at least 20 cycles of such advances and retreats of the ice masses. During the ice ages, the global average annual temperature was 5-10 degrees colder than today. A large part of the world's water was for long periods locked up in gigantic ice sheets. The water level in the World's oceans became very low, and the continents suffered from dust storms.
The graph shows the temperature in the Quaternary as a function of time, which progresses from right to left, from past to present. The Quaternary period is divided into Pleistocene, which is by far the largest part of The Quaternary and Holocene, which is the present since the end of last ice age.
Pleistocene lasted 2,576 thousand years, namely from 2,588,000 to 11,711 years before present when the Weichsel glaciation ended.
This graph has been prepared by manipulation of graph from Wikipedia
The scale to the left is the result of analysis of ice core drillings on the Russian Vostok station on Antarctica and shows the temperature on the surface of the ice as deviation from contemporary temperature (0), and the scale on the right shows isotope ratios, which are obtained by analysis of sediments from the seabed, as the amount of the heavy oxygen isotope oxygen-18 indicates the temperature, when the bottom layer was formed.
Note the increasingly pronounced temperature variations between actual glaciations and interglacial periods. Note also that the temperature is generally decreasing from Tertiary and down to the present day. In the first one and a half million years, a glacial cycle lasted about 41,000 years, while in the last 800,000 years glacial cycles lasted about 100,000 years each.
The dotted line at the temperature of 4 degrees below today's temperature represents the approximate temperature when the Weichsel ice age ended 11,711 years ago.
It has been defined that the Weichsel glaciation ended about 11,711 years ago, and then the temperature on the surface of the ice was about 4 degrees lower than today. If we draw a horizontal line across Pleistocene's temperature graph at 4 degrees lower than today's temperature, we will very roughly be able to separate the actual glacials from non-glacial periods by the same definition (see graph above).
Sea surface level in the World's ocean through 800,000 years - Murray-Wallace and Prazzoli have made some points showing estimates, while the other researchers have developed more or less comprehensive graphs. The assessments differ slightly, but the trend is roughly the same. In interglacial periods it was hot and some of the ice on the poles thereby melted causing increased sea surface levels. During the actual glaciations, large quantities of the planet's water were tied up as ice on the poles, and sea surface level dropped again. The difference in sea surface level between interglacial and glacials is about 120 meters. -
From "Eustatic Sea Level During Past Interglacials".
Thereby it can be seen that the temperature in the first one and a half million years of the Pleistocene was lower than today's but most of the time higher than 4 degrees below present temperature. Apart from cold periods of relatively short duration, we cannot denote the whole of the early period as an actual glaciation period out from our simple definition.
One must assume that the climate of Northern Europe by the beginning of the Pleistocene through long periods had resembled the climate of the first 1-2 thousand years of the hunter-Stone Age with temperature at the same level. The landscape was at that time characterized by an open and light birch forest, mixed with trees like aspen, willow, mountain ash and pine. The wildlife may have been something with reindeer, musk ox, bison and elk. This is just assumptions, we will never get any certain knowledge since all tracks have been erased by subsequent huge glaciers.
Reconstructed Northern European landscape from the late part of Early Pleistocene - Cold and rough, but for long periods without ice. It is believed that only about 800,000 years before present, glaciers arrived in South Scandinavia and Northern Europe.
In the early Pleistocene, it was cold, but not extremely cold. Periodically emerged relatively small glaciers, which may have been limited to northern Scandinavia, the Norwegian Mountains, northern Canada, Greenland and most likely some Arctic islands. The cycle time between cold periods and warm periods was 41,000 years. The temperature difference between cold and warm periods were about 4 degrees or less.
It can also be seen that during the last part of the early Pleistocene, i.e. between 1.8 and 0.8 million years before present, it became somewhat colder, and temperature variations between cold and warm periods became greater. More than half the time the average temperature was lower than 4 degrees below the present's and according to our rough definition real glaciation periods. In glacial periods the glacier's edge in Europe stood perhaps along the Norwegian coast and the Swedish lakes. The cycle time between cold and warm periods was still about 41,000 years.
The landscape of northern Europe may have been tundra, as today is known from northern Russia. But as mentioned above, we have no certain knowledge, since kilometer-thick glaciers have since removed all traces.
The graph shows variations in the frequency of oxygen isotopes between actual glaciations and interglacials during middle and late Pleistocene. It is compiled from analysis of oxygen isotopes in sediments on the seabed. (From Geoviden 2005, No 2 - see link below).
MIS means Marine Isotope Stages in the Pleistocene. Even numbers stand for cold periods, that is glaciations, and odd numbers stands for warm periods, that is interglacials.
Oxygen has two naturally occurring isotopes, namely, O-16 and O-18.
O-16 constitutes 99.762% and has 8 protons and 8 neutrons, in total 16. O-18 constitutes 0.2% and has 8 protons and 10 neutrons, a total of 18.
When water evaporates, water molecules, which contain the light isotope, will evaporate faster than water molecules containing the heavy isotope. Building up glaciers on the continents requires evaporation of a lot of water from the sea, which then accumulates as ice on land. The ice will be enriched with the light isotope, while the remaining water will be enriched with the heavy isotope. The ratio O-18/O-16 in seawater must, therefore, have been higher in glaciation periods than in the interglacials.
The Isotope ratio in organisms with shells (eg mussels) reflects the isotope ratio of the water at the time they lived. When they died, they sank to the bottom and became to sediments in the seabed.
In cores from the seabed, scientists can find the preserved shells, measure the isotopic ratio and thereby obtain an indication of the extent of ice on land when the organisms lived.
But 800,000 years ago the climate became sinister. The glacial cyclus time changed to 100,000 years, and the temperature difference between the actual glacials and interglacial periods increased to about 9 degrees measured on the glacier surface; at the equator, the difference was less dramatic. Kilometer thick glaciers extended far down in Europe and North America, down to 40 degrees latitude. During a great part of the last 800,000 years North America, South Scandinavia and Northern Germany were covered by ice. The Northern European ice sheet is called the Fenno-Scandinavian ice shield and the North American is called the Laurentide ice shield. Only in the short interglacial periods, the area was free of ice.
In the southern hemisphere glaciers existed in Argentina, New Zealand, Tasmania and Antarctica not to forget, besides a short-lived small glacier existed in the mountains of southern Australia.
Most believe that first in the period, called Cromer complex, the Pleistocene glaciers did reach South Scandinavia.
The various glacier advances and the associated interglacial periods are named differently in North America, Scandinavia and the Alpine area. Moreover, there seems to be disagreements about the sequence and duration of the glacier advances. Here we will stick to names used by Geological Institute of Copenhagen.
The maximum extent of the Elster, Saale and Weichsel glaciations.
At the glaciations' maximum extent they tied up large amounts of the planet's water as inland ice, and therefore sea surface level decreased up to 120 meters below today's level.
Traditionally the last 800,000 years of Pleistocene is divided into the glacial periods Cromer, Elster, Saale and Weichsel.
Cromer is named after a town in Norfolk, England. Originally it was thought that Cromer was an interglacial period, but later research has shown that the period contained 6 glacier-advances and as many interglacial periods, depending on how they are defined. How far the Cromer glacials advanced is not known in details. But at least one of Cromer's glaciations reached the river Don of Southern Russia (MIS 16).
Elster was a very hard glacial period. It is named after the city Elster of Saxony Anhalt in Germany. It lasted maybe from 480,000 to 400,000 years before present. In Europe, the ice reached to London and Slovakia. In North America, the ice reached as far south as the U.S. state of Kansas.
The Saale glaciation began about 380,000 years before present and ended 130,000 years ago. It is named after the river Saale, a tributary of the Elbe. Formerly it was assumed that the Saale was a single glaciation, but later research has shown that the period contained at least three glaciations and a similar number of interglacial periods. The Saale ice covered the entire Baltic Sea, northern Germany and the northern part of the British Isles. In Russia, the ice reached as far as the river Volga and the Ural Mountains.
Left: In North America, the Weichsel glaciers covered a much larger area than in Northern Europe. The Laurentide glacier was more than four times as large as the Fenno-Scandinavian glacier - In North America, the Weichsel glaciation is called Wisconsin.
Right: The extent of the Weichsel glaciers during Last Glacial Maximum (LGM) 18 - 20 thousand years ago - The ice covered mainly the British Isles, Scandinavia and the Baltic and the Barents Sea, whereas northern Russia was free of ice. - Svendsen 2004.
Weichsel was the last glaciation that began 117,000 years ago and ended 11,711 before present. It is named after the river Weichsel in Poland. In early Weichsel the glaciers were limited to the Scandinavian mountains and large parts of Southern Scandinavia and northern Europe was covered by tundra with a sparse growth of hardy herbs and low shrubs of dwarf birch and willow. Mammoth, woolly rhinoceros, bison, reindeer and musk ox lived on the tundra.
