It flickers, then it tips – study identifies early warning signals for the end of the African Humid Period

Posted on: 07 May 2024

Drill site, Chew Bahar. Image: Verena Foerster.

A future climatic tipping point, where the climate shifts suddenly and drastically to a new state, is a real and dangerous possibility as human activity forces change in the climate system.

This has happened in the past, with dramatic consequences for human populations, as shown by new data extracted from a 600,000-year core record of climate, taken from deep-drilled sediments of Chew Bahir, a dry lake in south Ethiopia.   

The new study, published today in leading international journal, Nature Communications. You can read it on the publisher's website.

The study reveals for the first time that the marked and permanent shift from humid to arid conditions in northern Africa 6,000 years ago was preceded by climatic “flickers” – a series of rapid alternations between wet and dry climate, each lasting 20-80 years, at approximately 160-year intervals.  

“This climate shift, known as the end of the African Humid Period, resulted in human populations becoming largely restricted to favourable habitats in mountains, oases and in the Nile valley,” said Henry Lamb, adjunct professor in Trinity’s Botany Department in the School of Natural Sciences, and part of the research team.

 “The long core record shows that similar flickering also occurred in advance of much earlier transitions in the climate, implying that such tipping points can occur naturally, long before human impact on the climate. Moreover, the flickers can be simulated in climate models, enhancing our understanding of the mechanisms involved. 

“We see climate flickers as warning signals of the potentially catastrophic effects that future tipping points would have on the biosphere, including human populations. Clearly, it is important to consider current climate fluctuations as possible warning signs of climate breakdown, not just in Africa, but also in other sensitive climate systems such as the North Atlantic.”

Pipes used for drilling in Chew Bahir. Image: Julian Ruddock.

The end of the African Humid Period (AHP) – a seismic change in North Africa

The transition from the AHP to dry conditions in North Africa 6000 years ago is the clearest example of a climate tipping point in recent geological history. This occurs when small perturbations trigger a large, non-linear response in the system and shift the climate to a different future state, usually with dramatic consequences for the biosphere.

That was the case in North Africa, where the grasslands, forests, and lakes favoured by humans disappeared, causing them to retreat to other areas. This of particular relevance for understanding current and future climate change, not least because it is an impressive example of how quickly and extensively climate change can affect human societies.

Climate researchers have identified two main types of tipping points: With the first type, processes slow at an increasing rate and the climate has a hard time recovering from disturbances until a transition occurs. The second type is characterised by a flickering between stable humid and dry climates that occurs shortly before the transition.

“The two types of tipping points differ with regard to the early warning signals that can be used to recognise them,” explains Prof. Martin Trauth, University of Potsdam, who led the study.  

“Researching and better understanding them is important if we want to be able to predict possible future climate tipping points caused by humans. While the slowdown seen in the first type of tipping point leads to a decrease in variability, autocorrelation, and skewness, the flickering in the second type leads to the exact opposite – and, in some cases, to the impending tipping point not being recognised.”

The project was funded, among others, by the German Research Foundation (DFG), the US National Science Foundation and the UK Natural Environment Research Council.

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