April 20, 2017
October 30, 2014
A number of abrupt changes in climate of Earth during past 15,000 years caused by natural events have been identified. These changes had profound effects within a short period on natural conditions, flora and fauna worldwide and consequently affected course of human civilisational history also. Some of these changes are :
The Younger Dryas stadial, also referred to as the Big Freeze, was a geologically brief (1,300 ± 70 years) period of cold climatic conditions and drought which occurred between approximately 12,800 and 11,500 years BP (between 10,800 and 9500 BC). The Younger Dryas stadial is thought to have been caused by the collapse of the North American ice sheets, although rival theories have been proposed.It followed the Bølling-Allerød interstadial (warm period) at the end of the Pleistocene and preceded the preboreal of the early Holocene. It is named after an indicator genus, the alpine-tundra wildflower Dryas octopetala. In Ireland, the period has been known as the Nahanagan Stadial, while in the United Kingdom it has been called the Loch Lomond Stadial and most recently Greenland Stadial 1 (GS1). The Younger Dryas (GS1) is also a Blytt-Sernander climate period detected from layers in north European bog peat.The Dryas stadials were cold periods which interrupted the warming trend since the Last Glacial Maximum 20,000 years ago. The Older Dryas occurred approximately 1,000 years before the Younger Dryas and lasted about 3000 years. The Oldest Dryas is dated between approximately 18,000 and 15,000 BP (16000 to 13000 BC).Abrupt climate changeThe Younger Dryas saw a rapid return to glacial conditions in the higher latitudes of the Northern Hemisphere between 12.9–11.5 ka BP, in sharp contrast to the warming of the preceding interstadial deglaciation. It has been believed that the transitions each occurred over a period of a decade or so, but the onset may have been faster. Thermally fractionated nitrogen and argon isotope data from Greenland ice core GISP2 indicate that the summit of Greenland was approximately 15 °C (27 °F) colder during the Younger Dryas than today. In the UK, coleopteran (beetle) fossil evidence suggests that mean annual temperature dropped to −5 °C (23 °F), and periglacial conditions prevailed in lowland areas, while icefields and glaciers formed in upland areas. Nothing of the size, extent, or rapidity of this period of abrupt climate change has been experienced since.Global effectsIn western Europe and Greenland, the Younger Dryas is a well-defined synchronous cool period. But cooling in the tropical North Atlantic may have preceded this by a few hundred years; South America shows a less well defined initiation but a sharp termination. The Antarctic Cold Reversal appears to have started a thousand years before the Younger Dryas, and has no clearly defined start or end; Peter Huybers has argued that there is fair confidence in the absence of the Younger Dryas in Antarctica, New Zealand and parts of Oceania. Timing of the tropical counterpart to the Younger Dryas – the Deglaciation Climate Reversal (DCR) – is difficult to establish as low latitude ice core records generally lack independent dating over this interval. An example of this is the Sajama ice core (Bolivia), for which the timing of the DCR has been pinned to that of the GISP2 ice core record (central Greenland). Climatic change in the central Andes during the DCR, however, was significant and characterized by a shift to much wetter, and likely colder, conditions. The magnitude and abruptness of these changes would suggest that low latitude climate did not respond passively during the YD/DCR.In western North America it is likely that the effects of the Younger Dryas were less intense than in Europe; however, evidence of glacial re-advance indicates Younger Dryas cooling occurred in the Pacific
Northwest. Other features seen include:
1. Replacement of forest in Scandinavia with glacial tundra (which is the habitat of the plant Dryas octopetala)
2. Glaciation or increased snow in mountain ranges around the world
3. Formation of solifluction layers and loess deposits in Northern Europe4. More dust in the atmosphere, originating from deserts in Asia5. Drought in the Levant, perhaps motivating the Natufian culture to develop agriculture.The Huelmo/Mascardi Cold Reversal in the Southern Hemisphere ended at the same time
