Study examines ‘pacemakers’ of highly variable ancient Nile floods, finding similarities to present times

The study finds that ‘flood variability was paced by similar climatic forcing as today’. The historical data suggested an ‘as yet unidentified driver on multi-decadal timescales’. The Sahara wasn’t a desert during the North African humid period that’s the subject of the study. These periods are known to have occurred regularly over long timescales. As well as El Niño influences, a 30-40 year cycle was also evident in the records. Using climate models to create scenarios for year 2100 has its issues.
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Global warming as well as recent droughts and floods threaten large populations along the Nile Valley, claims Phys.org.

Understanding how such a large river will respond to an invigorated hydrological cycle is therefore a pressing issue. [Talkshop comment – wherefore ‘therefore’?]

Insights can be gained by studying past periods with wetter and warmer conditions, such as the North African Humid Period 11 to 6 thousand years ago.

A research team of the German Research Centre for Geosciences GFZ, led by Cécile Blanchet, together with colleagues at the University of Innsbruck (Austria) and the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (Germany) analyzed a 1,500 year long annually-laminated sediment core. The study was published today in Nature Geoscience.

It reveals that wetter climates led to very strong and weak floods and a highly instable river system, which may have rendered the Nile valley uninhabitable.

Although intensified, flood variability was paced by similar climatic forcing as today, operating on annual—like El Niño—to multi-decadal timescales. This suggests that the occurrence of such extreme events might be predictable, helping to reduce risks for local populations.
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Sediment core analysis
Analysis of the cores comprised looking at the layers, counting and measuring them under the microscope—part of which was also done at home during the COVID-lockdowns. The chronology was constrained using a combination of annual layer counting and radiocarbon dating of fossil plankton buried in the layers.

Blanchet quickly realized that the thickness of the flood layers was varying drastically in time spans of 30–40 years, from being very small (0.3 mm) to being very thick (10 mm).

“It may sound not so much, but several millimeters deposited offshore is enormous,” says Blanchet. “However, we know that large rivers are complicated systems, which may retain or release sediments independently of the discharge, that is the amount of water in the river.

“So, it is not always possible to relate the volume of sediments transported, expressed in our record as the thickness of the layers, to the size of the flood. But we noticed as well that the size of the particles increased in thicker layers, which means that the thickness of the layers is a reliable indicator of the strength of past floods.”

From her observations, Blanchet concluded that the North African Humid Period was characterized by the occurrence of extremely strong and variable Nile floods.
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Comparison with Nilometer flood data from the past 2000 years
The researchers compared their flood-record data from the time between 9,470 and 7,940 years before present, with data from the ancient Egyptians, recorded between 622 and 1922 CE, especially with regard to the oscillations.

“The fact that ancient Egyptians measured the level of the Nile River in special buildings called ‘Nilometers’—see picture 2—year after year is really fascinating and they have offered us a unique record of past Nile floods in the last 2,000 years,” said co-author Ramisch, previously at GFZ and now at the University of Innsbruck.

“The resolution and length of both records being similar, we could apply similar statistical tools and derive the main temporal variabilities. It showed us that the drivers remain quite similar even though the climatic conditions were different.”

This comparison shows that similar climate drivers can have very different effects, namely a much higher amplitude of flood magnitude under wetter and warmer climates. This is one of the main findings of the study. And it has important implications for building reliable tools to forecast and reduce flood risks.

“I am sure that our findings will have direct applications and we are already working on providing constraints on flood magnitude based on the record we have. This is not trivial, but with the help of modelers and geomorphologists, I am confident that we will tackle this new challenge,” concludes Blanchet.

Full article here.

Source: https://tallbloke.wordpress.com/2024/07/04/study-examines-pacemakers-of-highly-variable-ancient-nile-floods-finding-similarities-to-present-times/