LGM - Last Glacial Maximum.
During its maximum 18 to 20 thousand - maybe 21 thousand - years ago the Weichsel ice covered mainly the British Isles, Scandinavia, the Baltic countries and the Barents Sea; whereas northern Russia was free of ice. The precipitation, which supplied building material to the glaciers in the form of snow, came from the Gulf Stream in the North Atlantic Ocean, and the
humidity was led into the land by the western wind. Certain places the glaciers built up to a height of 2-3 kilometers, and it is easy to imagine that when the
western wind was forced up over these "mountains", it would drop its snow there, and not much precipitation there would be left for northern Russia.
There has been found evidence of human activity in the arctic part of Russia that is dated to be 30,000-40,000 years old, probably, it originates from the Neanderthals. This indicates that there cannot have been glaciers at that time and there have not been any since.
The temperature during the ice ages relative to the present varied greatly between different places on Earth. At higher latitudes, temperature difference was far more dramatic than in areas closer to the equator. The cooling was more intense in the center of the continents than it was in coastal areas. For example, analyzes of cores from drillings in the central Greenland inland ice show that temperature during the "Last Glacial Maximum" here could be 23
degrees lower than it is today, while the temperature in the tropics around equator only was about 5 degrees lower at the same time.
Top: Pleistocene Park in eastern Siberia".
Bottom: Pleistocene Park is a nature reserve at the Kolyma river in northern Siberia, where Sergei Zimov and his colleagues are working to recreate the ecological system of the Pleistocene steppe - including wildlife in the form of reindeer, elk, wild horse, musk ox, ground squirrel, wolf, bear and many others.
It is known that the early part of the Weichsel glaciation did not represent any dramatic climate change in the region south of the Alps. A pollen-collection in South Italy at a place called the Lago Grande di Monticchio indicates that in the first half of the Weichsel glaciation 117 to 75 thousand years before present, the new ice age was hardly noticeable if there had been people there. In this location climate continued to be relatively hot until Weichel finally showed teeth with its "Last Glacial Maximum". Only a very cold period 85,000 before present lasting a few hundred years was able to put a mark on the environment of the Mediterranean.
Climate zones on mountain slopes in Pleistocene compared with the present. It was
generally colder in most of the Pleistocene, and therefore all climate zones were shifted downwards relative to present.
Pleistocene was essentially a cold period, which was reflected in the fact that
all climate zones were shifted towards equator compared to today. During Last Glacial Maximum tundra and mammoth steppe extended right down to the Alps, and the Mediterranean Sea was surrounded by a sparse stand of pine trees.
Climate zones on the mountain slopes were lower than today. In some places, the snowline could be almost 900 meters further down than in the present. In Africa, the now ailing glaciers on Mount Kenya, Kilimanjaro and on the Ruwenzori range between Uganda and Congo were greater. There were also glaciers on the mountains of Ethiopia and on the western Atlas Mountains.
Early in the Weichsel period occurred two milder periods, called Brørup and Odderade, each of which lasted about 10,000 years. They peaked almost 60,000 and 80,000 years ago. The Scandinavian ice sheet melted partially, the ice front retreated, and the sea surface level rose accordingly to 25-50 meters below today's level. The vegetation in Northern Europe and Denmark was characterized by open birch forest mixed with pine. Spruce migrated at the ends of the mild periods.
Seawater temperature variations between "Last Glacial Maximum" and present in the North Atlantic (CLIMAP 1976).
World's oceans sea surface temperatures were in average 4-5 degrees lower than
today, and the deep-sea temperature was 1-2 degrees lower than today's 2 degrees.
In some parts of the North Atlantic Sea, surface temperature was 10 degrees lower than today. There is some debate about the Pleistocene sea surface temperatures in tropical waters. Some believe that temperatures were the same as today, while others believe that they were about 2-3 degrees lower.
Many researchers believe that the overall climate of the Pacific in the Pleistocene can be characterized as a continuous El Nino.
The sun's heat is strongest around the equator. This leads to the ascending air and a constant low pressure around the equator. The trade wind is a wind that blows from the tropics of Cancer and Capricorn towards the equator from north and south seeking to fill this low pressure. Because of Earth's rotation, the trade winds are deflected making them into a north-easterly wind in the northern hemisphere and south-easterly wind in the Southern Hemisphere.
The sun heats the area at the Equator. The hot air rises, thereby forming a low pressure. Tradewinds blowing from the area between the tropics of Cancer and Capricorn seeking to fill the low pressure. The winds are deflected to the west by the Coriolis force, as the air comes from a region of lower rotational speed and is blowing against a region of higher rotation speed.
When it is not El Nino years, trade winds in the Pacific are blowing from east
to west thereby "pushing" warm surface water to the waters of Indonesia. At the same time, nutrient-rich cold bottom water rises at the Western Coast of South America. The high sea surface temperatures lead to high levels of precipitation in the Indonesian region, and the correspondingly lower sea surface temperatures at South America's west coast is causing a drier climate.
An El Nino occurs when there is a weakening of the trade winds, which equalizes the difference in water temperature between the sea at Indonesia and the sea at South America. Indonesia will not get the usual rainfall, and South America gets more torrential rain than usual, less ascending cold, nutrient-rich bottom water and thus poorer fishing.
It is the Sun's heating at the equator, which drives the trade winds and thus maintains the contemporary climate of the Pacific. It is easy to imagine that in a cold period, that Pleistocene really was, the Sun did not heat as much at the equator, as it does today, and therefore the trade-winds were less powerful, which could lead to permanent El Nino.
Australia in Pleistocene - Because of the low sea surface level in the world's oceans in Pleistocene both New Guinea and Tasmania were parts of the Australian mainland.
In Australia, occurred for long periods in the earlier parts of Pleistocene more precipitation in the inland areas than nowadays, in other periods the continent was mainly desert. In wet periods Lake Eyre covered 100,000 km2 and was surrounded by many smaller lakes. Scattered across the continent existed areas of rainforest. It is believed that the Aborigines immigration coincided with a wet period 60,000 to 40,000 years ago.
Before the last glacial maximum North Africa was in some periods greener than today, and the land was home to many species, among others many ostriches (Uriarte). The deep dry canyons, which are now called wadis, had in some periods permanent water flow. The prehistoric Niger flowed to the north and emptied in the Mediterranean.
A large part of present-day Israel and Jordan have been covered by a big lake, 250 kilometers long and 50 kilometers wide. Today it is shrunk to the Dead Sea.
A humid climate throughout most of Pleistocene has created many freshwater and brackish lakes in local depressions of present-day Syria, Iraq, Saudi Arabia, Iran and elsewhere in the now dry Middle East.
In "Great Bassin" in North America, which includes local depressions in the desert States Nevada, Utah, Oregon, California and New Mexico, were during the Weichsel period at least 68 large and small lakes. Today, only the Great Salt Lake and Pyramid Lake are left, and the former lakes have become desert.
In the Pleistocene existed Lake Bonneville, Lake Lahontan and many other lakes in the present desert states of south-western North America.
In Asia, the summer monsoon was weaker than today, but never the less there can still be found evidence of the existence of large lakes in the continent's interior during some periods of the past. The now-vanished salt lake Lop Nor in the Taklamakan desert was, for example, a big lake in some parts of the Pleistocene.
Today the jungle in Amazonas receives its rain from the trade winds that blow humidity over the land from the South Atlantic. But for long periods in the Pleistocene sea surface temperatures were lower than today, and most likely trade winds were correspondingly weaker and carried less moisture. Much of the area, which today is covered by the Amazonas jungle, was therefore at that time open grass steppe.
In Central America and in Colombia's coastal regions the tropical rainforests, in general, remained intact throughout the Pleistocene - probably due to the very high level of precipitation in these regions.
In tropical Africa, the temperature during most of the Pleistocene was about 5 degrees lower than now. Where we today find the tropical forests of the Congo and along the Guinea Bay, was then open savannah. Only along rivers and in particular humid coastal areas existed narrow strips of tropical jungle.
Landscape forms of the Peruvian-Bolivian high plateau called Altiplano shows that in the periods before about 28,000 and after 12,500 - 11,000 years before present, lakes covered areas, which was about four to six times as large as today. On this basis, one can conclude that precipitation in these periods must have been about 50 to 75% greater than today.
The Weichsel glaciation set in its final sprint about 30,000 years ago. The Period between 23,000 and 19,000 years before present is referred to as "Last Glacial Maximum" (LGM). It is estimated that about 30% of Earth's land masses were covered with ice. Glaciers could be between 1.5 and 3.0 km thick on the highest places, and thereby they tied up a very large part of the planet's water so that the sea surface level in the oceans dropped to 120 meters below today's level, some say 140 meters.
