Did the South Ocean promote the rise of humanity?
Human civilization would probably not be where it stands today without the help of the climate: since the end of the last ice age about 11,000 years ago, it has developed and spread rapidly. This was made possible by a particularly mild and stable climate, which our ancestors owed to a phase of slightly increased CO2 levels. Where this greenhouse gas came from back then was unclear. Now researchers could have found an answer – in the Southern Ocean.
Today, carbon dioxide is more likely to be considered “bad” as the cause of climate change. But about 11,000 years ago, it was probably this greenhouse gas that gave our ancestors a period of unusually stable, warm climates. For the age after the end of the Ice Age, called the Holocene, was one of the rare warm periods of the last million years. The reason for this was that while in other warm periods CO2 levels remained stable or even decreased, the CO2 content of the atmosphere in the Holocene rose by about 20 parts per million (ppm) – from 260 ppm in the early to 280 ppm in the late Holocene. “Considering today’s CO2 increase, this may not seem much,” says co-author Daniel Sigman of Princeton University. “However, scientists believe that this small but significant increase played a key role
“Leak” in the biological pump
But what triggered this decisive increase in CO2 at that time and where the greenhouse gas came from was previously unclear. Some researchers suspected that vegetation could be reduced; others considered the then strongly rising sea level or increased alkalinity of the seawater as possible causes. For their study, Sigman, first author Anja Studer from the Max Planck Institute for Chemistry in Mainz and her team set out to search for clues in the Southern Ocean. In this marine region, large amounts of cold deep water rise and with it, CO2 that has previously been absorbed and stored by algae and other organisms from other marine areas is returned to the surface of the ocean. “We often refer to the South Ocean as a ‘leak’ in the biological pump,” explains Sigman. How strong this “leak” was after the end of the ice age,
The evaluations showed that during the Holocene the surface water of the Southern Ocean was unusually rich in nitrogen. According to the researchers, this indicates that very large quantities of nutrient- and CO2-rich water from the deep ocean had risen to the surface of the Southern Ocean at that time. As a result, more greenhouse gas was released into the atmosphere – and that might have been enough to increase global CO2 levels by 20 ppm, as Studer and her colleagues explain. “Probably weakening the biological pump was not the only change that affected atmospheric CO2 during the Holocene,” the researchers explain. “But adding the changes to the terrestrial biosphere and the alkalinity of the ocean, you get a 20ppm net rise in that time.”
Cause of the “leak” still unclear
However, it is still unclear why, after the end of the ice age in the Southern Ocean, a great deal of CO2-rich deep water reached the top. The researchers suspect that a change in the large-scale wind currents played a role in this. For example, the western hemisphere of the southern hemisphere could have shifted to the south and strengthened. That would have led to more waves and a stronger water mixing of the Southern Ocean – and could have brought more deep water to the surface. “However, so far there is no clarity as to how the position and strength of the westerly winds actually changed at that time”, emphasize Studer and her colleagues. Here you have to continue research.
The results are also interesting for today and the future. Even now, the biological pump of the ocean plays a key role in the climate and CO2 levels in the atmosphere. “Using the results for the Holocene to predict how the rising water currents in the Southern Ocean will evolve in the future can also help to improve predictions on the future evolution of atmospheric CO2 and global climate,” says Sigman. A somewhat encouraging scenario would be that anthropogenic climate change could even boost the transport of CO2-rich water to the sea surface. This would bring additional greenhouse gas into the atmosphere and increase global warming.