Antarctic ice
© Thomas BauskaAir bubbles in Antarctic ice, akin to ice cores.
Scientists have discovered in Antarctic ice a strange link between past levels of atmospheric carbon dioxide and centuries-long global pandemics, reminding us of just how easily humans - or the lack thereof - can shape planet Earth.

Bubbles of air encased in ancient ice are like teensy time capsules, trapping tiny samples of gases from atmospheres thousands or even millions of years ago.

The best records for the past 2,000 years come from just two ice cores that have greatly influenced modeling studies of climate and carbon cycles in the Common Era: the Law Dome, an Antarctic ice 'hill'; and the West Antarctic Ice Sheet (WAIS) Divide ice cores.

Some 2,000 years ago, at the dawn of the Common Era, empires were rising, Mount Vesuvius was erupting, unknown souls were scratching scrolls of parchment, and humans kept warring across Europe. In the centuries prior and since, diseases such as the plague and syphilis decimated human populations, over and over again.

Covering this formative period, the Law Dome and WAIS Divide ice cores complement each other nicely, although there are some inconsistencies between the two in their measurements of atmospheric CO2.

For instance, the Law Dome core shows a rapid decrease in CO2 levels over a 90-year period that bottoms out at 1610 CE.

This parallels the aftermath of the first contact between the Old and New World, where scores of Indigenous people died from diseases introduced to the Americas by Christopher Columbus, and Europeans faced a similar fate on Columbus' return (though who seeded which diseases is contested by emerging evidence).

Comment: It also coincides with the Little Ice Age period:
The NASA Earth Observatory notes three particularly cold intervals. One began about 1650, another about 1770, and the last in 1850, all of which were separated by intervals of slight warming.[7]

It's thought that human population numbers plummeted so much with these pandemics that communities likely abandoned previously populated areas, allowing vegetation to regrow. These reforested areas would have absorbed huge amounts of CO2 from the atmosphere, hence the measurable drop in the Law Dome ice core.

Comment: Perhaps, but since significant CO2 fluctuations have been shown to rise and fall independent on human activity, isn't it likely that there's a much greater driver at play here?

Note that in our own time studies are claiming that our world is 'greening': NASA: Recent "Greening Earth" has had strong cooling effect on land

The WAIS Divide core, however, doesn't show the same dramatic dip, but rather a gradual decline in CO2 levels stretching further into the 17th century.

To rectify these discrepancies, paleoclimatologist Amy King of the British Antarctic Survey and colleagues measured CO2 levels in a separate ice core, the Skytrain Ice Rise ice core, which was drilled in 2018-2019 from the edge of the West Antarctic Ice Sheet.

King and colleagues analyzed sections of the Skytrain ice core hauled up from 83.2-104 meters (273-341.2 feet) deep, capturing gases dated from 1454 CE to 1688 CE.

Their analysis suggests that CO2 levels indeed declined through this period, likely because of large changes in human populations and land vegetation. However, this decline was more gradual than the Law Dome ice core suggests.

"Our analysis supports a more gradual decrease in CO2 of 0.5 ppm [parts per million] per decade from 1516 to 1670 CE," King and colleagues write in their paper, adding that roughly 2.6 gigatonnes of CO2 were absorbed per decade as human population numbers dwindled and forests regrew.

"This corroborates modeled scenarios of large-scale reorganization of land use in the Americas following New World-Old World contact."

The team then simulated atmospheric carbon fluxes based on each of the ice cores, and possible changes in land vegetation, based on admittedly coarse population estimates.

They conclude that the rapid decrease in CO2 at 1610 CE seen in the Law Dome ice core is "implausibly large" and "incompatible with even the most extreme land-use change scenarios".

Yet even so, there could be some as-yet-unknown climate-carbon feedback event, which could explain the minimum in the Law Dome record, the researchers note.

The study has been published in Nature Communications.