A satellite image of the Sahara by NASA World Wind
The discovery of ancient human burial site in Niger, Africa last Summer (by Paleontologist Paul Sereno, Univ. of Chicago) with graves possessing numerous artifacts and and even plant fibers and seeds, was indirect confirmation of what scientists have long known: that the Sahara region was once a lush, lake-strewn region hospitable to many early human groups. In addition, many other larger fauna, like hippos, populated the area.
How this transition to a most inhospitable, arid desert (known as desertification) occurred, and how long ago, has remained an open question. But a recent analysis of the sediments (published in Science, May 2008) from one of the only permanent lakes still existing in the Sahara region, Lake Yoa, in northern Chad, has begun to offer some answers.
The research team (Kropelin, et al) found evidence in the crystallized, lake bed sediments of an “ecosystem succession”–a move from a freshwater lake to a highly saline body of water–that lasted some 3 centuries. The process began with a gradual loss of tropical vegetation, followed by a transition to a grassier land cover, and then finally, to the sandy desert we know today. Based on rock dating techniques, the team claims that this transition started around 4000 thousand years ago (about the time the Great Sphinx was built).
One is tempted to wonder if this process could ever reverse itself, and thus restore the desert to its former, lush, green state. But in nature, so much change occurs as a result of an irreversible process that pushes a given ecosystem to a far-from-equilibrium state (i.e., away from a state that supported a “green” ecosystem). This effect is sometimes referred to as a “run away” effect (as in a “runaway greenhouse effect”).
No one knows why this desertification process started, of course. It is speculated that a change in weather patterns–such as a shift in wind currents that formerly brought regular cloud cover and precipitation–may have been to blame. There is evidence of an ancient river that extends for miles beneath the sands. This river most likely fed the lush vegetation that once covered the region. A dramatic enough shift in weather patterns could have started the long process of the river drying up, and with the water source for all the plant life.
This wind current or weather pattern shift might have been brought about by increased volcanism (volcanic ash in the upper atmosphere can block out sunlight, causing changes in atmospheric heating, and thus air flow). Or, perhaps something more “cosmic”– like a change in the Earth’s axial tilt towards the sun, that also coincided with a local warming trend. The combined effect may have triggered a positive feedback cycle that could not stop itself (or be compensated for by negative feedback).
But perhaps the answer will come from observations of current, human-caused desertification going on in many parts of the world, especially in former rain forest covered lands that have been chopped down or burned down. The top soil of a rain forest is, at most, only a few inches thick–not enough to reconstitute itself or sustain itself for long once exposed to heat and continuous farming.
The are examples from the past in which human activity led directly to deforestation, such as in the American Southwest, where it is believed that the ancient Anasasi people, over several centuries, cleared or burned enough of the original forest cover (a critical threshold of forest loss was reached) to trigger a runaway, desertification process. Only petrified wood remains in some areas to testify to this formerly forested region.
photo: NASA World Wind
About Michael Ricciardi
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