The prevailing theory is that the Younger Dryas was caused by significant reduction or shutdown of the North Atlantic “Conveyor”, which circulates warm tropical waters northward, in response to a sudden influx of fresh water from Lake Agassiz and deglaciation in North America. Geological evidence for such an event is thus far lacking. The global climate would then have become locked into the new state until freezing removed the fresh water “lid” from the north Atlantic Ocean. An alternative theory suggests instead that the jet stream shifted northward in response to the changing topographic forcing of the melting North American ice sheet, bringing more rain to the North Atlantic which freshened the ocean surface enough to slow the thermohaline circulation. There is also some evidence that a solar flare may have been responsible for the megafaunal extinction, though it cannot explain the apparent variability in the extinction across all continents. There is evidence that some previous glacial terminations had post glacial cooling periods similar to the Younger Dryas.Impact hypothesisA hypothesized Younger Dryas impact event, presumed to have occurred in North America around 12.9 ka BP, has been proposed as the mechanism to have initiated the Younger Dryas cooling. Amongst other things findings of melt-glass material in sediments in Pennsylvania, South Carolina, and Syria have been reported. These researchers argue that this material, which dates back nearly 13,000 years, was formed at temperatures of 1,700 to 2,200 °C (3,100 to 4,000 °F) as the result of a bolide impact. They argue that these findings support the controversial Younger Dryas Boundary (YDB) hypothesis, that the bolide impact occurred at the onset of the Younger Dryas. The hypothesis has been questioned by research that stated that most of the conclusions cannot be repeated by other scientists, misinterpretation of data, and the lack of confirmatory evidence. A review of the sediments that are found at sites. New research found that sediments claimed, by the hypothesis proponents, to be deposits resulting from a bolide impact were, in fact, dated from much later or much earlier time periods than the proposed date of the cosmic impact. The researchers examined 29 sites that are commonly referenced to support the impact theory to determine if they can be geologically dated to around 13,000 years ago. Crucially, only 3 of the sites actually date from that time. Although there may be several causes of the Younger Dryas, volcanic activity is considered one possibility. The Laacher in Germany was of sufficient size, VEI 6, with over 10 km3 (2.4 cu mi) tephra ejected, to have caused significant temperature changes in the northern hemisphere. Laacher See tephra is found throughout the Younger Dryas boundary layer. This possibility has been disputed by 14C analysis. In the view of Cambridge University volcanologist, Clive Oppenheimer, the magnitude of Laacher See was similar to the 1991 Mount Pinatubo eruption, and the effects were a year or two of northern hemisphere summer cooling and winter warming, and up to two decades of environmental disruption in Germany.End of the climate periodMeasurements of oxygen isotopes from the GISP2 ice core suggest the ending of the Younger Dryas took place over just 40–50 years in three discrete steps, each lasting five years. Other proxy data, such as dust concentration, and snow accumulation, suggest an even more rapid transition, requiring about a 7 °C (13 °F) warming in just a few years. Total warming in Greenland was 10 ± 4 °C (18 ± 7 °F).. The end of the Younger Dryas has been dated to around 11.55 ka BP, occurring at 10 ka bp (uncalibrated radiocarbon year), a “radiocarbon plateau” by a variety of methods, with mostly consistent results:11.50 ± 0.05 ka BP: GRIP ice core,
Greenland11.53 + 0.04 − 0.06 ka BP: Krakenes Lake, western Norway11.57 ka BP: Cariaco Basin core, Venezuela11.57 ka BP: German oak/pine dendrochronology11.64 ± 0.28 ka BP: GISP2 ice core,
The Younger Dryas is often linked to the adoption of agriculture in the Levant. It is argued that the cold and dry Younger Dryas lowered the carrying capacity of the area and forced the sedentary Early Natufian population into a more mobile subsistence pattern. Further climatic deterioration is thought to have brought about cereal cultivation. While there exists relative consensus regarding the role of the Younger Dryas in the changing subsistence patterns during the Natufian, its connection to the beginning of agriculture at the end of the period is still being debated.
