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Farmers May Have Kicked Off Local Climate Change 3,500 Years Ago

The views expressed are those of the author and are not necessarily those of Scientific American.

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congo-riverHumans 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


David Biello About the Author: David Biello is the associate editor for environment and energy at Scientific American. Follow on Twitter @dbiello.

The views expressed are those of the author and are not necessarily those of Scientific American.

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  1. 1. Alex Wild 4:31 pm 02/10/2012

    Years ago I was a Peace Corps volunteer in a frontier town in rural Paraguay. Several of the older farmers remarked that it used to rain more frequently before the forest had been cut and attributed the drier local weather to deforestation. Much like that suggested here.

    This is just anecdata, of course, but it’s at least topical.

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  2. 2. Paleoecologist 5:25 pm 02/10/2012

    Nice piece! A little extra context: William Ruddiman’s Early Anthropocene hypothesis suggests that humans may have been changing the atmosphere and the climate as early as 8,000 years ago, with the onset of deforestation (reducing CO2 uptake by plants) and rice cultivation and livestock rearing (which would have produced methane).

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  3. 3. Owl905 9:19 pm 02/11/2012

    If there’s a case for climate change under those circumstances, it points at a natural tipping point that led to erosion. For parallels, try the drying out of the Fertile Crescent, or the desert spread that led to the emergence of a Nile River civilization. The mere presence of farming in all of those cases simply doesn’t supply a scope and intensity to support the human-cause hypothesis. It’s as tentative as ascribing the red skies of Hanno’s Phoenician exploration to post-harvest stubble burning as proof of a great lost sub-Saharan empire. It’s fun, but it’s … premature.

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  4. 4. jrobinson 9:37 pm 02/12/2012

    But it works both ways…

    For example, the US currently has more forest than it did when Columbus arrived, for the simple reason that we put out forest fires. They used to regularly burn areas the size of Connecticut. There were a number of indian tribes that even practiced preventative “back-burning” and other management tasks to control the fires. There are species whose mating cycles were dependent on regular fires.

    (That’s the thing people don’t realize… when man doesn’t “manage” nature, nature does it herself. Usually by burning it all down regularly.)

    And it also means that forest fires AND humans used both destroy forests. Now, they don’t. Certainly, not anywhere near as much. Humanity, overall, has had a positive effect of forestry.

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  5. 5. brock2118 11:24 pm 02/12/2012


    I think it was powerful prayers to the Congo Rain Gods that did the trick.

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  6. 6. Miob80 2:05 am 02/13/2012

    Interesting article. The same has been known for the largest river in Europe: The River Rhine. See for instance Hoffman et al., 2009: Trends and controls of Holocene floodplain sedimentation in the Rhine catchment

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  7. 7. Patrick49 8:13 am 02/13/2012

    How about going back a few thousand years as found in the following excerpt from an article in the Smithsonian magazine titled:
    Uncovering Secrets of the Sphinx
    “The Sahara has not always been a wilderness of sand dunes. German climatologists Rudolph Kuper and Stefan Kröpelin, analyzing the radiocarbon dates of archaeological sites, recently concluded that the region’s prevailing climate pattern changed around 8,500 B.C., with the monsoon rains that covered the tropics moving north. The desert sands sprouted rolling grasslands punctuated by verdant valleys, prompting people to begin settling the region in 7,000 B.C. Kuper and Kröpelin say this green Sahara came to an end between 3,500 B.C. and 1,500 B.C., when the monsoon belt returned to the tropics and the desert reemerged. That date range is 500 years later than prevailing theories had suggested.
    Further studies led by Kröpelin revealed that the return to a desert climate was a gradual process spanning centuries. This transitional period was characterized by cycles of ever-decreasing rains and extended dry spells. Support for this theory can be found in recent research conducted by Judith Bunbury, a geologist at the University of Cambridge. After studying sediment samples in the Nile Valley, she concluded that climate change in the Giza region began early in the Old Kingdom, with desert sands arriving in force late in the era.”

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  8. 8. Adolphe FABER 4:51 am 02/14/2012

    Yet another example of climate change induced by the introduction of farming. Certainely AGW, i.e. warming caused by Man, but not via CO2!

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