Enlarge / A slice through an ice core showing bubbles of trapped air. (credit: British Antarctic Survey)

Did the massive scale of death in the Americas following colonial contact in the 1500s affect atmospheric CO₂ levels? That’s a question scientists have debated over the last 30 years, ever since they noticed a sharp drop in CO₂ around the year 1610 in air preserved in Antarctic ice.

That drop in atmospheric CO₂ levels is the only significant decline in recent millennia, and scientists suggested that it was caused by reforestation in the Americas, which resulted from their depopulation via pandemics unleashed by early European contact. It is so distinct that it was proposed as a candidate for the marker of the beginning of a new geological epoch—the “Anthropocene.”

But the record from that ice core, taken at Law Dome in East Antarctica, shows that CO₂ starts declining a bit late to match European contact, and it plummets over just 90 years, which is too drastic for feasible rates of vegetation regrowth. A different ice core, drilled in the West Antarctic, showed a more gradual decline starting earlier, but lacked the fine detail of the Law Dome ice.