Enlarge / Loads of lava: Kasbohm with a few solidified lava flows of the Columbia River Basalts. (credit: Joshua Murray)

As our climate warms beyond its historical range, scientists increasingly need to study climates deeper in the planet’s past to get information about our future. One object of study is a warming event known as the Miocene Climate Optimum (MCO) from about 17 to 15 million years ago. It coincided with floods of basalt lava that covered a large area of the Northwestern US, creating what are called the “Columbia River Basalts.” This timing suggests that volcanic CO₂ was the cause of the warming.

Those eruptions were the most recent example of a “Large Igneous Province,” a phenomenon that has repeatedly triggered climate upheavals and mass extinctions throughout Earth’s past. The Miocene version was relatively benign; it saw CO₂ levels and global temperatures rise, causing ecosystem changes and significant melting of Antarctic ice, but didn’t trigger a mass extinction.

A paper just published in Geology, led by Jennifer Kasbohm of the Carnegie Science’s Earth and Planets Laboratory, upends the idea that the eruptions triggered the warming while still blaming them for the peak climate warmth.

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.