Toba Catastrophe Theory - Volcanic Winter and Cooling

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 low δ18O 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.

According to Alan Robock, the Toba incident did not initiate an ice age. Using an emission of 6 billion tons of sulphur dioxide, his simulations demonstrated a maximum global cooling of around 15 °C, approximately three 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 (9,900 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.