Top: Landscape types and sea surface temperatures in Europe during Last Glacial Maximum. In what is now France, Germany and Poland there was predominantly tundra. In Russia and Ukraine extended the mammoth steppe. Only along the Mediterranean and the Black Sea grew some pine forest.
Bottom: Landscape types and sea surface temperatures in North America during Last Glacial Maximum. Note that the American tundra belt was not nearly as wide as the European. South of the tundra belt was different vegetation dependent on altitude, mostly different open pine forest. Only along the Gulf of Mexico grew a real deciduous forest. In the present south-western desert, states existed great lakes.
In most places, the climate became cool and dry. Many deserts emerged and other expanded. During Last Glacial Maximum Sahara was completely barren and dry and extended hundreds of kilometers against north and south over much larger areas than today.
Thick layer of loess in China, maybe in the northern parts of the provinces of Gansu, Shaanxi or Shanxi
During Last Glacial Maximum the British Isles, the entire Baltic area, Scandinavia - except Western Jutland - were covered by ice. South and Eastern Europe was an ice cold tundra roamed by mammoths and woolly rhinos. Present southern Russia and Ukraine consisted of steppe and prairie. Only in a few narrow strips along the Mediterranean and the Black Sea grew so-called boreal forest, which is a cool, light and open pine forest mixed with birch, rowan and other shrubs and trees.
Most places became very dry. Dust storms must have been much more common during cold periods of the Pleistocene than they are now. The cold and thus dry wind, which blew down from the glaciers eroded the tundra areas and transported fine-grained material called loess to more humid, vegetation-covered regions, where it was caught and deposited. Loess-layer thickness varies widely; in some places, it is more than 100 m thick. Loess from the arctic regions was deposited in central Europe, where it formed the so-called loess belt, which stretches from the Southern Belgium to the east. In Ukraine, Southern Russia, North China and the U.S. are also large areas of loess.
Steppe-tundra also called mammoth steppe was widespread in the Pleistocene on the middle latitudes of North America and Eurasia - It is a very cold and dry
type of vegetation consisting of mostly treeless open herbal vegetation with
scattered low shrubs and occasional stunted trees in places with a little
Loess-soil is one of the world's most fertile types of arable land because loess is very nutritious and have a high water-holding capacity. At the same time, it is easy to process, but however susceptible to erosion.
A zone of permafrost stretched southward from the edge of the ice cap. In North America, the permafrost zone was around a hundred kilometers wide, and in Eurasia is was several hundred kilometers. The annual average temperature at the edge of ice cap was minus 6 degrees, and at the edge of the permafrost belt, it was 0 degrees. This should be compared with Denmark's current annual average temperature, which is 8 degrees, and with the entire Earth's annual average temperature, which today is 14 degrees.
In Asia, it was also very cold during Last Glacial Maximum, but the continents were generally not covered by glaciers in the same comprehensive way as Europe and North America, probably due to the low precipitation in the cold northern areas.
Left: The sea surface level in World's ocean in late Pleistocene - measured on Barbados.
Right: The land bridge between Asia and America during the glacial maximum is called Beringia - The low sea surface level in the world's oceans during Last Glacial Maximum allowed early humans to migrate from Eurasia to America.
As it presently still is the case, the Pamir, Tien Shan, Himalayas and other
high boundary mountains surrounding the Tibetan plateau are covered by glaciers. Experts discuss whether there may have been ice on the actual Tibetan
Plateau or perhaps part of it.
North of the Tibetan Plateau in contemporary Siberia and Mongolia was the vast mammoth steppe. It was icy cold, but also dry. The permafrost area stretched as far south as to today's Beijing.
It is believed that the climate of Asia was determined by especially two things: the Pacific and especially the Indian Ocean was warmer than the Atlantic, and the Asian mountain ranges are extending in an east-west direction, in contrast to for example the American mountain ranges, which typically extend in the north-south direction.
Indonesia during Last Glacial Maximum - Most Indonesian islands were connected to the Eurasian continent by land. The country got the name Sundaland.
The high east-west going mountain ranges forced the summer monsoon to release its humidity on the mountain slopes as rain or snow. The proximity of the relatively warm oceans caused that the snow melted during the summer, and the water was led back to the ocean by rivers. Only a little humidity slipped across the Pamir and Himalaya mountains, but nevertheless sufficient to form the local glaciers on the Central Asian mountains.
During "Last Glacial Maximum" the Pacific, after everything that is said, was warmer than the Atlantic Ocean, and the Indian Ocean was warmer than the Pacific Ocean. That was because the Atlantic was directly connected with the Arctic Ocean through the waters around Iceland and Greenland, while the Pacific did not have a corresponding connection to the Arctic Ocean because the Bering Strait had been replaced by the land bridge Beringia.
The Indian Ocean extends very much under the burning tropical sun and receives by all accounts more solar heat per area unit on average, than both the Pacific and Atlantic. But the exchange of heat between the Indian and the Pacific oceans were obstructed during Last Glacial Maximum due the emerge of Sundaland that made the Indonesian islands connected to Eurasia by land.
The monsoon is a steady wind blowing from sea to land or from land over sea due to temperature differences between land and sea.
Summer monsoon in South East Asia.
In the summer, the Sun heats up both land and sea, but the temperature of land rises faster than the temperature of the sea. Rocks and soil have a poor thermal conductivity and small heat capacity, and therefore, the temperature on land rises rapidly. However, water and sea can absorb more heat from the same solar
radiation with less rise in temperature, as water has a good thermal conductivity and a large specific heat capacity, and moreover, the heat is rapidly distributed by waves and currents in the ocean.
When land masses are heated in the summertime, the heated air ascends, and a low pressure will emerge. The warm moist air from the ocean will flow in attempting to fill this low pressure, and this wind is the monsoon. In winter sea is warmer than land and the monsoon blows then from land to sea.
Glaciers on Himalaya.
The larger land masses, which borders on the larger the oceans, the more pronounced will the monsoon winds be. In principle, there may be monsoon winds all over the world, but the East Asian monsoon is the most famous because here a very large continent borders a very large and warm ocean.
Monsoon winds have been known always in Earth's climate history. Around 300 million years ago, all Earth's continents had gathered in a single huge super-continent named Pangaea, which of course was surrounded by an equally huge ocean, namely the rest of Earth's surface. It is believed that shores of Pangaea must have been exposed to even very strong monsoon winds.
But back to the Last Glacial Maximum in the Pleistocene.
Also at that time, the summer monsoon blew warm and humid air against the south-east Asian coast, as the air was forced upwards by the mountain slopes, it poured out its moisture in the form of abundant rain and snow. But however, it was so hot in South Asia along the coasts of the Indian Ocean and the Yellow Sea that snow melted during the summer, and the melt-water returned to the sea by the many rivers. Only a bit of moisture managed to slip over the mountains and become glaciers on Central Asia's high mountains.
Pollen analyzes from many places in Australia show that during LGM the climate was extremely dry. The desert stretched as far south as northern Tasmania, and a large area with less than 2 percent vegetation covered entire South Australia. Forests were largely limited to small sheltered areas along the east coast and the extreme southwestern part of Western Australia.
Landscape forms of the Peruvian-Bolivian Altiplano plateau indicate that in periods before 28,000 years before present, lakes were about four to six times as big as today. It can then be concluded that precipitation during these periods may have been about 50 to 75% higher than it is today. During the LGM the plateau can have been dry and dusty, like so many other parts of the world.
Variations in temperature and atmospheric CO2 concentration during the last 400,000 years from analyzes of drill cores carried out on the Russian Vostok Station on Antarctica. - The temperature is in Celsius as deviation from present temperature, and the CO2 concentration is volume parts per million in absolute numbers. Time progresses from right to left. It is precisely visible that the CO2 curve is slightly behind.
Thanks to the analysis of air bubbles, that are found in drill cores from Greenland and Antarctica, we know that the atmospheric concentration of greenhouse gases such as CO2 and methane has varied over the latest part of the Pleistocene. There is a striking correlation between CO2 and temperature. When it was cold, the CO2 content of the atmosphere was fairly low, and at higher temperatures, the CO2 content was also high.
There is actually a very good correlation between CO2 and temperature, which was a key message of Al Gore's film from 2006 entitled "An Inconvenient Truth". Gore thought it was self-evident that the varying levels of greenhouse gas CO2 in the atmosphere was the cause, and the temperature variation was the effect.
Al Gore in the film "An Inconvenient Truth".
"An Inconvenient Truth" won an Oscar in 2007 for best documentary. In 2007, Al Gore together with the UN climate panel were awarded the Nobel Peace Prize for their work to raise awareness of CO2 emissions and the resulting man-made climate changes.
