The study, which was led by researchers from the University of Cambridge and the British Antarctic Survey, examined volcanic ash deposits in the Southern Ocean that formed during periods of increased volcanic activity. The researchers found that the levels of atmospheric carbon dioxide (CO2) decreased markedly around the same time that volcanic activity increased, suggesting that there may be a link between the two.
CO2 is a greenhouse gas that helps keep the Earth warm, so decreased levels of CO2 would promote cooling. It is widely believed that changes in ocean circulation were also instrumental in precipitating the cold, glacial conditions of the ensuing ice age.
Scientists had previously thought that other factors, such as the Earth's orbit around the sun or the presence of extensive ice sheets on Antarctica, were the primary triggers of the ice age. However, this latest study suggests that volcanic activity may have been the key factor that initiated the process of cooling.
The study also found that the periods of increased volcanic activity were accompanied by decreases in the level of oxygen in the deep ocean. The researchers believe this is because the volcanic ash that settled on the ocean surface reduced the amount of sunlight reaching the depths of the ocean, slowing down the production of oxygen by organisms at the ocean's surface. The reduced oxygen levels would further decrease species diversity and alter ocean circulation patterns, thereby also potentially promoting the freezing of ocean surfaces.
The study findings are consistent with the hypothesis that certain patterns of volcanic activity can release enough of certain chemicals to reduce both CO2 and oxygen to tipping points at which they trigger rapid global cooling."
Lead researcher Dr. Robert Mulvaney explained: "Until now it has been a mystery what the initial driver was to start the global cooling and the shift into this glacial climate state 2.6 million years ago."
"It had already been noted that around one and a half million years before the cold onset there was a big increase in ice rafting activity in areas now adjacent to the East Antarctic Ice Sheet in the Southern Ocean that was somehow connected to these changes in atmosphere and ocean".
This latest evidence for the triggering of abrupt global cooling events from multiple large volcanic eruptions now makes a much stronger, clearer case, with important implications for predicting how rapid ice sheet loss over the next few centuries from global warming would affect the natural climate system."
