Transition from savannah to desert more rapid than previously thought

Village on the Nile, 1891. From original silver print in book titled: "A Month in Palestine and Syria, April 1891," (author unknown). Original 19th century book in the possession of Kimberly Blaker, New Boston Fine and Rare Books. Image: Wikimedia Commons (CC BY-SA 3.0)

Village on the Nile, 1891. From original silver print in book titled: "A Month in Palestine and Syria, April 1891," (author unknown). Original 19th century book in the possession of Kimberly Blaker, New Boston Fine and Rare Books. Image: Wikimedia Commons (CC BY-SA 3.0)

The Sahara was once a savannah, populated by large herds of wild animal, but today it belongs to the most arid regions of the Earth. Scientists have now discovered that the green vegetation covering the Sahara disappeared much faster than previously assumed. Drastic climate change took place there in the past 10,000 years, in the so-called Holocene. At the beginning of this period, the rainfall amounts were significantly higher than they are today.

The study by researchers from GEOMAR Helmholtz Centre for ocean research in Kiel and the Netherlands Institute for sea research (NIOZ) is now published in the open-access journal PLOS ONE.

Savannah to desert

In the understanding of the relationship between humans and their environment; the Sahara and the Nile Valley are particularly interesting regions. “It took only a few centuries or even decades, for the fertile savannah landscape to become a desert“, says Dr. Cecile Blanchet, lead author of the study.

The root cause of climate change in North Africa at the beginning of the Holocene was the African monsoon and the associated rain belt which shifted southward. “But if this process was relatively even, we know of large regional and temporal variations in the effects on North Africa. We don’t really understand the details of the transition“, says Prof. Dr. Martin Frank, palaeo oceanographer at GEOMAR and co-author of the study.

Sediment core

To better break down the processes, Dr. Blanchet and Professor Frank analysed a six-metre sediment core. They used the Kiel research vessel POSEIDON to obtain the core from the Nile Delta in the Mediterranean at a water depth of 700 metres. With a combination of different geochemical methods, they found traces of former vegetation and eroded soils in the core, which had been swept by the Nile in the course of thousands of years into the Mediterranean Sea. From this they were able to reconstruct high resolution plant growth, precipitation, the outgoing quantities of water and the erosion in the catchment area of the Nile during the last 9,500 years.

The evaluation of these analyses, the flows of the Nile and thus the rainfall, shows a drastic decrease in the vegetation around 8000 years ago. “Here a long-term climatic process had a very short-term significant impact, after a certain threshold has been exceeded“, says Dr. Blanchet.

Important to know precise sequence

The results are exciting for the scientists, because in the more distant catchment area of the Nile during the same period important steps of human development were taking place. “Therefore it is important to know the precise sequence of the transition from a relatively humid to an extremely dry environment and how that relates to humans” says co-author Professor Schouten.

Possibly, the rapid change in the vegetation forced people to abandon their way of life as hunters and gatherers, because the usual food sources were no longer there. “Instead it was probably advantageous to domesticate cattle or to practice agriculture“, says Dr. Blanchet. Ultimately the rapid desertification meant the rise of the early Egyptian Pharaonic dynasties.

Of course these relationships between human development and climate change should be examined further. But with our results, we have strong indications that even a slow running climate change can become faster and can trigger dramatic environmental changes. That is important information for the history of mankind, but also when understanding what could happen in the future“, said Dr. Blanchet.