The good correlation between CO2 and temperature was an important argument for the campaign against, what is called the anthropogenic climate change. It was, and still is, believed that when the CO2 concentration in the atmosphere thus can cause ice ages to come and go, so would an increase in the CO2 content, created by the modern industrial society, cause an uncontrolled and disastrous temperature increase, the so-called AGW "Anthropogenic Global Warming" (anthropogenic means man-made).
However, at this time the analysis of ice cores was still in its infancy. Since then more sophisticated methods of analysis have shown that CO2 and temperature do not correlate completely over the last 800,000 years of the Pleistocene. It has been demonstrated that the atmospheric CO2 content each time was a little behind. CO2 in the atmosphere always peaked 200 to 800 years after temperature maximum. In other words, there is much to suggest that the temperature was the cause, and atmospheric CO2 content was the effect, which is the complete opposite of AGW supporters claim.
An interglacial period between two Saale glaciations of 237,500 years ago - It is obvious that the CO2 maximum is 800 years behind temperature maximum. From the BBC broadcasting "The Great Global Warming Swindle".
We must assume that during cold periods, large quantities of CO2 was dissolved in the oceans. When the temperature rose, the World's oceans could no longer contain as much CO2 and the excess was eventually released into the atmosphere. The world ocean represents a huge volume of water, and all processes take a long time. It is not unreasonable to assume that it takes hundreds of years for the sea to adjust its amount of dissolved CO2 to a new temperature.
The world oceans can dissolve large amounts of CO2. As we know from cola and soft drinks, large amounts of CO2 can be dissolved in water. A liter of cola contains more than two liters of CO2 at normal pressure and temperature. If we heat the cola, CO2 will escape as bubbles, because warm water does not dissolve as much CO2 as cold water.
The Serbian engineer Milutin Milankovitch formed during the first world war the theory that the comings and goings of ice ages are due to small cyclic variations in Earth's orbit around the Sun. A theory that today has won wide acceptance because it fits so well with the observed variations of temperature through Earth's history.
The three Milankovitch parameters precession, axis tilt and eccentricity.
Milankovitch theory is based on three basic cycles in the Earth's motion
around the Sun. They are precession, axis tilt and eccentricity. These
parameters are created by gravitational forces of the Sun, Moon, Jupiter, Venus and other planets. As we know, Earth is not completely round but slightly flattened at the poles, and therefore the other celestial bodies can exercise their gravitational force on the bulge at the equator.
The precession is the shortest Milankovitch cycle. Consider a spinning top, which is close to a standstill. In a few seconds before it topples over, it will falter, and the upper end of its rotational axis will start to describe a circle. It is called axial precession. While a spinning top precesses a full circle in less than a second, larger tops precess slower. The Earth is a very large spinning top, it precesses once every 25,772 years. The precession was mentioned already in 120 B.C. by the Greek astronomer Hipparchus, who found a difference between his own observations and earlier Babylonian records from 4,000 BC. About 12,000 years from now, the Earth's axis will point to the star Vega instead of Polaris due to precession, and the northern hemisphere will experience summer in December and winter in June.
The axis tilt is another Milankovitch parameter. We know that the Earth revolves around its own axis, which is the reason why we have night and day. The axis is not vertically on Earth's orbital plane; Today it is inclined 23.5 degrees with it's vertical, which is the cause of the seasons. But this angle varies between 22.1 degrees and 24.5 degrees over a period of approx. 41,000 years. It will, therefore, take 41,000 years for the axis to move from the minimum to the maximum position and back again. The greater the tilt is, the more pronounced the seasons will be. With a big axis tilt the Earth will experience hot summers and cold winters, and with a small axis tilt the winters are less cold and the summers less warm.
The third Milankovitch parameter is Earth's orbital eccentricity. Earth's orbit around the sun is roughly circular, but it does not form a completely perfect circle, but an ellipse with the Sun in one focus.
Eccentricity is specified as a number between 0 and 1. For a perfectly circular orbit, eccentricity is 0. The elliptical eccentricity of Earth's orbit varies between near 0 and 0.06, and back again over a period of on average 100,000 years. At the same time, the entire ellipse is turning around the Sun. Today, the eccentricity of Earth's orbit is around 0,017; which is very close to a circle.
Between 120,000 and 90,000 years ago, eccentricity was around 0.04, and the difference in the incoming effect of the Solar rays between Earth positioned
respectively in maximum and minimum distance from the Sun was approx. 14-17 % . In present times, the difference is about 7%.
But there is another factor, which must be taken into account. Namely, when Earth in its elliptical orbit is near the sun, it will move faster, than it does, when it is farthest from the Sun. Thus the incoming Solar energy per unit of time indeed will be high at the focal point near the Sun, but because of Earth's increased speed it will not stay there for so long; therefore, the total difference in incoming solar energy, received by Earth between max. and min. distance from the Sun in an orbit of maximum elliptical eccentricity will be less than the 7%, that is only approx. 0.3%. Earth's increased speed near the Sun is described by Kepler's second law on the planet's movements, namely that radius vector (the line between the sun and the earth) sweeps equal areas in equal periods.
Eccentricity of Earth's orbit. The shape of Earth's orbit varies from nearly circular to somewhat eccentric and back again over a period of about 100,000 years.
It is obvious that the Milankovitch theory applies. At the beginning of Pleistocene one glacial cycle lasted 42,000 years, which is very close to the axis tilt cycle that is 41,000 years. Only 800,000 years ago began a series of ice ages with periods of 100,000 years, which is the cycle time of the eccentricity.
Eccentricity is the only one of the three Milankovitch parameters, which gives any difference in the total amount of solar energy that Earth receives because the distance to the Sun varies. Viewed from the Sun, Earth is a disk with a diameter of about 12.760 km, and no matter in which direction and how much its rotation axis is inclined, this disk receives the same amount of solar energy, when the distance to the sun is the same. If the Northern Hemisphere receives less solar energy, the southern hemisphere receives a near equivalent more.
But the Milankovitch theory does not deal with the total amount of solar energy that Earth receives. What matters is the incoming solar energy at the critical moment at the critical point, and this is the month of June at 65 degrees northern latitude.
Milankovitch believed that the Northern Hemisphere is climatically determining in relation to the southern because the north is dominated by large land masses, on which there can be built glaciers, while the southern hemisphere is dominated by sea. Solar heat in the month of June is critical, because the summer sun should be sufficient to melt ice and snow from the winter, otherwise it will accumulate from year to year and form glaciers.
Top: The critical area around 65 degrees north latitude is greatly covered by land masses on which inland ice can built up.
Below: If sun rays are coming straight down that they do at the equator, the incoming energy per area unit will be very high. When the same sunrays hit the Earth at an angle, which they do at high latitudes, they will spread over a larger area, and thus the incoming solar energy per area unit will be less.
A belt around the Earth along the 65. latitude just south of the ice sheet,
permafrost and permanent sea ice is the critical point because if inland ice is allowed to spread in this area, it will trigger some feedback mechanisms, which will further contribute to the Earth's cooling.
A comparison between the Milankovitch theory and found isotope distributions, which indicates the actual temperature. Time progresses from the bottom to the top. The three first columns from left to right represent the theoretical effect of the three Milankovitch parameters, which are eccentricity, tilt and precession, perhaps in watts/m2 at 65 degrees north latitude in June. The "combined signal" column is the sum of the first three columns added together.
We note that "combined signal" for year 0 is zero as well, which shows that the "combined signal" column represents deviation from present.
The fifth column on the far right is the oxygen-isotope ratio found in sediments on the sea floor - which indicate the temperature, or more accurately, the amount of water bound as inland ice.
The point of the diagram is that there is a reasonably good correlation between the total theoretical Milankovitch radiation from the Sun, and the sea temperature indicated by the found isotops, which proves that the Milankovitch theory is largely true, although it is probably not the only parameter, which determines Earth's temperature variation.
Ice and snow is dazzling white, and the sunlight that hits its surface, will to great extent be reflected back into space. New snow has an albedo of 0.8-0.95, sandy soil 0.25-0.45 and a water surface 0.05-0.08. That is to say
that most of the sun rays that hit snow, will be reflected back to space, while sun rays that hit soil or water, will largely heat these bodies, and only a smaller part of the heat will be reflected back into space.
An increased area of ice and snow will contribute to cool the Earth further, which may lead to even more ice and snow, and so on. Water vapor is the main atmospheric greenhouse gas. When the planet's temperature falls, the atmosphere can no longer contain as much water vapor, which causes loss of greenhouse effect and thereby further drop in temperature, resulting in even more loss of water vapor in the atmosphere - and so on. When the temperature of the World Oceans drop, the water can dissolve more CO2 taken from the atmosphere. Thereby the atmosphere loses further greenhouse effect, which causes the temperature to drop even more - and so on.
The incoming solar energy is often called insolation, and it may be given in ,for example, Watt/m2. The curves describe the insolation at 65. degrees northern latitude, passing through Iceland, northern Scandinavia and Russia, Siberia, Alaska and northern Canada.
