Toba Catastrophe Theory

It is one of the largest known explosive eruptions in the Earth's history. The Toba catastrophe theory holds that this event caused a severe global volcanic winter of six to ten years and contributed to a 1,000-year-long cooling episode, leading to a genetic bottleneck in humans. However, some physical evidence disputes the links with the millennium-long cold event and genetic bottleneck, and some consider the theory disproven.

Toba eruption theory
Artist's impression of the eruption from about 42 km (26 mi) above northern Sumatra
Volcano	Toba Caldera Complex
Date	c. 74,000 years BP
Location	Sumatra, Indonesia 2°41′04″N 98°52′32″E / 2.6845°N 98.8756°E / 2.6845; 98.8756
VEI	8
Impact	Impact disputed
Deaths	(Potentially) almost all of humanity, leaving around 3,000–10,000 humans left on the planet
Lake Toba is the resulting crater lake

In 1972, an analysis of human hemoglobins found very few variants, and to account for the low frequency of variation human population must had been as low as a few thousand until very recently. More genetic studies confirmed an effective population on the order of 10,000 for much of human history. Subsequent research on the differences in human mitochondrial DNA sequences dated a rapid growth from a small effective population size of 1,000 to 10,000, sometime between 35,000 and 65,000 years ago.

The large magnitude of Toba eruption has been known since 1939, and various techniques dated the timing of the event to 73,000 to 75,000 years ago. A study published in 1993 suggested that the eruption accelerated climate and environmental transition from the last interglacial period MIS-5 to the last glacial period MIS-4.

In 1993, science journalist Ann Gibbons posited that population growth was suppressed by the cold climate of the last Pleistocene Ice Age, possibly exacerbated by the Toba eruption. The subsequent explosive human expansion was believed to be the result of the end of the ice age. Geologist Michael R. Rampino of New York University and volcanologist Stephen Self of the University of Hawaiʻi at Mānoa supported her theory. In 1998, anthropologist Stanley H. Ambrose of the University of Illinois Urbana-Champaign laid out the scenario that the Toba eruption caused a human population crash, and the low population size was sustained by the global glacial condition of MIS-4 until the climate eventually transitioned to the warmer condition of MIS-3 around 60,000 years ago, during which rapid human expansion was recorded.

The most recent estimate of eruptive volume is 3,800 km³ (910 cu mi) dense-rock equivalent (DRE), of which 1,800 km³ (430 cu mi) was deposited as ash fall and 2,000 km³ (480 cu mi) as ignimbrite, making this eruption the largest during the Quaternary period. Previous volume estimates have ranged from 2,000 km³ (480 cu mi) to 6,000 km³ (1,400 cu mi). Inside caldera, the maximum thickness of pyroclastic flows is over 600 m (2,000 ft). The outflow sheet originally covered an area of 20,000–30,000 km² (7,700–11,600 sq mi) with thickness nearly 100 m (330 ft), likely reaching into the Indian Ocean and the Straits of Malacca. The air-fall of this eruption blanketed Indian subcontinent in a layer of 5 cm (2.0 in) ash, Arabian Sea in 1 mm (0.039 in), South China Sea in 3.5 cm (1.4 in), and Central Indian Ocean Basin in 10 cm (3.9 in). Its horizon of ashfall covered an area of more than 38,000,000 km² (15,000,000 sq mi) in 1 cm (0.39 in) or more thickness. In Sub-Saharan Africa, microscopic glass shards from this eruption are also discovered on the south coast of South Africa, in the lowlands of northwest Ethiopia, in Lake Malawi, and in Lake Chala.

The most recent two high-precision argon–argon datings dated the eruption to 73,880 ± 320 and 73,700 ± 300 years ago. Five distinct magma bodies were activated within a few centuries before the eruption. The implied prevailing wind from the ash distribution is consistent with the eruption taking place during summer. The eruption commenced with small and limited air-fall and was directly followed by the main phase of ignimbrite flows. The ignimbrite phase is characterized by low eruption fountain, but co-ignimbrite column developed on top of pyroclastic flows reached a height of 32 km (20 mi). The entire eruption was likely continuous without major break and may have only lasted 9 to 14 days. Petrological constrains on sulfur emission yielded a wide range from 10¹³ to 10¹⁵ g, depending on the existence of excess gas in the Toba magma chamber.

By analyzing climate proxies and simulating climate forcing, researchers can gain insights into the immediate climatic effects of the Toba eruption. However, there are limitations to both approaches. In sedimentary records where the Toba tuff does not serve as a marker horizon, it cannot pinpoint the exact section that records the environmental conditions immediately following the eruption. Meanwhile, in sedimentary records that do have the Toba tuff as a marker horizon, the sedimentation rate may be too low to capture the short-term climatic effects of the eruption. On the other hand, results of climate models entirely depend on the volatile budget of erupted magma, hence varies accordingly to the assumed volatile budget.

