Humans may have been causing climate change for much longer than we've been burning fossil fuels. In fact, the agrarian revolution may have started human-induced climate changes long before the industrial revolution began to sully the skies. How? Through the clearing of forests, which still remains the second-largest source of greenhouse gas emissions from human activity.

Sediment cores from the mouth of the Congo River—the deepest river in the world—suggest that humans may have played a significant role in changing the landscapes of Central Africa. That river curves through the world's second-biggest lingering tropical forest, but it and its tributaries also flow through the savannas so prized by modern-day safaris.

Scientists had previously thought that a climate shift from warm and humid to seasonally cooler and drier had helped create those savannas, which covered even more of Central Africa in the past. But the 40,000-year-old record preserved in the sediment cores tells a different story. Roughly 3,500 years ago the Congo River suddenly began dumping a lot more muck without any appreciable increase in rainfall to explain such weathering. One plausible explanation is the simultaneous arrival of the so-called Bantu people, who brought farming into the region.

They cultivated oil palm, pearl millet and yams, crops that need plenty of sunlight, which, of course, necessitated clearing forests. They also cut down trees for charcoal and as fuel for the fires of iron-smelting, which enabled them to make tools and weapons. Coupled with climate change, the result was savannas—and mutually reinforcing climate change.

At the same time, the presence of crops such as millet and yams suggests that climate had already changed given that they require alternating seasons of wet and dry. So it remains unclear whether changing climate conditions created the savannas that made Bantu-style farming possible or if Bantu-style farming created the conditions for savannas and changed the climate. What is clear is that "the environmental impact of human population in the central African rainforest was already significant about 2,500 years ago," as the researchers write in the paper presenting their findings published online in Science on February 9.

The same story is being repeated today in the same area. Forest is being cleared for agriculture to feed a swelling population, though locals are caught up in regional wars. At the same time, exploitative mining is ongoing for resources such as coltan, the mineral compound that offers up the element tantalum, critical in the manufacture of the tiny circuits that make smaller cell phones possible. Once again the Congo River is discharging a record in sediment of humanity's forest-destroying ways—and one that has been retold, with local variations, on every continent.

Image: Courtesy of NASA