Researchers have known for some time that the CO2 concentration in the atmosphere suddenly decreased in the 16th and 17th centuries. However, it was unclear how that happened. But British researchers now think they have figured it out: pandemics brought into the New World by European colonists appear to have caused the decline in CO2.
Researchers from the British Antarctic Survey after they took another look at an ice core obtained a few years ago in Antarctica. Such an ice core is a kind of time capsule that – thanks to air bubbles that have become trapped in the ice – can reveal more about how the atmosphere worked long ago (see box).
Ice core
An ice core consists of all kinds of layers of ice, which once started as snow layers. As more of these snow layers form – due to successive snow showers – the lower layers are compressed by the weight of the upper snow layers, transforming them into layers of ice. Air that has become trapped between the snowflakes during snow showers is captured in the ice. And by sampling those air bubbles in the ice, researchers can get a picture of what the atmosphere looked like when that snow layer (later transformed into ice) was formed. And in this way the researchers can look quite far back: ice cores often consist of ice layers that were formed many centuries or even thousands of years ago.
1454-1688
For the new research, scientists used a 651-meter-long ice core that was drilled from the edge of the West Antarctic ice sheet, the underside of which contains ice thousands of years old. However, for this study, the researchers did not have the ambition to look that far back; they focused on a more recently formed part of the ice core, namely the part that consisted of ice layers that formed between the years 1454 and 1688. The aim was to find out more about the rather mysterious drop in CO2 concentrations that took place in the sixteenth and seventeenth centuries.
Gradual decline
To this end, the researchers analyzed air bubbles that had become trapped in the ice layers and could provide an insight into the composition of the atmosphere in the sixteenth and seventeenth centuries. They specifically looked at how much CO2 and methane the air bubbles contained. And what they found was that CO2 concentrations decreased by about 0.5 ppm (parts per million) per decade between 1516 and 1670. And thus the amount of CO2 in the atmosphere decreased much more gradually than assumed.
America
But it doesn’t stop there. In their study, the researchers can also link this gradual decrease in the amount of CO2 in the atmosphere to a cause. The decrease therefore corresponds neatly with the decrease that models predict if land use changes drastically. And that happened exactly in the sixteenth and seventeenth centuries, namely in America. The European settlers arrived there around that time. And they brought with them diseases that the indigenous people of the Americas had never before been exposed to – such as smallpox, measles and the plague. It led to huge epidemics among the indigenous population, causing many deaths. And this also had consequences for land use: fields that were previously cultivated by flourishing indigenous societies became fallow because those societies no longer flourished and even shrank. That didn’t last long; surrounding forests advanced and appropriated the fields. And because these new areas of forest removed significant amounts of CO2 from the atmosphere, the CO2 concentration fell, the researchers say in the magazine Nature Communications.
“It is truly sobering to confirm that the last – if not the only – time in history that human activities caused CO2 concentrations to decrease rather than increase is linked to the devastating loss that can be associated with colonization of America,” says researcher Thomas Bauka.
