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4.2-kiloyear event
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4.2-kiloyear event
The 4.2-kiloyear event (also known as the 4.2 ka event) was an aridification event (long-term drought), 4,200 years ago (4,200 years BP). It was one of the most severe climatic events of the Holocene epoch. It defines the beginning of the current Meghalayan age in the Holocene epoch.
Starting around 2200 BC, it most likely lasted the entire 22nd century BC. It has been hypothesised to have caused the collapse of the Old Kingdom in Egypt, the Akkadian Empire in Mesopotamia, and the Liangzhu culture in the lower Yangtze River area. The drought may also have initiated the collapse of the Indus Valley Civilisation, with some of its population moving southeast to follow the movement of their desired habitat, as well as the migration of Indo-European-speaking people into India. Some scientists disagree with that conclusion, citing evidence that the event was not a global drought and did not happen in a clear timeline.
Modelling evidence suggests that the 4.2 ka event was the result of a significant weakening of the Atlantic meridional overturning circulation (AMOC), disrupting global ocean currents and generating precipitation and temperature changes in various regions. The Intertropical Convergence Zone (ITCZ) abruptly shifted southward. Evidence suggests increased El Niño–Southern Oscillation (ENSO) variability also played a role in generating the climatic conditions associated with the event. Explosive volcanism in Iceland has also been proposed as a cause, though the low sulphur content of Icelandic volcanoes has led other studies to suggest it had a negligible impact on global climate.
A phase of intense aridity around 2200 BC is recorded in physical evidence across North Africa, the Middle East, the Red Sea, the Arabian Peninsula, the Indian subcontinent, and midcontinental North America. Glaciers throughout the mountain ranges of western Canada advanced around that time. Iceland also experienced glacial advance. Evidence has also been found in an Italian cave flowstone, the Kilimanjaro ice sheet, and in Andean glacier ice. The onset of the aridification in Mesopotamia around 2100 BC also coincided with a cooling event in the North Atlantic, known as Bond event 3.
In 2018, the International Commission on Stratigraphy divided the Holocene epoch into three periods, with the late Holocene from around 2250 BC onwards designated as the Meghalayan stage/age. The boundary stratotype is a speleothem in Mawmluh cave in India, and the global auxiliary stratotype is an ice core from Mount Logan in Canada. However, justification for this division is debated as the event was not a global drought and did not happen within a clear timeframe. Jessica Tierney, a paleoclimatologist at the University of Arizona in Tucson, states that proponents of the new partitioning mistakenly "lumped together evidence of other droughts and wet periods, sometimes centuries away from the event."
In Ireland, there is little definitive record of the 4.2 ka event outside of a brief isotopic excursion in some cave speleothem records. The manner in which this climatic event manifested itself in the region is thus unclear. In Great Britain as in Ireland, the nature of the 4.2 ka event is ambiguous and unclear. The yew tree's abundance declined in eastern England.
Analysis of sediments from Lake Spore reveals that in Poland, winters became colder between 4250 and 4000 BP, with this cooling likely responsible for a podzolisation (generation of boreal forest soil type) event around 4200 BP, whereas summer temperatures remained constant. Humidity levels were not affected by the 4.2 ka event. Northeastern Poland became drier, as indicated by declining lake levels.
On the Iberian Peninsula, in general the climate between 2800 and 1100 cal BC is quite stable and relatively humid. A reconstruction of precipitation shows two rapid, pronounced dry phases from 2350 to 2200 cal BC (4.3 - 4.15 ka BP) and from 2100 to 2000 cal BC (4.05 - 3.95 ka BP). The dry phases were followed by a shift towards wetter conditions, suggesting a more complex pattern of climate change than other regions during the 4.2 ka event.
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4.2-kiloyear event
The 4.2-kiloyear event (also known as the 4.2 ka event) was an aridification event (long-term drought), 4,200 years ago (4,200 years BP). It was one of the most severe climatic events of the Holocene epoch. It defines the beginning of the current Meghalayan age in the Holocene epoch.
Starting around 2200 BC, it most likely lasted the entire 22nd century BC. It has been hypothesised to have caused the collapse of the Old Kingdom in Egypt, the Akkadian Empire in Mesopotamia, and the Liangzhu culture in the lower Yangtze River area. The drought may also have initiated the collapse of the Indus Valley Civilisation, with some of its population moving southeast to follow the movement of their desired habitat, as well as the migration of Indo-European-speaking people into India. Some scientists disagree with that conclusion, citing evidence that the event was not a global drought and did not happen in a clear timeline.
Modelling evidence suggests that the 4.2 ka event was the result of a significant weakening of the Atlantic meridional overturning circulation (AMOC), disrupting global ocean currents and generating precipitation and temperature changes in various regions. The Intertropical Convergence Zone (ITCZ) abruptly shifted southward. Evidence suggests increased El Niño–Southern Oscillation (ENSO) variability also played a role in generating the climatic conditions associated with the event. Explosive volcanism in Iceland has also been proposed as a cause, though the low sulphur content of Icelandic volcanoes has led other studies to suggest it had a negligible impact on global climate.
A phase of intense aridity around 2200 BC is recorded in physical evidence across North Africa, the Middle East, the Red Sea, the Arabian Peninsula, the Indian subcontinent, and midcontinental North America. Glaciers throughout the mountain ranges of western Canada advanced around that time. Iceland also experienced glacial advance. Evidence has also been found in an Italian cave flowstone, the Kilimanjaro ice sheet, and in Andean glacier ice. The onset of the aridification in Mesopotamia around 2100 BC also coincided with a cooling event in the North Atlantic, known as Bond event 3.
In 2018, the International Commission on Stratigraphy divided the Holocene epoch into three periods, with the late Holocene from around 2250 BC onwards designated as the Meghalayan stage/age. The boundary stratotype is a speleothem in Mawmluh cave in India, and the global auxiliary stratotype is an ice core from Mount Logan in Canada. However, justification for this division is debated as the event was not a global drought and did not happen within a clear timeframe. Jessica Tierney, a paleoclimatologist at the University of Arizona in Tucson, states that proponents of the new partitioning mistakenly "lumped together evidence of other droughts and wet periods, sometimes centuries away from the event."
In Ireland, there is little definitive record of the 4.2 ka event outside of a brief isotopic excursion in some cave speleothem records. The manner in which this climatic event manifested itself in the region is thus unclear. In Great Britain as in Ireland, the nature of the 4.2 ka event is ambiguous and unclear. The yew tree's abundance declined in eastern England.
Analysis of sediments from Lake Spore reveals that in Poland, winters became colder between 4250 and 4000 BP, with this cooling likely responsible for a podzolisation (generation of boreal forest soil type) event around 4200 BP, whereas summer temperatures remained constant. Humidity levels were not affected by the 4.2 ka event. Northeastern Poland became drier, as indicated by declining lake levels.
On the Iberian Peninsula, in general the climate between 2800 and 1100 cal BC is quite stable and relatively humid. A reconstruction of precipitation shows two rapid, pronounced dry phases from 2350 to 2200 cal BC (4.3 - 4.15 ka BP) and from 2100 to 2000 cal BC (4.05 - 3.95 ka BP). The dry phases were followed by a shift towards wetter conditions, suggesting a more complex pattern of climate change than other regions during the 4.2 ka event.
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