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The climate flickers before tipping

Study identifies early warning signals for the end of the African Humid Period / Publication in ‘Nature Communications’

Tipping points in the climate system can be the result of a slow but linear development. They can also be accompanied by a ‘flickering’, with two stable climatic states that alternate before a final transition occurs – and the climate tips permanently. A study by the research team of the Universities of Potsdam and Cologne in Germany, Aberystwyth in Wales, Addis Abeba in Ethiopia and other universities confirms this for the end of the African Humid Period (AHP) and the transition to the pronounced aridity that is typical for the region today. The researchers analysed several sediment cores measuring up to 280 metres from the Chew Bahir Basin in southern Ethiopia, which act as a ‘record’ of 620,000 years of East African climate history. The results of the study, which have just been published in Nature Communications, show that at the end of the African Humid Period, intense dry and wet events alternated regularly over a period of around 1,000 years before a dry climate prevailed around 5,000 years ago.

A better understanding of the various tipping points and, above all, their typical early warning signals could prove essential for further climate change research and for the improvement of the modelling. The research team found the transition from the African Humid Period to dry conditions in North Africa to be the clearest example of climate tipping points in recent geological history. These dynamics occur when small perturbations trigger a large, non-linear response in the system and shift the climate to a different future state. This has dramatic consequences for the biosphere. In North Africa the grasslands, forests and lakes favoured by humans disappeared. This forced them to retreat to areas like the mountains, oases and the Nile Delta.

Slower or more dramatic transition?

Climate research has 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. Ultimately a transition occurs. The second type is characterized by a flickering between stable humid and dry climates that accompanies the transition.  “The two types of tipping points differ with regard to the early warning signals that can be used to recognize them. Researching and better understanding them is important if we want to be able to predict possible future climate tipping points caused by humans,” explained Professor Dr Martin Trauth from the Institute of Geosciences at the University of Potsdam. “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 recognized.”

The large-scale project is led by Trauth together with colleagues from the partner universities and funded by the German Research Foundation (DFG). The researchers are analysing lake sediments obtained by means of scientific deep drilling in the Chew Bahir Basin, a former freshwater lake in eastern Africa.  “For the current study, six shorter (9 to 18 metres) and two long (280 metres) drill cores were evaluated, which can be used to reconstruct the past 620,000 years of climate history in the region,” explained Dr Verena Foerster-Indenhuck from the Institute of Geography Education at the University of Cologne.

In the short cores from Chew Bahir, each of which lasted twenty to eighty years and recurred at intervals of 160 years (±40), the team observed at least 14 dry events. Later in the transitional phase, starting 6,000 years ago, seven wet events occurred in addition to the dry events, which were of a similar duration and frequency.  “These high-frequency, extreme wet-dry events represent a pronounced ‘climate flickering’ that can be simulated in climate models and can also be observed in earlier climate transitions in the environmental records from Chew Bahir. This indicates that transitions with flickering are characteristic of this region,” said Trauth.

Very similar transitions can also be found in the older sections of the sediment cores. In particular, the changeover from humid to dry climate around 379,000 years ago looks like a perfect copy of the transition at the end of the African Humid Period. “This is interesting because this transition was natural, so to speak, as it occurred at a time when human influence on the environment was negligible,” said co-author Professor Dr Stefanie Kaboth-Bahr at the Institute of Geological Sciences of Freie Universität Berlin. “Among others, this observation refutes the claim that the end of the AHP was accelerated by human activities, as proposed by American colleagues.” Using the high-resolution climate data from the sediment deep core, the scientists want to carry out further research on the behaviour of the sensitive climate system at transitions and work out the differences between gradual and flickering climate tipping dynamics even more precisely in order to use similar ‘early warning signals’ for a better understanding of climate change.

Media Contact:
Dr Verena E. Foerster-Indenhuck
University of Cologne
Institute of Geography Education
+49 221 470 4634

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