The failure of North Atlantic thermohaline circulation is used to explain rapid climate change in some science fiction writings as early as Stanley G. Weinbaum’s 1937 short story “Shifting Seas” where the author described the freezing of Europe after the Gulf Stream was disrupted, and more recently in Kim Stanley Robinson’s novels, particularly Fifty Degrees Below. It also underpinned the 1999 book, The Coming Global Superstorm. Likewise, the idea of rapid climate change caused by disruption of North Atlantic ocean currents creates the setting for 2004 apocalyptic science-fiction film The Day After Tomorrow. Similar sudden cooling events have featured in other novels, such as John Christopher’s The World in Winter, though not always with the same explicit links to the Younger Dryas event as is the case of Robinson’s work.
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Bond events are North Atlantic climate fluctuations occurring every ≈1,470 ± 500 years throughout the Holocene. Eight such events have been identified, primarily from fluctuations in ice-rafted debris. Bond events may be the interglacial relatives of the glacial Dansgaard–Oeschger events, with a magnitude of perhaps 15–20% of the glacial-interglacial temperature change.Gerard C. Bond of the Lamont–Doherty Earth Observatory at Columbia University, was the lead author of the 1997 paper that postulated the theory of 1,470-year climate cycles in the Holocene, mainly based on petrologic tracers of drift ice in the North Atlantic.The existence of climatic changes, possibly on a quasi-1,500 year cycle, is well established for the last glacial period from ice cores. Less well established is the continuation of these cycles into the holocene. Bond et al. (1997) argue for a cyclicity close to 1470 ± 500 years in the North Atlantic region, and that their results imply a variation in Holocene climate in this region. In their view, many if not most of the Dansgaard–Oeschger events of the last ice age, conform to a 1,500-year pattern, as do some climate events of later eras, like the Little Ice Age, the 8.2 kiloyear event, and the start of the Younger Dryas.The North Atlantic ice-rafting events happen to correlate with most weak events of the Asian monsoon for at least the past 9,000 years, while also correlating with most aridification events in the Middle East for the past 55,000 years (both Heinrich and Bond events). Also, there is widespread evidence that a ≈1,500 yr climate oscillation caused changes in vegetation communities across all of North America.For reasons that are unclear, the only Holocene Bond event that has a clear temperature signal in the Greenland ice cores is the 8.2 kyr event.The hypothesis holds that the 1,500-year cycle displays nonlinear behavior and stochastic resonance; not every instance of the pattern is a significant climate event, though some rise to major prominence in environmental history. Causes and determining factors of the cycle are under study; researchers have focused attention on variations in solar output, and “reorganizations of atmospheric circulation.” Bond events may also be correlated with the 1800-year lunar tidal cycle.
List of Bond events
Most Bond events do not have a clear climate signal; some correspond to periods of cooling, others are coincident with aridification in some regions.
No. Time (BP) Notes
- 1. 0 ≈0.5 ka See Little Ice Age
- 2.1 ≈1.4 ka See Migration Period
- 3.2 ≈2.8 ka Early 1st millennium BC drought in the Eastern Mediterranean, possibly triggering the collapse of Late Bronze Age cultures.
- 4.3 ≈4.2 ka See 4.2 kiloyear event; collapse of the Akkadian Empire, end of the Egyptian Old Kingdom. 
- 5.4 ≈5.9 ka See 5.9 kiloyear event;
- 6.5 ≈8.2 ka See 8.2 kiloyear event;
- 7.6 ≈9.4 ka Erdalen event of glacier activity in Norway, as well as with a cold event in China.
- 8.7 ≈10.3 ka
- 9. 8 ≈11.1 ka
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The 8.2 kiloyear event is the term that climatologists have adopted for a sudden decrease in global temperatures that occurred approximately 8,200 years before the present, or c. 6,200 BCE, and which lasted for the next two to four centuries. Milder than the Younger Dryas cold spell that preceded it, but more severe than the Little Ice Age that would follow, the 8.2 kiloyear cooling was a significant exception to general trends of the Holocene climatic optimum. During the event, atmospheric methane concentration decreased by 80 ppb or 15% emission reduction by cooling and drying at a hemispheric scale.A rapid cooling around 6200 BCE was first identified by Swiss botanist Heinrich Zoller in 1960, who named the event Misox oscillation (for the Val Mesolcina). It is also known as Finse event in Norway. Bond et al. argued that the origin of the 8.2 kiloyear event is linked to a 1,500-year climate cycle; it correlates with Bond event 5.