The calculated insolation that is received solar radiation at 65 degrees north latitude in June through past and future from
Time progresses from left to right. Time is in "kilo-years" thus to be added three zeros. 0 represents present day. The red curve represents insolation calculated for all three Milankovitch parameters. The green is the contribution from the axis tilt only.
As you can see, there does not appear to be any major changes over the next ten thousand years, then it may even be warmer - shall we believe Milankovitch theory and this calculation of insolationen.
According to some calculations of the Milankovitch parameters (see above) the future of mankind can be expected to be fairly happy, at least concerned with the climate. Astronomical calculations show that the insolation on 65 degrees northern latitude will increase gradually over the next 25,000 years. The upcoming eccentricity about for the next 100,000 years will not have a major effect. Changes in the Northern Hemisphere summer insolation will be dominated by changes in axial tilt. The Milankovitch theory tells us that interglacial periods will be ended by very deep insolation minima, which will start the glaciers; However, the next 50,000 years is not expected to show any decline in insolation on 65 degrees north latitude that can give rise to a new ice age. - But, it is almost too good to be true that the periodic alternation between glacial and short interglacial periods that have been prevalent in several million years, so convenient to be canceled.
There are a few, who expect that it will be a little cooler. In an often cited 1980 paper by Imbrie and Imbrie it is predicted that the long-term cooling trend, that began in the Stone Age about 6,000 years ago, will continue for the next 23,000 years. However, a recent report by Berger and Loutre indicates that the current warm climate may last another 50,000 years.
Temperatures established on the basis of analyzes of crystals from Devils Hole in Nebraska compared with the theoretical Milankovitch insolation. - The left
vertical axis shows the frequency of the heavy oxygen isotope in the crystals, and the vertical axis on the right shows the insolation in wat/m2. It can be seen that the correlation between temperature and insolation is significantly reduced compared with the curves from inland ice and seabed sediments.
But however, American researchers have introduced a snake in Paradise. In
a place called Devils Hole in Nebraska, they have found some crystals, whose
isotopic composition they have analyzed. Based on this analysis, they have
reconstructed past temperatures, and it has been found that the resulting
graph is quite different from the familiar graphs drawn on the basis of
drilling cores from inland ice and sediments in the seabed. The researchers even believe that the results from Devils Hole are the most accurate, we have. If this is the case, there is no longer a particularly good correlation between the Milankovitch insolation and world's temperature; and it is this
correlation, which is an important proof of the correctness of the Milankovitch theory.
Milankovitch's theory does not explain why Earth's temperature fell steadily
through 65 million years.
One can also argue that the theory does not explain why it as a whole had become so cold. The Milankovitch theory does not explain why Earth's temperature dropped from the Jurassic and Cretaceous periods' around 20 degrees to the current 14 degrees of our present interglacial period and the 5 degrees of the actual ice ages. In the Jurassic and Cretaceous Earth's orbit probably also was eccentric, and Earth's rotation axis probably showed both precession and axial tilt, yet came no ice ages through more than 200 million years. There seems to be one or more very general parameter that controls Earth's temperature and climate, and the Milankovitch parameters only overlies the dominant parameters.
If we disregard eccentricity, the theory predicts that when it is cold on the northern hemisphere, so it should be correspondingly hot in the southern hemisphere, and vice versa. Anyway, the cold should come first in the northern hemisphere. However, all studies show that glaciers in the northern and southern hemispheres were synchronous, when there was Ice Age in the northern hemisphere, there was also ice age in the southern hemisphere. There are also no indications that the ice ages started on the northern hemisphere and then spread to the south.
About 800,000 years ago a shift occurred in the dominant period length of glacial cycles from 41,000 years to 100,000 years. Milankovitch's theory offers no explanation for this because there were no significant changes in Earth's orbit parameters at this time. Moreover, the 100,000-year period correlates not so well with the mathematical predictions of insolation, as the previous 41,000 year periods do.
MIS - Marine Isotope Stages in Pleistocene. Equal numbers represent glaciations, and odd numbers represent warm periods, mostly interglacials.
The climate of the Pleistocene was characterized by a series of glaciations, where glaciers extended deep down into Europe and North America. In the first one and a half million years, the glacial cycles lasted the around 41,000 years, while in the last 800,000 years they lasted approximately 100,000 years. The glaciations were separated by relatively short lasting warming periods called interglacials so that an actual glaciation period typically lasted 90,000 years and the following interglacial period 10,000 years. The climate and duration of the interglacials varied however, some were warmer than the present and others were colder, most lasted about the ten thousand years, but some lasted nearly 25,000 years.
In the present, we live in such an interglacial period, called the Holocene. Scandinavia is an area that can be expected to be covered by ice during a coming glaciation period, as the land previously had been many times during the last 800,000 years.
Therefore we must have a strong interest in exploring how climate will evolve through the rest of our interglacial period, and when the period will finish.
MIS means "Marine Isotope Stage" and refers to a sequence of periods defined and described by isotope analyzes of drill cores from the seabed.
MIS periods are numbered so that even numbers refer to cold periods, that is glaciations, while odd numbers refer to warm periods, that is mainly interglacials. Here we will consider only the odd numbered.
Precisely when an ice age ends and an interglacial period begins and vice versa may highly depend on definitions, as temperature changes often take place gradually. There is no absolute truth. For example, if one defines the start of the Holocene at the end of Younger Dryas, then the duration be 11,700 years until now. However, if one defines the start at the beginning of the Bølling Allerød warm period, our interglacial has lasted about 14,800 years so far.
The basic timeline for MIS below is essentially taken from Wikipedia, which in turn has got them from various international databases.
Scientists distinguish interglacials, with temperatures close to today's, from other warm periods with less high temperatures, which are called interstadials. There are 103 MIS periods in the Pleistocene. Many MIS numbers have been expanded by addition of letters; for example, MIS 5, which is the Eemian interglacial, has been expanded to MIS 5a, MIS 5b, MIS 5c, MIS 5d and MIS 5e, which latter is the original Eemian interglacial.
Holocene is our present interglacial period. It started about 11,700 years ago at the end of the Weichsel glaciation. In the very early Holocene the climate in Europe was cool with an open herb and shrub vegetation and scattered birch and pine, but in the course of 1,000 years temperature rose to about 3 degrees above present temperature, and Europe became covered with deciduous forests. In the endless forests, that covered Southern Scandinavia lived European pond turtle and Dalmatian pelican, who today live only in southern Europe. After a few more thousand years world sea surface level rose about 25 meters to approximately current level. In the last 2,000-3,000 years, the climate had become colder and wetter, but with distinct fluctuations. Bronze and Middle Ages were warm periods, while the years 1600-1700 AC was a very cold period called the Little Ice Age.
Holocene is not yet completed, and it is uncertain when a new glaciation will begin.
Terraces with rice fields in the province of Yunnan in China. - Previously, most of Earth's surface was covered by forest. The trees tied up a large amount of the biosphere's carbon, and therefore the atmospheric carbon content was not very high. Now the forests largely have been burned or they have rotted away, and the carbon exists as CO2 in the atmosphere - say advocates of the "early anthropogenic hypothesis"
In 2003, the American Professor William Ruddiman proposed the "early anthropogenic (man-made) hypothesis", when he suggested that humans began altering the concentration of greenhouse gases in the atmosphere already thousands of years before the industrial era. Deforestation and intensification of rice cultivation led to increases in atmospheric CO2 and CH4 levels, he said. By comparing the natural tendencies of the three previous interglacial periods, he estimated that already in late Holocene, concentrations of these gases in the atmosphere were increased because of human activity by 35-40 ppmv (parts per million - volume) of CO2 and around 230-250 ppbv (parts per billion - volume) of CH4. The increased concentrations of greenhouse gases countered the "natural" cooling trend and thereby prevented the global climate from entering a new ice age.
The "Early anthropogenic hypothesis" came quickly under intense criticism, especially with regard to the extent to which the development of the Holocene greenhouse gas concentrations can be attributed to human activities.
Left: Findings from mammoths in Scandinavia from MIS 3 - The findings are marked with small mammoths. The squares are sites where the climate in MIS 3 has been explored, e.g. Sokli in Finland. - From "Re-dating the Pilgrimstad
Interstadial with OSL" by Helena Alexanderson, Timothy Johnsen and Andrew S.
Right: Drill core from Sokli in northern Finland - The first vertical column on the left is the age of the different layers plus minus uncertainty. The depth in meters goes without saying. The nature of the vegetation is derived from analysis of pollen in the different layers. The column on the far right is the MIS periods, numbers in parentheses are the boundaries between the various MIS - it can be seen that during most of the MIS 3, there has been tundra in Sokli. From "Present-day temperatures in northern Scandinavia during the last glaciation" by K.F. Helmens and others. See the link below.
