How could climate change affect the behavior of one of the most important rivers in the world? A look into the history of the Nile using core analysis has now provided clues to this question. Under the warm and humid conditions around 9,000 years ago, the river was prone to very severe flood fluctuations and extreme flooding. The study results could now help to better prepare for future challenges in the wake of climate change, say the researchers.
It is the famous lifeline for millions of people in northeast Africa: the Nile carries enormous amounts of water from the equator to the Mediterranean and is known for its annual floods. Seasonal rainfall in its catchment area leads to flood phases, which, when moderate, provide irrigation and fertilization: for thousands of years, the regular Nile floods have been crucial for the food supply of the people in the region. But how will the activity of the mighty river develop in the future? The recent drought and flood episodes in Egypt, Ethiopia and Sudan raise fears of an increase in precipitation variability that could shape the river. Climate models also predict that global warming could lead to increased monsoon rainfall in this region in particular.
How did the Nile once behave?
To shed light on how this trend might affect the Nile’s river system, it is worth looking into the past. There have already been times with wetter and warmer climatic conditions: During the so-called North African humid period, around 11,000 to 6,000 years ago, there was significantly increased rainfall in the Nile catchment area. To get clues about how the river behaved in this era, researchers led by Cécile Blanchet from the GeoForschungsZentrum (GFZ) in Potsdam examined a sediment core from the seabed of the estuary in the Mediterranean. It was dated to the North African humid period and covers a time span of 1,500 years. The core shows layered structures that were created by the deposition of suspended particles that the river once carried during its seasonal floods, the researchers explain.
As they report, the examination of the material showed that the thickness of the layers reflecting the flooding events varied drastically in some cases: from 0.3 to ten millimeters. “That may not sound like much, but several millimeters deposited off the coast is enormous,” says Blanchet. The researchers see the thickness of the layers, which contain larger particles as they become wider, as a reliable indicator of the strength of the Nile floods of the past. The results thus showed that the North African humid period was characterized by extremely strong, but also weak Nile floods. Especially in the period from 9200 to 8600 years ago, large amounts of material were often deposited off the coast, reflecting unusually severe flooding events with strong erosion activity. The wetter climate apparently led to very strong variability and an extremely unstable river system, which may have made the Nile Valley uninhabitable, the researchers conclude.
Tracking influencing factors
The team then used modeling to investigate which driving forces could have led to the strong fluctuations in the North African humid era. The results suggested that the El Niño Southern Oscillation (ENSO) played a role even then. “ENSO originates in the Pacific region and is transmitted to other parts of the world through atmospheric teleconnections,” says co-author Monica Ionita from the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research in Bremerhaven. “It was fascinating to see that we were able to find this variability in both the model and the data,” says the scientist.
In principle, similar drivers of variability are also emerging over the last 2000 years, the researchers report. This was shown by comparing the flood data from the era of the North African humid period with historical records of the Nile floods. “Because the resolution and length of the two records are similar, we were able to use similar statistical methods and derive the most important temporal fluctuations. This showed us that the influencing factors are quite similar, although the climatic conditions were different,” says co-author Arne Ramisch, formerly at the GFZ. This means that similar climate drivers can have a specific effect: Under wetter and warmer climatic conditions, the amplitude of the flood extent increases.
The researchers believe that their study results are important for assessing flood risks in the wake of current climate developments. They could lead to the development of methods for predicting and reducing flood risks. “I believe that our results could have direct applications,” Blanchet concluded.
Source: Article: Helmholtz Centre Potsdam – German Research Centre for Geosciences GFZ, Technical article: Nature Geoscience, doi: 10.1038/s41561-024-01471-9