Conservation Comment: How Genghis Khan changed the climate

A statue of Genghis Khan, who is regarded as the founder of Mongolia's national culture. Photo / Supplied

Science is not a story of absolutes and indisputable facts. As Karl Popper explained, the difference between a scientific theory and a non-scientific theory is that a scientific theory can be tested and proven false.

Straightforward science tells us that carbon dioxide in the atmosphere acts like a blanket, reducing the escape of heat.

Just like putting an extra blanket on the bed in winter quickly feels more snug, if the level of carbon dioxide in the atmosphere increases the average temperature of the surface of the earth will increase. We are seeing atmospheric temperature rises at the moment which most scientists attribute to extra carbon dioxide in the atmosphere from the use of fossil fuels.

So what if we humans can reduce the level of carbon dioxide? Have we done it in the past?

In Mongolia the birthday of Genghis Khan, who is regarded as the founder of the national culture, is a national holiday. He was actually born about 1162. Over the next 250 years he, and the dynasty that he founded, created the largest empire the world has ever seen, taking in most of Asia and threatening Europe.

The downside of this empire building was the body count. Huge tracts of land were depopulated. The agriculture that had supported the people of these areas disappeared and the land went back to forest. Reforestation sequestered vast amounts of carbon from the atmosphere. Ice core records tell us a drop in atmospheric temperature also occurred at this time.

The population of North America before the mass arrival of Europeans is estimated at perhaps 80 million. This population was devastated by diseases from Europe, causing the collapse of agriculture over huge areas of what is now the United States. These lands, as with central Asia a century earlier, went back to forest.

According to Stanford University Professors Richard Nevle and Denis Bird, the resulting transformation of atmospheric carbon into wood resulted in atmospheric cooling that peaked in the period 1500 to 1750 with what has been dubbed "The Little Ice Age".

Some historians suggest that pressure on food and other supplies caused by more severe winters at that time were a root cause of violent unrest in Europe and one of the, per capita, bloodiest wars in history, the English Civil War. There is not space here to explore the results of these conflicts which still affect us today.

Similar work by Julia Pongratz of the Carnegie Institution's Department of Global Ecology suggests similar, smaller, effects on climate resulting from the Black Death plagues and the fall of the Ming Dynasty in China.

Professor William Ruddiman of the University of Virginia has pointed out that the level of atmospheric greenhouse gases stopped accurately following the patterns of the Milankovitch cycles (caused by predictable, gravity induced variations in the orbit and inclination of the earth) at the point 8000 years ago when humans first started intensive agriculture.

We can only survive if we work alongside the Earth not against it.

* Frank Gibson is a recently retired teacher of physics and maths who intends to use his spare time to do physics and maths.