According to a new climate model, the Sahara desert should have formed 500 years earlier than it did. The influence of hunter-gatherers and pastoralists may explain the delay in desertification.

The Sahara only became the desert it's known as today some 5,500 years ago. Some 8,000 years ago, the band stretching across North Africa was green, home to diverse vegetation and populations of hunter-gatherers.

Changes in the tilt of Earth's orbital axis cause paths of seasonal monsoons in Africa to shift. These shifts explain why the Sahara alternates between dry and wet across long time-scales.

Scientists have previously argued over-exploitation and degradation by humans accelerated the Sahara's last transition from grassland to desert, but the latest findings -- published this week in the journal Nature Communications -- suggest the opposite is true.

"The possibility that humans could have had a stabilizing influence on the environment has significant implications," Chris Brierley, a geographer at the University College London, said in a news release. "We contest the common narrative that past human-environment interactions must always be one of over-exploitation and degradation."

Scientists designed a model to predict when the African Humid Period should have ended. Researchers populated their model with data on vegetation, precipitation and atmospheric CO2. The simulations showed the "Green Sahara" should have turned to desert 500 years earlier than it did -- not later.

The discovery suggests human activities could have delayed the region's transition to desert.

"The fact that societies practicing 'pastoralism' persisted in this region for so long and invested both economically and ideologically in the local landscape, does not support the scenario of over-exploitation," Brierley said. "Our study shows that increasing human population and sustainable pastoralism did not accelerate -- and may even have delayed -- the decline of the Green Sahara."

Around 1,000 years before the Sahara turned to desert, the region experienced an increase in the number of pastoralists, nomadic or semi-nomadic cattle-herders.

Research suggests the Sahara's herders were adept at adapting to environmental change and managing scarce natural resources.

"The spread of domestic animals across the Sahara occurred at a time of increasing climatic instability, and yet, these pastoralist populations thrived," King's College London researcher Katie Manning said. "It is likely that strategies used by contemporary traditional herders, such as seasonal movement and selective grazing, were also used by these early pastoralists, helping to maintain an otherwise deteriorating ecosystem."

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