Toba catastrophe theory

Thanks to Alan MasonThe Toba supereruption (Youngest Toba Tuff or simply YTT) occurred between 69,000 and 77,000 years ago at Lake Toba (Sumatra, Indonesia), and it is recognized as one of the earth’s largest known eruptions. The related catastrophe theory holds that this supervolcanic event plunged the planet into a 6-to-10-year volcanic winter, which resulted in the world’s human population being reduced to 10,000 or even a mere 1,000 breeding pairs, creating a bottleneck inhuman evolution. Some researchers argue that the Toba eruption produced not only a catastrophic volcanic winter but also an additional 1,000-year cooling episode.

The Toba event is the most closely studied supereruption. In 1993, science journalist Ann Gibbons first suggested a link between the eruption and a bottleneck in human evolution. Michael R. Rampino of New York University and Stephen Self of the University of Hawaii at Manoa quickly lent their support to the idea. The theory was further developed in 1998 by Stanley H. Ambrose of theUniversity of Illinois at Urbana-Champaign.

Supereruption

The Toba eruption or Toba event occurred at what is now Lake Toba about 73,500 years (± 3,000 years) or 73,000 years (± 4,000 years) ago. The Toba eruption was the latest of the three major eruptions which occurred at Toba in the last 1 million years. The last eruption had an estimated Volcanic Explosivity Index of 8 (described as “mega-colossal”), or magnitude ≥ M8; it thus made a sizeable contribution to the 100 × 30 km caldera complex. Dense-rock equivalent estimates of eruptive volume for the eruption vary between 2,000 km3 and 3,000 km3, but the most frequently quoted DRE is ~2,800 km3 (7 × 10 18g) of erupted magma, of which 800 km3 was deposited as ash fall. It was two orders of magnitude greater in erupted mass than the largest volcanic eruption in historic times, in 1815 at Mount Tambora in Indonesia, which caused the 1816 “Year Without a Summer” in the northern hemisphere.

Although the eruption took place in Indonesia, it deposited an ash layer approximately 15 centimeters thick over the entirety of South Asia. A blanket of volcanic ash was also deposited over the Indian Ocean, and the Arabian and South China Sea. Studies, based on deep-sea cores retrieved from the South China Sea, recently extended the known distribution of the eruption, and suggest that the ~2,800 km3 calculation of the eruption magnitude is a minimum value or even an under-estimate.

Volcanic winter and cooling

The apparent coincidence of the eruption with the onset of the last glacial period attracted the scientists’ interest. Michael L. Rampino and Stephen Self argued that the eruption caused a “brief, dramatic cooling or ‘volcanic winter'”, which resulted in a global mean surface temperature drop of 3–5°C and accelerated the glacial transition from warm to cold temperatures of the last glacial cycle. Zielinski showed Greenland ice core evidence for a 1,000-year cool period with lowδ18O and increased dust deposition immediately following the eruption. He further suggested that this 1,000-year cool period (stadial) could have been caused by the eruption, and that the longevity of the Toba stratospheric loading may account at least for the first two centuries of the cooling episode. Rampino and Self believe that global cooling was already underway at the time of the eruption, but the procedure was extremely slow; YTT “may have provided the extra ‘kick’ that caused the climate system to switch from warm to cold states.” Oppenheimer discounts the arguments that the eruption triggered the last glaciation, but he accepts that it may have been responsible for a millennium of cool climate prior to the Dansgaard-Oeschger event 19.

According to Alan Robock, the Toba incident did not initiate an ice age. Using an emission of 6,000 million tons of sulfur dioxide, his simulations demonstrated a maximum global cooling of around 15 °C, approximately 3 years after the eruption. As the saturated adiabatic lapse rate is 4.9 °C/ 1,000 m for temperatures above freezing, this means that the tree line and the snow line were around 3,000 m (9,000 ft) lower at this time. Nevertheless, the climate recovered over a few decades. Robock found no evidence that the 1,000-year cold period seen in Greenland ice core records was directly generated by the Toba eruption. Nevertheless, he argues that the volcanic winter would have been colder and longer-lasting than Ambrose assumed, which strengthens his argument for a genetic bottleneck. Contrary to Robock, Oppenheimer believes that estimates of a surface temperature drop of 3–5°C after the eruption are probably too high; a figure closer to 1°C appears more realistic. Robock criticized Oppenheimer’s analysis, arguing that it is based on simplistic T-forcing relationships.

Despite the different approaches and estimates, scientists agree that a supereruption like the one at Lake Toba must have led to very extensive ash-fall layers and injection of noxious gases into the atmosphere, having severe worldwide effects on climate and weather. Additionally, the Greenland ice core data display an abrupt climate change around this time, but there is no consensus that the eruption directly generated the 1,000-year cold period seen in Greenland or triggered the last glaciation.

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