The Hot Zone

How Genghis Khan may have cooled the planet

Posted by Alison Hawkes on January 21, 2011
Category : The man made climate

If human consumption and population growth can be linked to warming the climate, there’s certainly a sensible argument to be made that a reversal in the trend could cool the planet down.

War, invasion, disease epidemics, and societal collapse — all events that are devastating to humans — may actually have helped drop temperatures momentarily, according to a study published this week in the journal, The Holocene. It’s kind of a morbid perspective, and one that pits humans squarely as the enemy of a stable climate. But there it is, worth a little chewing on.

Lead author Julia Pongratz from the Department of Global Ecology at Stanford University used a climate-carbon cycle model to look at several devastating events in human history: Genghis Khan in the Mongol invasion (1200-1380 AD), the Black Death (1347-1400AD), the conquest of the Americas (1519-1700 AD), and finally the fall of the Ming Dynasty (1600-1650 AD).

Pongratz’s theory was that a drop in population, or the disruption of war, would have taken agricultural land out of production, returning these lands instead to forest cover and natural ecosystems that store carbon. The researchers came up with their estimates on temperature cooling by using pre and post population estimates from each of these major events, using estimates on the amount of agricultural land each person would have required, and then calculating the additional CO2 uptake from that land lying fallow.

So, for example, the Mongol invasion, the population dropped by about 30 percent in the impacted area, which resulted in the regrowth of 142,000 km2 of forests and a reduction in global carbon emissions of 183 megatons.

According to the researchers, that’s not enough to have any significant global cooling impact –  not more than 1 part per million concentration of CO2 in the atmosphere. That’s because even as some part of the planet was heavily impacted by the event, elsewhere in the world human populations could have been expanding and taking down more forested land for agriculture.

There would also have been a lag time in carbon uptake after such events as trees and vegetation need 100 years or more to mature. Previous plant material also has to decompose, taking away the net CO2 storage effects of new plant cover.

Nevertheless, the study brings up an interesting point about human impacts on the climate. The Industrial Revolution is normally pointed to as the event that catapulted the planet into long term climate change. But humans have been contributing to higher and higher CO2 levels since the advent of agriculture and the burning of fires.

The corrective forces of war and disease helped buffer the impacts of humans on CO2 levels in those earlier days. But people had learned they could alter their environment, a revelation that brings us to today.

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