High mortality events like the Mongol invasions in Asia, the plague in 14th century Europe, the conquest of the Americas and the fall of the Ming Dynasty in China had minor global effects on atmospheric carbon dioxide (CO2), according to the study published in the online issue of The Holocene.

"We found that during the short events such as the Black Death and the Ming Dynasty collapse, the forest re-growth wasn't enough to overcome the emissions from decaying material in the soil," said lead author Julia Pongratz of the Carnegie Institution’s Department of Global Ecology.

"But during the longer-lasting ones like the Mongol invasion and the conquest of the Americas there was enough time for the forests to re-grow and absorb significant amounts of carbon."

Pongratz - along with colleagues of the Max Planck Institute in Germany and of the Carnegie Institution - compiled a detailed reconstruction of global land cover over the time period from 800 AD to 1850 to determine the impact of historic events on the CO2 cycle.

Increasing deforestation over centuries released huge amounts of carbon dioxide into the atmosphere when trees and other vegetation were burned or decayed.

Christian Reick of the Max Planck Institute in Germany told Deutsche Welle that even though forest regenerations led to local CO2 uptake, "deforestation simultaneously continued elsewhere in the world and erased local effects."

"Even big events such as the Mongol invasions didn't have a visible effect on the carbon cycle," he added.

Learning from the past

The study found that during Mongol invasions in Asia between 1200 and 1380, re-growth of forests stockpiled nearly 700 million tons of CO2 absorbed from the atmosphere. That's as much as the world's total annual footprint from gasoline today.

"People started to interfere with the carbon cycle way ahead of the industrial revolution. It was not until the first half of the 20th century when emissions produced by fossil fuels outnumbered emissions caused by agriculture," Reick told Deutsche Welle.

"Today about a quarter of the net primary production on the Earth's land surface is used by humans in some way, mostly through agriculture," Pongratz said. She added the study could help with current climate issues.

"There is a large potential for our land-use choices to alter the global carbon cycle. In the past we have had a substantial impact on global climate and the carbon cycle, but it was all unintentional." Pongratz said.

"Based on the knowledge we have gained from the past, we are now in a position to make land-use decisions that will diminish our impact on climate and the carbon cycle."

Author: Sarah Steffen
Editor: Anke Rasper