The Sahara is the world’s largest desert, one that geologist Dr. Sebastian Lüning – having participated in scientific expeditions there over the course of a decade – is very familiar with. Lüning and Fritz Vahrenholt recently wrote a post about a new study that examines climate cycles of the Sahara region throughout the Holocene.
Contrary to what many may believe today, the climate of the Sahara desert is far from being steady. Rather, it has gone through profound cyclic changes over the last 10,000 years. For example it was much greener 8000 to 5500 years ago, a time when it was teeming with wildlife.
What caused the “green Sahara”? The average temperature back then was approx. 1°C higher than today (with atmospheric CO2 concentrations 35% lower). The cause was likely the solar intensity maximum on the northern hemisphere due to the Earth’s orbital orientation (Milankovitch Cycles).
A warmer planet made the Sahara wetter in the past. Is that the case today? How have the Sahara and its neighboring regions changed over the recent years? Listening to the prophesies of some scientists influenced by special interests, you’d think man-made climate change was causing the Sahara to expand catastrophically.
But Lüning and Vahrenholt write that the real picture is entirely different (translated, edited excerpt from the German):
Bedouins in the Sahara report that they’ve seen unusually large amounts of rain over the last years. In a study that was authored by Jonathan Seaquist et al appearing in the journal Biogeosciences in 2009, using satellite data, they were able to show that photosynthesis activity in the Sahel Zone between 1982 and 2002 increased significantly. The living conditions of plants improved markedly during this time period.
There’s good evidence that the extreme, temporary dry phase in the Sahara/Sahel area beginning in the mid 1960s was driven by the rhythm of ocean cycles. These cycles led to a natural changeover between dry and wet phases on a scale of decades.
Added to this are the natural cycles that range in length from a few to many centuries. A US group of scientists led by Christopher Bernhardt from the US Geological Survey has taken a close look at the post-Ice Age history of drought periods at the north edge of the Sahara. Their results were published in May 2012 in the journal Geology. The study is based on a 28-meter sediment core extracted from the Burullus Lagoon of the Egyptian Nile. The scientists were especially interested in the younger, upper part of the sediment core, which covers the last 7000 years. Using the pollen composition from 74 single probes, Bernhardt and his colleagues were able to reconstruct changes in the lagoon’s vegetation, which ultimately reflected fluctuations in water feed-in from the Nile. Determining the age was done with the help of seven radio-carbon dating, which were compared to biostratigraphic fossil types.
As expected, the authors were able to show the warm-wet phases of the “green Sahara” in their sediment cores. Near the end of the green Sahara periods, the intertropical Convergence Zone moved southwards and summer monsoons weakened.
The climate suddenly began to fluctuate with a millennial frequency. The scientists found an entire series of very distinct phases of drought with little water feed-in by the Nile occurring 6000-5500, 5000, 4200 and 3000 before today. Interestingly, dry phases dominated at other parts of the globe at the same times, thus allowing us to assume a mutual driver was impacting over the globe and not just regionally. The scientists suspect that the intertropical Convergence Zone shifted globally. Some of the dry periods mentioned are known from historical reports from Egypt and the Middle East, and contributed to the collapse of large civilizations at the time.
Obviously the climate of the region over the last thousands of years was everything but stable. Significant fluctuations occurred and their pattern urgently needs to studied further. Only when the natural pattern and its driving factors are clear will it be possible to determine man’s industrial impact after 1850. Currently, however, every change occurring is being attributed to man, which is completely unscientific because of the evidence of strong pre-industrial climate fluctuations.
So what was the trigger of the repeating Nile dry periods of the last 7000 years? What exactly is behind the suspected shifting of the intertropical Convergence Zone?
Christopher Bernhardt and his colleagues left this point open. A comparison to the solar-driven millennium cycles, such as the one described by Gerard Bond of the North Atlantic and other places on Earth, bring us the first solid clues. The dry period of 6000-5500 years ago coincides with the cold period described by Bond and designated “Number 4“ – at a time when solar activity was especially weak. Similarly the dry period of 4200 years ago coincided with Bond’s low solar activity cold period “Number 3“.
Weak sun, low temperatures – little rain in the Sahel/Sahara? Actually that would fit with a strong sun – high temperatures – more moisture in the Sahel/Sahara.
If you consider the existing uncertainties in dating and simple linear age model, we could get some strong leads. More research in this area is urgently needed.
Figure 1: Drought periods in the Nile catchment area are indicated by the gray zones, During these times the amount of water in the Nile was reduced (peaks to the left in the curve), based on pollen analyses. Figure from Bernhardt et al. (2012).”