Climate proxy

The Toba tephra layer in marine sediments coincides with the δ¹⁸O MIS 5a to 4 boundary, marking a climatic transition from warm to cold caused by a change in ocean circulation and a drop in atmospheric CO₂ concentration, also known as the Dansgaard-Oeschger event. Geologist Michael R. Rampino and volcanologist Stephen Self hypothesized that Toba eruption accelerated this shift. Testing this hypothesis required higher resolution sedimentary records.

Two marine sediment cores Toba marker horizon retrieved^{[clarification needed]} from the Northern Indian Ocean and the South China Sea either showed no pronounced cooling or a 0.8–1.0 °C (1.4–1.8 °F) cooling in the centuries following eruption. The core resolution^{[clarification needed]} was insufficient to ascertain that the cooling was caused by the Toba eruption since the two events could be decades or centuries apart in the core. However, a severe cooling of only a few years is not expected to appear in these sediment records of centennial resolution. Nonetheless, the marine sedimentary records support that Toba had only a minor impact on the time scales longer than a century.

In Greenland ice cores, a large sulfate spike that appeared between Dansgaard–Oeschger event 19 and 20 was possibly related to Toba eruption. The δ¹⁸O values of the ice cores indicate a 1,000-year cooling event immediately following the sulfate signal. However, high-resolution δ¹⁸O excluded the possibility of a more-than-a-century-long cooling impact of the eruption and ruled out that Toba triggered the cooling as it was already underway.

Insufficient resolution in marine sediments bearing the Toba tuff has hindered the assessment of any short-term effects that may have lasted for less than a century.

In 2013, a microscopic layer of Toba ash was reported in sediments of Lake Malawi. Together with the high sedimentation rate of the lake and Toba marker horizon, several team have reconstructed the local environment after Toba eruption at subdecadal resolution of ~6–9 years. The sediments in core display no clear evidence of cooling and no unusual deviations in concentrations of climate-sensitive ecological indicators. These results imply that the duration of the Toba cooling must have been either shorter than the sampling resolution of ~6–9 years or too small in magnitude in East Africa.

Climate modeling

The mass of sulfurous gases emitted during Toba eruption is a crucial parameter when modeling its climatic effects.

Assuming an emission of 1.7 billion tonnes (1.9 billion short tons) of sulphur dioxide, which is 100 times the 1991 Pinatubo sulphur, the modeled volcanic winter has maximum global mean cooling of −3.5 °C (−6.3 °F) and gradually returns within the range of natural variability 5 years after the eruption. An initiation of 1,000-year cold period or ice age is not supported by the model.

In a 2021 study, two other emission scenarios, 0.2 billion tonnes (0.22 billion short tons) and 2 billion tonnes (2.2 billion short tons) of sulphur dioxide which are 10 and 100 times of Pinatubo respectively, are investigated using state-of-art simulations provided by the Community Earth System Model. Maximum global mean cooling is −2.3 °C (−4.1 °F) for a 0.2 billion tonnes SO₂ release and −4.1 °C (−7.4 °F) for a 2 billion tonnes SO₂ release. Negative temperature anomalies return to less than −1 °C (−1.8 °F) within 3 and 6 years for each emission scenario after the eruption.

Petrological studies of Toba magma constrained that the mass of sulfuric acid aerosols from Toba eruption represents about 2–5 times the sulfuric acid aerosols generated during 1991 Pinatubo eruption. The studies suggest that previous modelings of global temperature perturbations following Toba eruption were excessive. Ice core records of atmospheric sulfur injection during the period during which the Toba eruption occurred contain three large injections that are 10–30 times the Pinatubo sulfur.

Genetic bottleneck in humans

The Toba eruption has been linked to a genetic bottleneck in human evolution about 70,000 years ago; it is hypothesized that the eruption resulted in a severe reduction in the size of the total human population due to the effects of the eruption on the global climate. According to the genetic bottleneck theory, between 50,000 and 100,000 years ago, human populations sharply decreased to 3,000–10,000 surviving individuals. It is supported by some genetic evidence suggesting that today's humans are descended from a very small population of between 1,000 and 10,000 breeding pairs that existed about 70,000 years ago.