The strongest evidence for the event comes from the North Atlantic region; the disruption in climate shows clearly in Greenland ice cores and in sedimentary and other records of the temporal and tropical North Atlantic. It is less evident in ice cores from Antarctica and in South American indices. The effects of the cold snap were global, however, most notably in changes in sea level during the relevant era.
The 8.2 kiloyear cooling event may have been caused by a large meltwater pulse from the final collapse of the Laurentide ice sheet of northeastern North America—most likely when the glacial lakes Ojibway and Agassiz suddenly drained into the North Atlantic Ocean. The same type of action produced the Missoula floods that created the Channeled scablands of the Columbia River basin. The meltwater pulse may have affected the North Atlantic thermohaline circulation, reducing northward heat transport in the Atlantic and causing significant circum-North Atlantic cooling. Estimates of the cooling vary and depend somewhat on the interpretation of the proxy data, but drops of around 1 to 5 deg C (1 to 11 deg F)[clarification needed Not equivalent] have been reported. In Greenland, the event started at 8175 Before Present, and the cooling was 3.3 deg C (decadal average) in less than ~20 years, and the coldest period lasted for about 60 years, and the total duration was about 150 years. Further afield, some tropical records report a 3 deg C (5 deg F) cooling from cores drilled into an ancient coral reef in Indonesia.  The event also caused a global CO2 decline of ~ 25 ppm over ~ 300 years. However, the dating and interpretation of this and other tropical sites are more ambiguous than the North Atlantic sites.
Drier conditions were notable in North Africa, while East Africa suffered five centuries of general drought. In West Asia and especially Mesopotamia, the 8.2 kiloyear event was a three-hundred year aridification and cooling episode, which may have provided the natural force for Mesopotamian irrigation agriculture and surplus production that were essential for the earliest class-formation and urban life. However multi-centennial changes around the same period are difficult to link specifically to the approximately 100-year abrupt event as recorded most clearly in the Greenland ice cores.
The initial melt water pulse caused between 0.5 and 4 m (1 ft 8 in and 13 ft 1 in) of sea-level rise. Based on estimates of lake volume and decaying ice cap size, values of 0.4–1.2 m (1 ft 4 in–3 ft 11 in) circulate. Based on sea-level data from below modern deltas, 2–4 m (6 ft 7 in–13 ft 1 in) of near-instantaneous rise is estimated, in addition to ‘normal’ post-glacial sea-level rise. Melt water pulse sea-level rise was experienced fully at great distance from the release area. Gravity and rebound effects associated with the shifting of water masses meant that the sea-level fingerprint was smaller in areas closer to the Hudson Bay. The Mississippi delta records ~20%, NW Europe records ~70% and Asia records ~105% of the global averaged amount. The cooling of the 8.2 kiloyear event was a temporary feature; the sea-level rise of the meltwater pulse was permanent.
In 2003, the Office of Net Assessment at the United States Department of Defense was commissioned to produce a study on the likely and potential effects of a modern climate change. The study, conducted under ONA head Andrew Marshall, modelled its prospective climate change on the 8.2 kiloyear event, precisely because it was the middle alternative between the Younger Dryas and the Little Ice Age.
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The 5.9 kiloyear event was one of the most intense aridification events during the Holocene Epoch. It occurred around 3900 BC (5,900 years BP), ending the Neolithic Subpluvial and probably initiated the most recent desiccation of the Sahara desert.Thus, it also triggered worldwide migration to river valleys, such as from central North Africa to the Nile valley, which eventually led to the emergence of the first complex, highly organized, state-level societies in the 4th millennium BC. It is associated with the last round of the Sahara pump theory.A model by Claussen et al. (1999) suggested rapid
desertification associated with vegetation-atmosphere interactions following a cooling event, Bond event 4. Bond et al. (1997) identified a North Atlantic cooling episode 5,900 years ago from ice-rafted debris, as well as other such now called Bond events that indicate the existence of a quasiperiodic cycle of Atlantic cooling events, which occur approximately every 1,470 years ± 500 years. For some reason, all of the earlier of these arid events (including the 8.2 kiloyear event) were followed by recovery, as attested by the wealth of evidence of humid conditions in the Sahara between 10,000 and 6,000 BP. However, it appears that the 5.9 kiloyear event was followed by a partial recovery at best, with accelerated desiccation in the millennium that followed. For example, Cremaschi (1998) describes evidence of rapid aridification in Tadrart Acacus of southwestern Libya, in the form of increased aeolian erosion, sand incursions and the collapse of the roofs of rock shelters. The 5.9 kiloyear event was also recorded as a cold event in the Erhai Lake (China) sediments.In the Middle East the 5.9 kiloyear event contributed to the abrupt end of the Ubaid period. It was associated with an abandonment of unwalled villages and the rapid growth of hierarchically structured walled cities, and in the Jemdet Nasr period, with the first book-keeping scripts.
