Ancient emissions: did early civilisations alter global climate?


Climate Change

Fire emissions

Credit: Carl Osbourn/Wikimedia

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Massive burning of forests for agriculture thousands of years ago may have increased atmospheric carbon dioxide enough to alter the global climate and usher in a warming trend that continues today, US researchers have suggested.

In a study published last week in the journal Quaternary Science Reviews, researchers said that today's six billion people use about 90 per cent less land per person for growing food than was used by far smaller populations early in the development of civilization.

Those early societies likely relied on slash-and-burn techniques to clear large tracts of land for relatively small levels of food production.

"They may have inadvertently altered the climate," said climate scientist and lead author William Ruddiman, from the University of Virginia.

"They used more land for farming because they had little incentive to maximize yield from less land, and because there was plenty of forest to burn."

Early populations likely cleared five or more times the land they actually farmed at any given time, he said. They would burn forests, plant and grow crops until yields began to decline, then burn off another area of forest to start again, ever expanding the cleared areas as their populations grew.

It was only as populations grew much larger, and less land was available for farming or for laying fallow, that societies adopted more intensive farming techniques and slowly gained more food yield from less land.

Ruddiman said that with the highly efficient and intensive farming of today, growing populations are using less land per capita for agriculture, allowing forests to return in many parts of the world.

But the positive environmental effects of this reforestation, however, are being cancelled out by our large-scale burning of fossil fuels, he said.

This is not the first time Ruddiman has put forward the idea that humans began altering the global climate thousands of years ago. Five years ago he suggested human activity could in part account for rises in carbon dioxide that began about 7,000 years ago.

But so far his theories have been met with criticism, with other climate scientists suggesting early populations were just too small to create enough carbon dioxide to alter the climate, and that truly large-scale land clearing and resulting emissions have only occurred during the industrial era.

"[But] many climate models assume that land use in the past was similar to land use today; and that the great population explosion of the past 150 years has increased land use proportionally," said Ruddiman's co-author, Erle Ellis, an ecologist who specialises in land-use change.

"We are proposing that much smaller earlier populations used much more land per person, and may have more greatly affected climate than current models reflect."