Central Africa's rainforests may have been lost 3,000 years ago because of human land-use for agriculture.
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SYDNEY: Central Africa's shift from rainforests to savannas 3,000 years ago may have been caused by human actions, according to new research.
Publishing in Science today, a team of scientists have analysed sediment samples from the mouth of the Congo River made from the chemical breakdown of rocks and minerals in the Congo Basin over the past 40,000 years. This chemical breakdown, or weathering, is dependent on rainfall, vegetation and land-use so provides an accurate record of changes over time.
Despite the consensus among scientists that climate change caused Africa's rainforest loss, this research found the loss may have also been caused by increased human land-use for agriculture.
"Between about 40,000 and 3,000 years ago, the intensity of weathering was strongly related to rainfall, with the following relationship: the more it rained in Central Africa, the more the soils were weathered," said co-author Germain Bayon from the French Research Institute for Exploration of the Sea. "After 3000 years ago, the rate of weathering intensified strongly while the climate was becoming progressively dryer. This decoupling suggests that the increase of weathering rates could not be simply related to climate change."
Rapid disappearance of rainforests
Previous research indicates that Central Africa was once covered in rainforests full of evergreen trees, but a sudden change occurred around 3,000 years ago that caused these forests to disappear quickly and be overtaken by savannas of grassy lands with sparse trees.
Scientists thought this rapid takeover was caused by changes in climate, with drier conditions and lower rainfall levels beginning 1,000 years before the rainforests disappeared. However, this wasn't the only change occurring.
At this time Bantu-speaking farmers were expanding across Africa. Now spread across Africa with hundreds of languages, including Swahili, they originally migrated out from the modern Nigeria-Cameroon border and reached the Congo River area 3,000 years ago. This migration was associated with the introduction of agriculture into the Central African rainforest where farmers began cultivating savanna crops, such as yams, which could have affected soils and sediments in the area.
Searching a kilometre under water
Collecting sediment samples from 914 m under the surface of the Congo River, each sample was taken at 5 cm intervals and correlated to hundreds of years of geological activity. The researchers measured the presence of elements known to indicate levels of weathering and soil movement into the riverbed.
Sample analysis found that from 40,000 to 3,500 years ago the rainfall in Africa was strongly linked to levels of weathering and breakdown of rocks and minerals in the area.
However, around 3,500 years ago this pattern decoupled - despite the move to drier conditions in Africa, weathering of rocks and minerals intensified more rapidly than at any point in the last 40,000 years. The results indicated that climate change wasn't the only factor that affected the weathering of rocks and minerals or the loss of rainforests, and the researchers pointed to the arrival of the Bantu-speaking farmers and their agricultural methods as another cause of rainforest loss.
As these farmers cleared the land of rainforests to cultivate their crops, this would have caused soil erosion and increased weathering of rocks and minerals. "This work perhaps provides support to the emerging view that humans have altered their environment from a long time before the start of the Industrial Era," said Bayon.
The researchers had quality data, though it would be good to see if the same trends were found elsewhere in Africa, said John Foden, a geologist at the University of Adelaide in South Australia, who was not involved in the research. "This is quite exciting and highlights the power of this type of geochemical approach."
Scientists are fear mongers
Its no wonder since science brought us 26 years of needless panic with climate blame and their pesticides and cancer causing chemicals that scientists are called "lab coat consultants". You fear mongers are no worse than "say anything" to get elected politicians.