How Long Have Humans Been Changing The Climate?

A long time, researchers claim:

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.

Interesting.  The post also mentions clearing for agriculture and coltan mining in the region today are still changing the climate.