The Weichsel Glaciation lasted about 90,000 years, but Southern Scandinavia and Northern Europe were only completely covered by ice about 10,000 years under LGM, which means "Last Glacial Maximum". In the rest of the Weichel period, northern Europe was either partially covered by ice, much of the land covered by tundra with July temperatures of 8-13 degrees, or most of the land was an open forest with birch and pine with temperatures that shortly could be pretty close to today's.
In a lake at Härnösand in middle-Sweden have been found sediments from 63 - 61,000 years before present, that is from around the start of MIS 3 that is contemporary with the Brørup interstadial. Pollen analyzes show that different kinds of pondweed grew in the lake. Along the shores grew willows and the landscape was characterized by scattered vegetation with birch, spruce and pine. It has been concluded that the average temperature for July was 10-11 degrees. Härnösand is located at 63 degrees north latitude, and it has today a July average temperature of about 15-16 degrees.
Pilgrimstad, Hærnøsand, Oulu, Sokli and Yamozero.
Another interstadial in the MIS 3 period began between 50,000 to 40,000 years before present with a 12.5 degrees temperature increase in Greenland. It made the Scandinavian ice shield to retreat to the Scandinavian mountains. The ice-free part of Scandinavia was characterized by numerous lakes, scattered permafrost, sparse tundra vegetation and possibly scattered forest of pine and birch. It has been found that the July temperature in Sokli in northern Finland were between 10 and 13 degrees; which should be compared with the area's current average July temperature of about 13 degrees. Sokli is located north of the Arctic Circle at 67 degrees northern latitude.
The Milankovitch insolation was quite favorable during this period.
In recent times, however, doubts have been raised about whether it really was so cold between Eem and Holocene. In northern Russia, just south of the Arctic Circle a drilling core has been taken from the bottom of the lake Yamozero at the Timan Mountains. It shows that in the MIS 5a, 58,000 years before present, the lake was surrounded by pine forest; on the dry and more fertile soil, a little from the lake grew deciduous trees. This indicates that summers were warmer in northern Russia than they are today. MIS 5a is also called Odderade interstadial.
No later than 40,000 before present, the ice must very quickly have spread southward to arrive at Klintholm in Denmark perhaps between 35,000 and 30,000 before present. A mammoth molar found at Pilgrimstad in Jamtland has been dated to 34 to 29,000 years before present, indicating that the country was ice-free when the mammoth lived and that the ice has spread even faster southward.
The last 150,000 years temperature derived from analysis of ice cores. 5a to 5e represent MIS 5a to MIS 5e, that are different stages in MIS 5.
The Eemian interglaciation is interesting because it is our nearest previous
interglacial period, in which we can mirror our own Holocene interglacial and seek answers to the question of, how long it will last before the glaciers come back.
The Eemian corresponds to MIS 5 that is the second last interglacial period, which has been dated to about 130,000 to 116.000 years before present. The duration of the Eemian naturally depends much on how you define the start and finish, but it is usually assumed to have lasted 11-15.000 years. It should be compared with the duration of our current interglacial, which until now has lasted about 11,711 years if one defines the start at end of Younger Dryas. If, however, one defines the start of Holocene at the beginning of the Bølling-Allerød warm period, it will have lasted about 14,800 years. This corresponds roughly to half a Milankovitch precession cycle, that is 12,886 years.
The most common duration of actual glaciations during the last million years has been 90,000 years, and the most common duration of interglacial periods have been 10,000 years. One can thus say that Emian was a relatively long interglacial period, and the Holocene is already longer than average.
Milankovitch parameters through 300,000 years. The lower green graph is the sum of the theoretical insolation contributions from axial tilt, eccentricity and precession. It is interesting that the interglacials periods appear to start when insolation peaks. Moreover, there seems to have been more insolations maxima, which were larger or same size as Holocene but did not give rise to interglacials. Interglacials seem to been finished by very low insolation bottoms.
In the Eemian interglaciation, the Milankovitch parameters favored a big difference between summer and winter, and it is therefore assumed that the temperature in the summer was several degrees higher than today. All the ice cores ever drilled in the Greenland ice sheet, which includes ice from the Eemian, have indicated temperatures higher than today's, usually in the range 3-5 degrees higher than present, the latest drilling indicated 8 degrees.
It could be so hot in the Eemian, that the surface of the Greenland inland ice began to thaw, and surface-water penetrated into the lower layers of glacial ice, which today can be seen as refrozen melt-water in the cores. That kind of refreezing of surface water has occurred very rarely in the last 5,000 years. Only in 2012 could be seen that the ice began to thaw forming surface water.
In a drilling in the seabed off the coast of Greenland pollen from pine in layers from Eemian was three times as frequent as in layers from Holocene.
But the hot period did not last very long. After less than 5,000 years temperature began to fall declining steadily during about 10,000 years down to the beginning of the Weichsel Glaciation. Sea surface level decreased, deciduous forest was gradually replaced by pine forests, which in turn was replaced by tundra.
The climate of the Eemian was generally warmer and wetter than today. Climate belts were shifted to the north. Along the Thames and Rhine lived hippos, and Europe and the Scandinavian Peninsula was covered with forest up to Nordkap. Deciduous trees as elm, hazel, hornbeam and oak grew in Europe so far north as at Oulu in northern Finland at the Gulf of Bothnia. Some sources note that linden trees grew in the Netherlands and England.
Top: The Greenland inland ice in the Eemian interglacial. Many believe that it was partially melted, and this water contributed to the higher surface level in the World's Oceans. Today the ice cap is also about 3 km. high at its highest point.
Bottom: Vegetation in the Eemian interglacial at Grande Pile in France. By steppe means mammoth steppe. Boreal forest is an open forest of pine and birch, which today is known from northern Russia. Temperate forest is deciduous, distinguishing between cold and warm temperate forest. Ognon I Stadial I, Saint Germain I1, Melisey II, Saint Germain Ic, Montaigu event, Saint Germain Ia and Melisey are different periods of warm and cold weather in the Eemian. Zeifen is a short warm period, which was a forerunner to the Eemian, in the same way as Bølling-Allerød warm period was a forerunner of the Holocene. Eemian is the traditional part of the Eemian, where the temperature was warmer than the Holocene. 5a, 5b, and so on denote MIS 5a, 5b MIS and so forth, which are sub-divisions of MIS 5. "Oxygen isotope Iberian margin" refers to analyzes of sediments from the seabed off the Spanish coast concerning the frequency of the heavy oxygen isotope, which tells how much water was bound in inland ice and thus indicates the temperature. Far right are some approximate datings. - From "Climate, vegetation and CO2 dynamics during the Eemian interglacial (MIS 5e) in Europe ", see the link below.
In North America, the forests spread north to the southern part of Baffin Island. The boundary between forest and prairie was several hundred kilometers further west than it does today.
The sea surface level was 3.5-7 meters higher than today. There are different opinions about from where all this water came. The latest estimate is that about 25% came from a partial melting of the Greenland ice sheet, and the rest came from a melting of the Antarctic Ice cap.
It was previously thought that the Eemian interglacial had a uniformly warm climate through more than 10,000 years, but however, recent research has shown that the hot weather did not last so long, and it was interrupted by several long-lasting cold periods.
The salty Eemian Sea lay where the southern Baltic is now. It had wide connections to both the Atlantic and also to the Arctic Ocean through the White Sea, thus that the Scandinavian peninsula, Finland and the Kola Peninsula was a large island. Large parts of the North European lowland was covered by a shallow sea.
In the French lake Le Bouchet, located in 1,2 km altitude in central France, scientists have found pollen, which shows that the lake was surrounded by a forest of hornbeam 240,000 years ago in the MIS 7 period. At Pianico in Northern Italy, they also found traces of a stand of hornbeam.
Interglacial periods in 800,000 years.
MIS 7 lasted only a few thousand years. It is traditionally considered as one of the shortest and coldest interglacial periods. Findings from England in layers from MIS 7 indicate a temperate and dry steppe climate. This is supported by
other English findings that come from a species of steppe elephant with straight tusks called Palaeoloxodon antiquus, as well as from several species of woolly mammoths.
A core sample from northwestern Greece shows evidence of four forested periods. The forest periods were separated by intervals with very few tree pollen, which is interpreted as an expression of relatively short cold and dry periods. Pollen distributions indicate that summers were more rainy than today, and winters were colder than in both the Eemian and Holocene. At the end of the interglacial, the climate changed to more cold and dry conditions.
In Batajnica loess area in Serbia scientists have found remains of mollusks in a layer from MIS 7, indicating a temperate dry steppe climate in mid and late of the interglacial. In a few places in the Mediterranean have been found evidence of an equatorial, that is tropical, vegetation.
Studies on Bermuda has shown signs of an increased sea surface level compared to present of about 2.5 meters, which indicates at least one warmer period.
