How a warming Earth encouraged human settlement and expansion

A period of global warming jump-started the Holocene period around 11,000 years ago which allowed for human expansion, agriculture, and a stable climate.

As temperatures warmed, glaciers retreated, humans could spread out over a larger landscape, and the higher concentrations of CO2 in the atmosphere made permanent settlement possible.

The climate during the Holocene was unusually stable with atmospheric carbon levels increasing from 260 parts per millions (ppm) to 280 parts per million.

While the warming over the past 11,000 years has been well documented, the explanation for the sudden warming trend remained unknown.

An international collaboration of researchers led by Princeton University and the Max Planck Institute for Chemistry conducted a study examining Southern Ocean upwelling as the possible explanation for the Holocene’s warming.

Activity in the Southern Ocean weakens the oceans’ abilities to store carbon and the study proposes that this is what caused the warming starting 11,000 years ago.

The world’s oceans contain huge stores of CO2 and there is concern that warming temperatures will release these carbon stores back into the atmosphere, driving further climate change.

That’s why it’s important to understand what was behind the stable interglacial warming of the Holocene as it could help better predict climate models and show the effect that an increase in fossil fuels will have on ocean circulation and atmospheric CO2 levels in the future.

The results were published in the journal Nature Geoscience.

“We think we may have found the answer,” said Daniel Sigman, a member of the research team. “Increased circulation in the Southern Ocean allowed carbon dioxide to leak into the atmosphere, working to warm the planet.”

Sigman and the researchers theorized that a leak in the Southern Ocean, venting carbon from phytoplankton populations deep below the Ocean’s surface, was responsible for the Holocene warming.

To study this possibility, the researchers collected and analyzed diatoms, foraminifers, and deep-sea coral fossils from different areas of the Southern Ocean.

Diatoms and foraminifera are shelled microorganisms and researchers can measure the nitrogen isotope ratios trapped within the fossils to model the nutrient levels of the Southern Ocean at the time the fossils were alive. This can help show temperatures, oxygen levels, and the climate during that time as well.

After analyzing the isotope ratios, the measurements indicated that during the early years of the Holocene, the Southern Ocean saw an increase in nutrients and carbon dioxide and this weakened the ocean’s ability to contain its carbon stores and CO2 leaked into the atmosphere.

This could explain the 20 parts per millions rise in atmospheric carbon. The researchers say the same processes responsible for warming the Holocene are happening now, made worse by the increase in greenhouse gas emissions from anthropogenic activities.

“This process is allowing some of that deeply stored carbon dioxide to invade back to the atmosphere,” said Sigman. “We’re essentially punching holes in the membrane of the biological pump. If the findings from the Holocene can be used to predict how Southern Ocean upwelling will change in the future, it will improve our ability to forecast changes in atmospheric carbon dioxide and thus in global climate.”

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By Kay Vandette, Earth.com Staff Writer