Proponents of the genetic bottleneck theory (including Robock) suggest that the Toba eruption resulted in a global ecological disaster, including destruction of vegetation along with severe drought in the tropical rainforest belt and in monsoonal regions. A 10-year volcanic winter triggered by the eruption could have largely destroyed the food sources of humans and caused a severe reduction in population sizes. These environmental changes may have generated population bottlenecks in many species, including hominids; this in turn may have accelerated differentiation from within the smaller human population. Therefore, the genetic differences among modern humans may reflect changes within the last 70,000 years, rather than gradual differentiation over hundreds of thousands of years.

Other research has cast doubt on a link between the Toba Caldera Complex and a genetic bottleneck. For example, ancient stone tools at the Jurreru Valley in southern India were found above and below a thick layer of ash from the Toba eruption and were very similar across these layers, suggesting that the dust clouds from the eruption did not wipe out this local population. However, another site in India, the Middle Son Valley, exhibits evidence of a major population decline and it has been suggested that the abundant springs of the Jurreru Valley may have offered its inhabitants unique protection. Additional archaeological evidence from southern and northern India also suggests a lack of evidence for effects of the eruption on local populations, leading the authors of the study to conclude, "many forms of life survived the supereruption, contrary to other research which has suggested significant animal extinctions and genetic bottlenecks". However, some researchers have questioned the techniques utilized to date artifacts to the period subsequent to the Toba supervolcano. The Toba Catastrophe also coincides with the disappearance of the Skhul and Qafzeh hominins. Evidence from pollen analysis has suggested prolonged deforestation in South Asia, and some researchers have suggested that the Toba eruption may have forced humans to adopt new adaptive strategies, which may have permitted them to replace Neanderthals and "other archaic human species".

Additional caveats include difficulties in estimating the global and regional climatic impacts of the eruption and lack of conclusive evidence for the eruption preceding the bottleneck. Furthermore, genetic analysis of Alu sequences across the entire human genome has shown that the effective human population size was less than 26,000 at 1.2 million years ago; possible explanations for the low population size of human ancestors may include repeated population bottlenecks or periodic replacement events from competing Homo subspecies.

Genetic bottlenecks in other mammals

Some evidence points to genetic bottlenecks in other animals in the wake of the Toba eruption. The populations of the Eastern African chimpanzee, Bornean orangutan, central Indian macaque, cheetah and tiger, all recovered from very small populations around 70,000–55,000 years ago.

The exact geographic distribution of anatomically modern human populations at the time of the eruption is not known, and surviving populations may have lived in Africa and subsequently migrated to other parts of the world. Analyses of mitochondrial DNA have estimated that the major migration from Africa occurred 60,000–70,000 years ago, consistent with dating of the Toba eruption to around 75,000 years ago.^{[citation needed]}

• Early human migrations – Spread of humans from Africa through the world
• Most recent common ancestor – Most recent individual from which all organisms in a group are directly descended
• Quaternary extinction event – Extinctions of large mammals in the Late PleistocenePages displaying short descriptions of redirect targets
• Recent African origin of modern humans – "Out of Africa" theory of the early migration of humans
• Timeline of volcanism on Earth
• Wallace Line – Line separating Asian and Australian fauna

• Prothero, Donald R. (2018). When Humans Nearly Vanished: The Catastrophic Explosion of the Toba Volcano. Washington: Smithsonian Books. ISBN 978-1588346353. OCLC 1020313538.

• Population Bottlenecks and Volcanic Winter
• "Toba Volcano by George Weber". Archived from the original on April 22, 2011. Retrieved June 1, 2006.
• "The proper study of mankind" – Article in The Economist
• Homepage of Professor Stanley H. Ambrose, including bibliographic information on the two papers he has published on the Toba catastrophe theory
• Mount Toba: Late Pleistocene human population bottlenecks, volcanic winter, and differentiation of modern humans by Professor Stanley H. Ambrose, Department of Anthropology, University Of Illinois, Urbana, USA; Extract from "Journal of Human Evolution" [1998] 34, 623–651
• Journey of Mankind by The Bradshaw Foundation – includes discussion on Toba eruption, DNA and human migrations
• Geography Predicts Human Genetic Diversity ScienceDaily (Mar. 17, 2005) – By analyzing the relationship between the geographic location of current human populations in relation to East Africa and the genetic variability within these populations, researchers have found new evidence for an African origin of modern humans.
• Out of Africa – Bacteria, As Well: Homo Sapiens And H. Pylori Jointly Spread Across The Globe ScienceDaily (Feb. 16, 2007) – When man made his way out of Africa some 60,000 years ago to populate the world, he was not alone: He was accompanied by the bacterium Helicobacter pylori...; illus. migration map.
• Magma 'Pancakes' May Have Fueled Toba Supervolcano
• Youtube video "Stone Age Apocalypse"