Around 5.5 thousand years ago in the mid Holocene, climate, and therefore the habitat for humans, changed dramatically in northern and northeastern Africa. At the termination of the so-called African Humid Period (15–5 thousand years ago) previously green Sahara turned yellow, humans left the region and migrated eastwards into the Nile Valley, which was previously uninhabitable. Extensive marshlands with large populations of crocodiles and disease-transmitting mosquitoes made life there unbearable. The drier climate changed everything – and heralded one of the world's early great civilizations.

In our study, we take a closer look at this climate transition at a resolution of just 10 years, a time scale that affects humans. In short (<20 m) sediment cores that we drilled in 2009–2010 in the Chew Bahir Basin, a mud flat in southern Ethiopia, we were able to measure how long the transition from humid to dry climate lasted: ~885 years (Trauth et al., 2018). That is long enough for people to adapt or flee, for example from the green Sahara to the Nile Valley to start a life there. However, we also noticed that this transition was punctuated by several 20–80 year long droughts, which recurred approximately every 160 years (Fig. 1a). We mentioned this in a publication, but did not follow it up because we were initially unsure whether these droughts were actually real. After all, we cannot measure climate directly, but infer climate from so-called climate proxies. In our case, it is the potassium concentration of the sediment, which is incorporated into clay minerals in an increasingly dry climate (Foerster et al., 2018).

In 2014, we drilled much deeper in the Chew Bahir Basin as part of the ICDP program, almost 300 meters twice, and with these new cores we can look back 620 thousand years into the past. Such cores are subjected to very extensive analyses and a wide variety of climate proxies are obtained, but we first looked again at the potassium concentrations. And to our surprise, we found an almost perfect copy of the mid Holocene transition, but 379 thousand years ago, which is at a sediment depth of more than 200 meters (Fig. 1b). Now we have taken a closer look at these two transitions, and in fact another ten similar transitions scattered over the 620 thousand years in our Chew Bahir cores. We realized that the recurring droughts were part of a pronounced climate flicker, because the droughts were followed by similarly long, extremely wet intervals. If you look at the entire almost thousand-year transition, you can see further similarities. For example, the climate is extremely stable before and after the transition. It is initially quite consistently humid, without any major changes, before it starts to flicker, before it begins to tilt and slowly becomes drier. At the end, the flickering calms down and the climate stabilizes at a much drier level.

While we were working on our cores, a new topic was increasingly coming to the attention of the scientific community: climate tipping points and possible early warning signals. Had we possibly found a first, clear example from the geological past, which was previously known mainly from theoretical considerations and climate modeling? When we started to create the first drafts of text and figures for the new paper, a paper by climate modelers Peter Hopcroft and Paul Valdes, whose modeling results were very similar to our paleodata, seemed to be a good fit (Hopcroft and Valdes, 2021). The two colleagues did not need much persuading – and are now members of our team and co-authors on the paper. We now had an interdisciplinary team that gave us the opportunity to shed light on all aspects of climate change in northeastern Africa: from reconstructing the climate from the sediments, understanding the physical processes from the data and the model results, to the possible effects on the living conditions of people in the region.

The last aspect in particular, which naturally interests us the most – how humans reacted to the change in climate, especially the flickering during the transition – was especially difficult to define. We talked a lot about this topic with the archaeologist in our team, but she remained very cautious about making overly bold statements. A lot of thought has been given in the past to the effects of a more or less rapid transition from wet to dry, but the phenomenon of flickering is still relatively new. The flickering of climate would have had a much more dramatic impact than the slow climate transition spanning tens of generations. It could perhaps explain why the archaeological findings in the region are so different, even contradictory, at times of the transition. Confirmation of the existence of flickering events as precursors several times in the past, however, may also provide insights into possible early warning signals for future large scale climate tipping points.

References

Foerster, V., Deocampo, D., Asrat, A., Günter, C., Junginger, A., Krämer, H., Stroncik, N., Trauth, M.H. (2018) Towards an understanding of climate proxy formation in the Chew Bahir basin, southern Ethiopian Rift. Palaeogeography, Palaeoclimatology, Palaeoecology, 501, 111-123, DOI: 10.1016/j.palaeo.2018.04.009.

Foerster, V., Asrat, A., Bronk Ramsey, C., Brown, E.T., Chapot, M.S., Deino, A., Duesing, W., Grove, M., Hahn, A., Junginger, A., Kaboth-Bahr, S., Lane, C.S., Opitz, S., Noren, A., Roberts, H.M., Stockhecke, M., Tiedemann, R., Vidal, C.M., Vogelsang, R., Cohen, A.S., Lamb, H.F., Schaebitz, F., Trauth, M.H. (2022) Pleistocene climate variability in eastern Africa influenced hominin evolution, Nature Geoscience, DOI: 10.1038/s41561-022-01032-y.

Hopcroft, P. O., Valdes, P. J. (2021) Paleoclimate-conditioning reveals a north Africa land–atmosphere tipping point. Proc. Natl. Acad. Sci. U. S. A., 118, DOI: 10.1073/pnas.2108783118.

Trauth, M.H., Foerster, V., Junginger, A., Asrat, A., Lamb, H., Schaebitz, F. (2018) Abrupt or Gradual? Change Point Analysis of the Late Pleistocene-Holocene Climate Record from Chew Bahir, Southern Ethiopia. Quaternary Research, 90, 321-330, DOI: 10.1017/qua.2018.30.

Trauth, M.H., Asrat, A., Fischer, M.L., Hopcroft, P.O., Foerster, V., Kaboth-Bahr, S., Kindermann, K., Lamb, H.F., Marwan, N., Maslin, M.A., Schaebitz, F., Valdes, P.J. (2024) Early Warning Signals of the Termination of the African Humid Period(s), Nature Communications, DOI: 10.1038/s41467-024-47921-1.