This interglacial period was also rather short, but however warmer than the Holocene, as long as it lasted.
In early MIS 9, the forests in England consisted of oak, elm, ash and several species of the evergreen yew indicating a temperate climate. Later the forest became dominated by oak and pine.
Bones of brown bear have been found in layers from MIS 9; that is an animal, who prefers a temperate climate.
Original forests with oak and pine.
At Cudmore Grove on the coast of Essex scientists have reconstructed the climate of MIS 9 from pollen analysis. It turned out that July temperature was about 2 degrees warmer than today's temperatures in the south-east part of England, while in January the temperature was a little colder.
Studies of coastal terraces along the Thames and elsewhere have shown that the sea surface level was a few meters higher than today's level.
Some studies have shown that sea temperature around Australia was about 4 degrees warmer in MIS 9 than it is today, which made it possible for corals to create the foundation for the Great Barrier Reef.
The American Professor William Ruddiman suggested in 2007 that, among the last four interglacial periods, the MIS 9 should be considered as the closest analogue to Holocene (MIS 1) based on the phase between axial tilt and precession and the resulting strong half-yearly insolation - It's not good news for contemporary residents of Earth's northern regions, as this interglacial only lasted the usual about 10,000 years.
Holstein is interesting because it is one of the interglacial periods that resemble Holocene most in terms of Milankovitch constellations. Both today and 400,000 years ago, Earth's orbit eccentricity was small, and the orbit was and is very close to a circle.
The MIS 11 - MIS 1 analogy is not perfect. Time above represent Holocene and below Holstein. The red graphs (grey) represent MIS 1 and the black graphs MIS 11. To the left is the precession curves shifted a few thousand years, so that they almost cover each other, but this renders the axial tilt curves quite far apart. To the right, the axis tilt curves cover each other but so it goes very wrong with precession curves.
From "The MIS 11 - MIS 1 analogy, southern European vegetation, atmospheric methane and the "early anthropogenic hypothesis" by P. C. Tzedakis, E. W. Wolff and others.
Loutre and Berger believed that MIS 11 represents the closest astronomical
analog to MIS 1 (Holocene) and therefore a study of that can tell us about our climatic future and in particular answer the question about, how long Holocene will last yet.
Although similar, the insolations-graphs of the two periods are not identical. An ongoing discussion among scientists is about, whether one should emphasize the precession or the axis tilt (obliquity) in the comparison between MIS 1 and MIS 11. One must imagine that you have MIS 1's insolations-curves on a piece of clear plastic that you can put over the MIS 11 curves to make them fit. But if precession curves cover each other, the axis tilt curves become quite different and vice versa. If one puts emphasis on precession it would predict that the Holocene is fast approaching its completion, it is said; as a half Milankovitch precession cycle is 12,886 years. If one emphasizes axis inclination, it will predict that the Holocene is a double period that spans two insolations-maxima that Holstein did. And in that case people can look forward to even 10,000 happy years of sun and warm, it is said.
Loutre and Berger believe that a comparison of vegetation trends between MIS 1 and MIS 11 favors a precessional alignment, suggesting that Holocene is approaching its completion of natural causes.
Comparison of five different interglacial periods.
For each interglacial period, time progresses from right to left. Example wise the graphs of Holocene (MIS 1) starts 20,000 years before present.
a) The theoretical Milankovitch insolation (solar radiation) in June on 65. degrees north latitude in W/m2 - It appears that MIS 11 has two maxima, while all the other MIS have only one.
b) Percentage of pollen from heather, from Portugal - generally heathland areas seems to be greatest when insolation is less.
c) Percentage of pollen from trees in Portugal that can be classed to a temperate climate - It is seen that generally, forest areas seem to be greatest when insolation is greatest.
d) Atmospheric CH4 concentrations as recorded in the EDC (Edica Dome C) ice cores from Antarctica. - It is seen that the curve correlates nicely with the insolation and also with the frequency of pollen from temperate trees. Evidently, when it is hot, the biological activity is great, and thus the atmospheric CH4 is greatest. However, MIS 1 is an exception, as the graph of CH4 escapes and rises just about introduction of the agriculture around 5,000-4,000 before present (3,000 to 2,000 BC).
From "The MIS 11- MIS 1 analogy, southern European vegetation, atmospheric methane and the "early anthropogenic hypothesis" by P. C. Tzedakis, E. W. Wolff and others. See link below.
Pollen analysis have shown that the Holstein interglacial in Europe for long
periods was characterized by mixed temperate forest, which indicates a similar temperature level as in the present. In an old lake bed near Dethlingen at Luneburger Heide in northern Germany have been found pollen from the Holstein period. They show that the land was covered by temperate forest almost
throughout the period except for start and finish and in a few cold spells. The findings show that the Northern European forests consisted of pine, fir, alder and oak.
In a core taken from the seabed off Greenland was found twenty times as many pollen from fir than the amount typically found in layers from Holocene suggesting that Greenland at least in part of the Holstein period has been quite green.
The climate of Holstein interglacial was not perfectly stable, which cannot be expected of an interglacial period with several maxima.
A pollen-based reconstruction of the temperature in the Holstein interglacial in Central Europe based on pollen from different sites is pointing to July temperature of 17.5 to 19.7 degrees, which is very similar to today's temperature.
Pollen analysis from drilling in lake bed from MIS 11 at Luneburger Heide. - The depth to the left in meters. Thus the oldest layer is at the bottom. The scales at the bottom of each kind of pollen is the percentage that this species accounts of each sample taken. The bright green is trees and shrubs, the yellow are herbs and dark green is algae.
Note that the period ended rather abruptly by that the more heat demanding trees, alder and oak, declined and pine, heather and grass became more predominant. Note also a strong cold spell in around 29 meters depth, and a less marked cold period in 26 meters depth, where alder, yew, hazel and oak declined and fir, birch and grass became more dominant.
From "Vegetation dynamics and climate variability during the Holsteinian interglacial based on a pollen record from Dethlingen (northern Germany)"
Pollen analyzes from the south of Portugal shows that in the early phases of the Holstein interglacial the land was covered by an open forest consisting of juniper, pine, birch and oak. When it became warmer, a Mediterranean flora with drought-tolerant shrubs and deciduous oaks was common. Later, deciduous trees became more dominant indicating a temperate climate, and the last part of the interglaciation is characterized by heathland with conifers and an increase in the amount of herbs.
Comparison between MIS 7, MIS 13 and MIS 15 - From top to bottom the graphs show:
Atmospheric CO2 measured in Antarctic ice cores from EDC (Edica Dome C).
Deuterium (2H) in Antarctic ice core from EDC, which indicates the temperature.
The ratio of oxygen isotopes O16 and O18 in cores from the seabed - Global Stack. This ratio indicates the temperature.
The ratio of oxygen isotopes O16 and O18 in drill core from the seabed in the Indian Ocean.
The ratio of oxygen isotopes O16 and O18 in drill core from the seabed in the South Pacific.
Sea Surface Temperature.
Pollen graph from Tenaghi Philippon in northeastern Greece.
Biological Silicon from a drilling in the bottom of Lake Baikal in Siberia.
Insolation at 65 degrees north latitude in July calculated after the Milankovitch theory.
Below the separate insolation contributions from precession and axial tilt. Eccentricity is not shown.
From "Interglacial and glacial variability from the last 800 ka in marine, ice and terrestrial archives" by N. Lang, E. W. Wolff and other.
Some ancient coastlines in Alaska, Bermuda, Indonesia and the Bahamas, which lies about 20 meters above today's sea surface level, is by some scholars interpreted so that Holstein was a very hot period. Other researchers believe, however, that these high coast lines are results of land uplift.
MIS 13 was a long interglacial period with two insolation maxima. It lasted about 30,000 years. The Indian monsoon was very strong, and on the Tibetan Plateau prevailed unusually wet conditions. Drill cores from Zoige Basin in eastern Tibet indicate a hot and wet climate. Although insolation was prolonged, it was not very strong, and compared with the present time large areas must have been covered by ice through long periods.
However, recently, namely in 2008, have been demonstrated large amounts of pollen in cores taken from the seabed off the coast of Greenland. Pollen originated from second half of MIS 13. There were three times as many pollen from fir than the quantity, one finds from the Holocene. The pollen volume from MIS 13 was more than from any other interglacial period except for MIS 11, which suggests that Greenland has been relatively warm and lush - at least some of the time, and the ice sheet has likely been somewhat reduced.