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The 4.2 kiloyear BP aridification event was one of the most severe climatic events of the Holocene period in terms of impact on cultural upheaval. Starting in ≈2200 BC, it probably lasted the entire 22nd century BC. It is very likely to have caused the collapse of the Old Kingdom in Egypt as well as the Akkadian Empire in Mesopotamia. The drought may have also initiated southeastward habitat tracking within the Indus Valley Civilization.A phase of intense aridity in ≈4.2 ka BP is well recorded across North Africa, the Middle East, the Red Sea, the Arabian peninsula, the Indian subcontinent, and mid continental North America. Glaciers throughout the mountain ranges of western Canada advanced at about this time. Evidence has also been found in an Italian cave flowstone, and in the Kilimanjaro Ice sheet, Andean glacier ice. The onset of the aridification in Mesopotamia near 4100 B.P also coincided with a cooling event in the North Atlantic, known as Bond event 3.
In ca. 2150 BC the Old Kingdom of Ancient Egypt was hit by a series of exceptionally low Nile floods, which was instrumental in the sudden collapse of centralized government in ancient Egypt. Famines, social disorder, and fragmentation during a period of approximately 40 years were followed by a phase of rehabilitation and restoration of order in various provinces. Egypt was eventually reunified within a new paradigm of kingship. The process of recovery depended on capable provincial administrators, the deployment of the idea of justice, irrigation projects, and an administrative reform.The aridification of Mesopotamia may have been related to the onset of cooler sea surface temperatures in the North Atlantic (Bond event 3), as analysis of the modern instrumental record shows that large (50%) interannual reductions in Mesopotamian water supply result when subpolar northwest Atlantic sea surface temperatures are anomalously cool. The headwaters of the Tigris and Euphrates Rivers are fed by elevation-induced capture of winter Mediterranean rainfall.The Akkadian Empire—which in 2300 B.C. was the second civilization to subsume independent societies into a single state (the first being ancient Egypt at around 3100 BC) —was brought low by a wide-ranging, centuries-long drought. Archaeological evidence documents widespread abandonment of the agricultural plains of northern Mesopotamia and dramatic influxes of refugees into southern Mesopotamia around 2170 BC. A 180-km-long wall, the “Repeller of the Amorites,” was built across central Mesopotamia to stem nomadic incursions to the south. Around 2150 BC, the Gutian people, who originally inhabited the Zagros Mountains, defeated the demoralized Akkadian army, took Akkad, and destroyed it around 2115 BC. Widespread agricultural change in the Near East is visible at the end of the third millennium BC. Resettlement of the northern plains by smaller sedentary populations occurred near 1900 BC, three centuries after the collapse.In the Persian Gulf region, there is a sudden change in settlement pattern, style of pottery and tombs at this time. The 22nd century BC drought marks the end of the Umm al-Nar period and the change to the Wadi Suq period.The drought may have caused the collapse of Neolithic Cultures around Central China during the late third millennium BC. At the same time, the middle reaches of the Yellow River saw a series of extraordinary floods. In the Yishu River Basin, the flourishing Longshan culture was hit by a cooling that made the paddies shortfall in output or even no seeds were gathered. The scarcity in natural resource led to substantial decrease in population and subsequent drop in archaeological sites. About 4000 cal. yr BP the Longshan culture was displaced by the Yueshi culture which was relatively underdeveloped, simple, and unsophisticated.
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