HUMANPAST.NET

Environment                  70,000 BC
Africa
Southwest Asia
Egypt
Indus Valley
China
Europe
South America
Mesoamerica
North America
Other

In General

I have reviewed the evidence for three displacements of the earth's crust during the last 110,000 years. They seem to have occurred at intervals of 30,000 to 40,000 years. There are indications that they may have occurred at this rate through much of the Pleistocene Epoch. From the evidence we now have it seems futile to try to determine the locations of the poles in the more remote cases. With every step backward in time the evidence naturally becomes thinner. (132)

What is now clear is that during the last ice age, and the period that followed it, the climate was much more chaotic than it has been in recent millennia. Generally, the climate was much more variable. Sudden changes occurred from time to time. Collapse of parts of the ice sheets, or release of meltwater lakes that built up behind the ice, led to cataclysmic changes. Armadas of icebergs or floods of icy freshwater swept out into the North Atlantic altering the circulation of the ocean at a stroke and with it the climate of the neighbouring continents. With a flick of the climatic switch, Europe and much of North America could be plunged back into icy conditions, having only just emerged from the abyss of the preceding millennia. Conversely, the stability of the glacial conditions could be interrupted by a re-establishment of the flow of warm water to higher latitudes in the North Atlantic, bringing surprising temporary warmth to the northern continents. (145)

...the isotopic temperature records show some 20 interstadials, ...between 15 and 100 kya. Typically the events start with an abrupt warming of Greenland of some 5 to 10°C over a few decades or less. This warming is followed by a gradual cooling over several hundred years, and occasionally much longer. This cooling phase often ends with an abrupt final reduction of temperature back to cold ('stadial') conditions. (145)

The truly notable feature is the additional emphasis given to the prolonged cold period from 70 to 63 kya, to the cold spell at around 36 kya and to the frequency of extreme cold from 30 to 15 kya. (145)

Changes in the sea level during the last 100 kyr. (145)

The climate then warmed up around 80 kya with the Odderade interstadial. Then, in terms of sudden climate change, the real action appears to have started around 74 kya. This timing is of particular interest as it coincides with some suggested dates for the movement of modern humans out of Africa.  (145)

There is an additional reason for looking closely at the events around 70 to 74 kya. This is the eruption of the supervolcano Toba, which is usually dated at about 74 kya. Toba was gigantic. The caldera that resulted from this eruption is 100 km long and 60 km wide. It ejected about 3000 cu. km of material. In the central Indian Ocean, some 2500 km downwind of Toba, a 35-cm-thick layer of ash was deposited. Furthermore, the eruption was rich in sulphur, which results in the formation of long-lasting sulphuric acid aerosol clouds in the stratosphere. This would have increased its climatic impact, the dust from the eruption would have dropped out of the atmosphere in a matter of months whereas these sulphuric acid aerosols would have remained aloft for several years. The impact of Toba is estimated to have been about a 5°C temperature drop, and possibly 15°C summer cooling in the temperate to high latitudes within a year or so and lasting for several more years. The cooling would have led to unseasonable frosts in many parts of the world and the disruption of growing seasons.  (145)

What can be said is that the bottleneck appears to have occurred around 70 to 80 kya, give or take 5 to 10 kyr. In particular, the event that has been linked to the massive eruption of Mount Toba in Indonesia at 71 kya is a likely candidate. This super­volcano could have produced what is sometimes referred to as a 'volcanic winter' that lasted several years and could conceivably have significantly altered global climate for the next 1000 years. The dramatic cooling resulting from this eruption could have brought famine and death to many modern humans, leading to an abrupt decrease in our ancestors' populations: in short, a bottleneck. The genetic evidence can be interpreted as indicating that the global human population crashed to no more than 15000 to 40000 people, and possibly lower. (145)

Africa

 

Southwest Asia

 

Egypt

 

Indus Valley

 

China

 

Europe

 By 75,000 years ago glacial conditions had begun in Europe, and a great ice sheet formed, perhaps on a land bridge connecting the British Isles with Iceland and Greenland. After a few thousand years it had grown thick enough to move out from the center of thickest accumulation by gravity. It covered Ireland, filled the Irish Sea basin and swept across the Welsh mountains into England about 55,000 years ago. Its advance, it appears, required a period of 20,000 to 25,000 years, comparable to the time that was apparently required for the growth of the Wisconsin ice sheet. ...a polar shift occurred at about the time this ice sheet reached its maximum: the shift from the Greenland Sea to Hudson Bay. It is easy to connect the two events: The shift brought the advance to an end and initiated the glacial retreat. This retreat probably started long before 42,000 years ago, and by about 37,000 years ago the ice sheet had left the Irish Sea basin. The decline of the Irish Sea ice sheet seems to have required a period of time similar to the time required for the decline of the Wisconsin ice sheet. The parallel is extremely interesting. (132)

Recent absolutely dated speleothem results from southwestern France show an intensely cold period extending from 67.4 to 61.2 kya. Here it is estimated that the average temperature dropped by 13.6°C between 75 kya and 67 kya. The speleothem then stopped growing until 61 kya, presumably because permafrost formed above the cave. (145)

South America

 

Mesoamerica

 

North America

 Several lines of solid evidence suggest that during the last ice age the North Pole was located in or near Hudson Bay. ...the best guess for the site of the pole seems to be approximately 60 degrees North Latitude and 83 degrees West Longitude. The first line of evidence that the last North American ice cap was a polar ice cap is based on the shape, size, and peculiar geographical location of the ice sheet. (132)

Studies of stalagmites from a cave in southeastern Missouri show remarkably little variation after a marked cooling around 55 kya. Prior to this, conditions appear to have oscillated more frequently. The warmest temperatures occurred around 57 kya and there were short-lived cooling events around 64, 71 and 74 kya. Beetle assemblages suggest that, following the cooling around 55 kya, the mean July temperature was 7.5-8°C lower than present and mean January value was 15-18°C lower than present. During what is termed the 'mid-Wisconsin' interstadial, dating from 43.5 to 39 kya, there was a rapid and intense warming. At the peak of this event, about 42kya, July temperatures were only 1-2°C lower than modern. (145)

Other

 There is an additional reason for looking closely at the events around 70 to 74 kya. This is the eruption of the supervolcano Toba, which is usually dated at about 74 kya. Toba was gigantic. The caldera that resulted from this eruption is 100 km long and 60 km wide. It ejected about 3000 cu. km of material. In the central Indian Ocean, some 2500 km downwind of Toba, a 35-cm-thick layer of ash was deposited. Furthermore, the eruption was rich in sulphur, which results in the formation of long-lasting sulphuric acid aerosol clouds in the stratosphere. This would have increased its climatic impact, the dust from the eruption would have dropped out of the atmosphere in a matter of months whereas these sulphuric acid aerosols would have remained aloft for several years. The impact of Toba is estimated to have been about a 5°C temperature drop, and possibly 15°C summer cooling in the temperate to high latitudes within a year or so and lasting for several more years. The cooling would have led to unseasonable frosts in many parts of the world and the disruption of growing seasons.  (145)