Artistic reconstruction of life and landscape at Pakefield in a Cromer
interglacial period. The early humans are Homo Heidelbergensis - painted by
At Pakefield south of Lowestoft in Suffolk in England have been found remains of a rich fauna of herbivorous mammals, including voles, mice, European hamster, beaver, the now extinct giant beaver, wild boar, fallow deer, roe deer and three species of the now extinct Irish giant deer, giant elk, a now extinct species of bison, two species of horse, an extinct rhinoceros, and the greatest of all, an elephant with straight tusks, an early mammoth, that was a smaller ancestor of later ice ages' woolly mammoths. All these animals were hunted by various predators, including lion, spotted hyena, wolf, bear and not forgetting early humans, Homo heidelbergensis.
From plants, insects (especially beetles) and other fossil remains from Pakefield, one can deduce that the landscape consisted of marshy areas in a floodplain with reed beds and flooded areas with alder trees that bordered on a slow-flowing meandering river. There was plenty of aquatic plants, including white water lilies and freshwater fish such as pike, sude and roach. The temperate deciduous forest including oak, hornbeam, elm, maple. Other trees and shrubs grew on drier soils. The open natural forest alternated with open areas with grass and herbs.
The presence of a few frost-sensitive species, such as hippopotamus, water chestnut, water fern, a species of heather and certain beetles, suggesting that summers was fairly warm, and the winters were probably milder than today.
Boreal forest, which is an open cool forest with pine, birch and a few other
deciduous trees. It is assumed that in the cooler interglacials large parts of
Europe and North America have been covered by this type of forest.
MIS 15 lasted about 20,000 years but was interrupted by a very serious cold period. It is assumed that the climate at European latitudes was temperate.
The melting of inland ice occurred very slowly, and maximum sea level
materialized late in the period. The glacier-covered areas in MIS 15 were
probably greater than in the present.
Together with MIS-13 was MIS-17 the coldest interglacial period during the last 800,000 years.
Near the town of Cromer in East Anglia in England have been found a lot of bones of mammals from MIS 17, including teeth from squirrels, hamsters and voles. In the neighborhood of Cromer were also bones of many large animals that lived in the forests of the middle Pleistocene interglacials, including wild boar, fallow deer, roe deer, elk, wolf and bear, all of which are animals that usually live in an open woodland. In Denmark has been found a frontal bone from a red deer from a Cromer interglacial.
In the interglacial periods of the Cromer-complex elm, beech, yew, ivy and holly were common trees in the forest. A kind of duckweed, called Azolla, grew in small shallow lakes
PC Tzedakis writes that MIS 1 - MIS 19 is a closer and more convincing
astronomical analogy than MIS 1 - MIS 11, which leads to a significant different conclusion about the expected natural duration of our present interglacial and the scope of the human contribution to the Holocene methane content in the atmosphere.
Top: Comparison of MIS 19 and MIS 1 (Holocene). The upper horizontal time scale in red relates to MIS 1. The lower horizontal time scale in black relates to MIS 19 - both in thousand years. The top red numbers in parentheses represents the future. Graphs relating to MIS 1 are red and graphs related to MIS 19 are black.
(a) The theoretical contribution to insolation from precession.
(b) The theoretical contribution to insolation from axis tilt.
(c) The frequency of the heavy oxygen isotopes in the cores from the sea floor taken from the international database "LR04 benthic stack". This number indicates temperature. The red graph representing the MIS 1 can for natural reasons only be displayed until present (0).
(d) the frequency of heavy hydrogen, deuterium, as found in the drill core from Edica Dome C in Antarctica. This number also indicates the temperature.
(e) The concentration of atmospheric methane also from the EDC ice core.
It appears that there is an excellent correlation between parameters for the two periods.
From: "The MIS 11 - MIS 1 analogy, southern European vegetation, atmospheric methane and the early anthropogenic hypothesis" by P. C. Tzedakis
Bottom: Full respect for science, but I would suggest adding a simple engineering interpretation of the graphs.
The graphs of MIS 19 and MIS 1 are very similar, however, the MIS 1 graphs (c) and (d) are at a slightly higher level. We allow ourselves to extrapolate the red graphs a few thousand years into the future with the same slope as MIS 19; it cannot be totally wrong, as all warm periods have this shape, the heat comes quickly and fades slowly away.
MIS 1 started 11,700 years ago. We draw a vertical line down through the curves and call it Start MIS 1 .
The graphs (c) and (d) indicate the temperature. When the temperature drops below the temperature that MIS 1 started with, we can say that MIS 1 will end.
Where the vertical line Start MIS 1 , which represents 11,700 before present, intersects the temperature indicating graphs, it should be the temperature, which divides interglacials from glacial periods.
From these points of intersection, we draw two horizontal iso temperature lines, which represents same temperature.
Where the iso temperature lines intersect the extrapolated red graphs of MIS 1, it can be expected that the temperature falls below the temperature that separate interglacials from glacial periods (Ice ages).
From these two intersections, we draw two vertical lines up to the time scale at the top, which we call End MIS 1 .
We read that graph (d) indicates that MIS 1 will conclude in 5,000 years, and graph (c) indicates that MIS 1 will conclude in 12,000 years. The average is 8.500 years with a quite considerable uncertainty. So we must support Tzedakis that MIS 1 will likely end in about 10.250 years. Perhaps he is a little optimistic. We must expect that Earth's temperature will decline very slowly through all this time, overlaid by many more short-termed heating and cooling periods.
P.C. Tzedakis concludes that: "The alignment of the two interglacials
suggests that the Holocene has another quarter of an obliquity cycle to run its natural course".
The period of axial tilt is 41.000 years, and a quarter is 10.250 years. So we can look forward to more 10,000 years in sun and warm weather before the cold again turns up in earnest.
We can see that the MIS 19 graphs, which indicate temperature, have all warm periods characteristic shape, namely that the heat came quickly and then slowly vanished. The quarter of an axis tilt cycle, that MIS 19 lasted more beyond MIS 1's current age, is characterized by a steady decline in temperature during 5 to 10,000 Year - overlaid by more short-term heating and cooling periods. If the MIS 1 - MIS 19 analogy is valid, that many think it is, we may expect Holocene also evolving in this way. Very slowly, over thousands of years - unnoticed for each generation - the temperature will fall against actual glacial conditions. It can be supported by a simple engineering approach. This view is also supported by an often-cited 1980 paper by Imbrie and Imbrie, which predicts that the long-term cooling trend, that began in the Stone Age about 6,000 years ago, will continue through the next 23,000 years.
Further Tzedakist concludes: "The divergence between atmospheric methane concentrations and temperate tree populations in the late Holocene would appear to favor the view of Ruddiman (2003, 2007) that the CH4 rise after 5.000 years before present reflects anthropogenic emissions." This means support of Ruddiman's "early anthropogenic hypothesis".
It's a bit hard to see in the comparison of the graphs of MIS 1 and MIS 19, but in one of the graphs above from the same source MIS 1, MIS 5e, MIS 7e, MIS 9e and MIS 11c are compared. Here it is seen that the graphs of CH4 follow the graphs of pollen from temperate trees quite well, except for MIS 1, where the graph of CH4 escapes and rises just about the time for introduction of agriculture around 5,000 to 4,000 before present (3,000 to 2,000 BC).
The only thing that is left of the super volcano is Lake
Toba, located in the northern central Sumatra.
Toba is a super volcano on Sumatra in Indonesia, which had an outbreak 73,000 years before present. The mass of the spewed lava, ash and gases was 100 times greater than from the largest volcanic eruptions in recent history, which was the eruption of Mount Tambora in Indonesia, which had an outbreak in 1815 and was the cause of 1816 as "the year without summer" on the northern hemisphere.
The super volcano Toba spread ash in the atmosphere and throughout South Asia in an about 15 cm thick layer, which still can be traced both in India and in the South China Sea. It is assumed that countless animals and early humans were eradicated on this occasion.
The Toba outbreak coincided with the beginning of one of the Weichsel Ice Age's particular cold periods, and many believe that the eruption caused a "volcanic winter", which resulted in a decrease in the sea surface temperature of 3-5 degrees that thereby accelerated the deterioration of the ice age climate. Analyses of Greenland ice cores show that the outbreak was followed by a thousand-year period of increased dust in the atmosphere and low temperatures.
The MIS 11 - MIS 1 analogy, southern European vegetation,
atmospheric methane and the - early anthropogenic hypothesis - P. C. Tzedakis. (pdf)
Interglacial and glacial variability from the last 800 ka in marine, ice and terrestrial archives N. Lang and E. W. Wolff. (pdf)
Sprucing Up Greenland Eric J. Steig and Alexander P. Wolfe - Perspectives. (pdf)
First complete ice core record of last interglacial period shows the climate of Greenland to be significantly warmer than today Anthony Watts - Watts Up With That?
Intriguing climatic shifts in a 90 kyr old lake record from northern Russia Mona Henriksen, Jan Mangerud med flere - BOREAS. (pdf)
Can we predict the duration of an interglacial? P. C. Tzedakis, E. W. Wolff med flere. (pdf)
Earth's Climate History (Kindle Edition) by Anton Uriarte.