You wouldn’t necessarily look to Genghis Kahn as being an environmentalist, but the facts are clear, he did a lot for increasing the storage of carbon dioxide as he and his Mongol hordes decided to head out and see what was on the other side of the wall.
A new study published in the online issue of the journal The Holocene by Julia Pongratz of the Carnegie Institution’s Department of Global Ecology has analysed the effect that historical events have had on the storage of carbon dioxide in biomass compared to the overarching increase in human-led deforestation.
“It’s a common misconception that the human impact on climate began with the large-scale burning of coal and oil in the industrial era,” says Pongratz. “Actually, humans started to influence the environment thousands of years ago by changing the vegetation cover of the Earth‘s landscapes when we cleared forests for agriculture.”
Pongratz compiled a detailed reconstruction of global land cover from 800AD to the present, and used a global climate-carbon cycle model to track the impact of land use changes on the global climate over the years studied. Pongratz looked specifically at four major historical events over the time period; the Mongol invasions in Asia (1200-1380), the Black Death in Europe (1347-1400), the conquest of the Americas (1519-1700), and the Fall of the Ming Dynasty in China (1600-1650).
“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,” says Pongratz. “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.”
It is a natural assumption that during plagues and wars the croplands are deserted, for one reason or another, giving the environment around a chance to move back in, if given enough time. Most events did not last long enough, or were neutralised by continued deforestation and clearing in other parts of the world.
However, according to Pongratz’ study, the Mongol invasions allowed depopulated lands to regrow so much that they absorbed nearly 700 million tonnes of carbon from the atmosphere, an amount equivalent to the today’s total annual demand for gasoline.
“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,” she says. “So 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. 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. We cannot ignore the knowledge we have gained.”