If humans are responsible for speeding the climate change currently underway, it may not be the first time. Scientists say a long time ago in Central Africa, early farmers may have contributed to the disappearance of rainforests. The question is being raised in the journal Science.
Scientists say about 3,000 years ago some of the rainforests were "abruptly replaced" by savannas – broad grasslands dotted with shrubs and trees. It was thought that climate change was the reason. But now research suggests that climate change alone could not be responsible for the sudden shift -- that perhaps people played a part.
Dr. Germain Bayon, who works for a French research institute, said much less is known about the agricultural history of Africa than either Europe or Asia.
"The onset of agriculture had already an impact on the environment, whereas in Africa, I guess because there have been (a lot fewer ) studies done, the link was not very clear," he said.
Telltale mud
Bayon and his colleagues have been studying sediment at the mouth of the Congo River.
"Basically, we analyzed the sediment core, which was recovered off the Congo River. Sediments have been accumulating at this site... which provide an integrated record of the particles, which have been discharged by the Congo River through time," he said.
The sediment tells the story of the Congo Basin climate.
"The climate in Central Africa was much more humid between about 10,000 years ago and 5,000 years ago. And then the climate started to deteriorate. So, it's only after about 4,000 years ago that the climate started to become dryer and of course the vegetation responded to this climate change," said Bayon.
That's when the savannahs started to appear. But Bayon said there may be other major contributing factors, such as erosion and the chemical weathering of soil.
"So, basically, the more it rains in Africa, the more the soils were being eroded. And what we showed is that from 3,000 years ago the weathering erosion signal became completely decoupled from the climatic signal. And this we think is a sign that this event was not only triggered by a change in the climate. And we need to take something else into account," he said.
Migration of the Bantu
Enter the Bantu people from what is now the border area between Nigeria and Cameroon. Bayon said they brought farming and iron smelting to the Congo Basin. Evidence shows one of the main crops was pearl millet. And that in itself says a lot about the climate.
"Pearl millet to be cultivated actually doesn't like the warm climate, humid climate. It requires alternating between a dry season and a wet season. It shows that the key factor for introducing agriculture into the rainforest was the establishment of this more pronounced seasonality. This alternates between wet and dry seasons," he said.
To grow pearl millet and other crops, the Bantu needed open fields. That meant clearing large areas of rainforest. That in turn exposed the land to erosion and resulted in the telltale sediment at the mouth of the Congo River. So, it's possible, said Bayon, the farmers along with climate change helped the rainforests to disappear.
The period lasted between 1,000 and 1500 years. Then things started to change again. The rainforests began to return. About the same time, the Bantu left for other parts of Africa. The question is: did the Bantu leave because the rainforests started to regrow or did the rainforests regrow because the Bantu left and took their farming with them?
"This, I must admit," said Bayon, "is something which is a bit puzzling and which is not well known at present, I think."
Bayon said, however, what the evidence does show is that even 3,000 years ago humans could have a major impact on the environment. It's now known that agriculture contributes to carbon emissions and that trees help trap that carbon and keep it from the atmosphere. During the Bantu's stay in Central Africa, there was more agriculture and fewer trees.