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Where Did the Plants and Animals of the Verdant Mid-Holocene Sahara Go?
Volume 5, Number 6: 6 February 2002

Nine thousand years ago, the Sahara was carpeted in green and supported an abundance of animal life, but ever-so-subtle changes in earth's orbit around the sun and the tilt of the planet's axis of rotation were conspiring to cool the Northern Hemisphere by gradually reducing the amount of solar radiation it received each summer. In Northern Africa and the lands that border on the Eastern Mediterranean, this cooling was accompanied by a concomitant decrease in rainfall, leading to a drop in lake levels; and as moisture became an increasingly scarce commodity somewhere in the vicinity of 7000 years ago, the region's vegetation was forced to embark upon the long and tortuous path of desiccation that ultimately transformed the once-verdant landscape into what is today the nine million square kilometers of barren sand and rock we call the Sahara.

How did the earth dispose of this major piece of the biosphere? Where did all the carbon that once comprised the region's plants and animals go?

In a study published in the December 2001 issue of Geology, a group of researchers from France and Israel describe what they believe to be the answer. Analyzing a sediment core they extracted from the continental slope of the southeast sector of the Mediterranean Sea, Schilman et al. (2001) developed a 3600-year history of carbon isotope changes that exhibited a gradual trend of decreasing 13C in the remains of both planktonic and benthic foraminifera; and they proposed that the likely driving force for the long-term decline was the introduction of increasing amounts of dissolved carbon that were supplied to the Mediterranean by the Nile River, as the once-lush biota of the region slowly disintegrated.

The 13C decline was not monotonic, however, with indications of finer-scale increases and decreases in Nile basin productivity, as the region experienced alternating periods of biological rejuvenation and setback. Nevertheless, the basic trend was downward; and the scientists noted there was "a good correspondence between the temporal 13C change and the global rise of atmospheric CO2," which led them to suggest "a causal link between the two records."

Assuming the 13C trend they observed over the past three and a half millennia could be extrapolated back in time in regular fashion to 7000 years before the present (BP), Schilman et al. proposed that "the regional biosphere during the aridification process released 195 Gt [gigatons] of C [carbon] to the atmosphere and gradually increased the concentration of atmospheric CO2, as shown in [the CO2 vs. time plot derived from the work of Indermuhle et al. (1999)]," which depicts an atmospheric gain of about 20 ppm CO2 from 7000 years BP to just before the start of the Industrial Revolution [see also Carbon Dioxide (History - The Last 10,000 Years) in our Subject Index].

This story illustrates how readily great quantities of carbon can travel the downward path that leads from complex communities of living plants and animals to lifeless reservoirs of deep-sea mud and atmospheric gas. It also, however, provides an opportunity to contemplate the reverse phenomenon, whereby lifeless carbon in the form of coal, gas and oil - which once lived - is resurrected to fuel the machines that sustain the modern world, while it is simultaneously returned to the atmosphere as CO2, from whence the life-giving gas is assimilated by plants that use it to construct their tissues and reproduce themselves, as well as provide the food that sustains all animals, including humans, thereby enabling their progeny also to spread abroad in the land.

This observation provides us the opportunity to consider many of our actions in an important new light. Will we continue to sustain the planet's positive carbon trajectory, whereby ever more carbon is daily being transferred from lifeless to living reservoirs via the generally-ignored, but abundantly-proven, biologically-positive effects of industry-produced CO2 that is fueling the great Greening of the Earth that is currently in progress? Or, will we choose the opposite, and take the road that leads to less life? That is the central question upon which the nations of the earth will soon be casting their votes.

Pray God they vote for life.

Dr. Sherwood B. Idso
Dr. Keith E. Idso
Vice President

Indermuhle, A., Stocker, T.F., Joos, F., Fischer, H., Smith, H.J., Wahlen, M., Deck, B., Mastroianni, D., Tschumi, J., Blunier, T., Meyer, R. and Stauffer, B. 1999. Holocene carbon-cycle dynamics based on CO2 trapped in ice at Taylor Dome, Antarctica. Nature 398: 121-126.

Schilman, B., Almogi-Labin, A., Bar-Matthews, M., Labeyrie, L., Paterne, M. and Luz B. 2001. Long- and short-term carbon fluctuations in the Eastern Mediterranean during the late Holocene. Geology 29: 1099-1102.