About five and a half thousand years ago, North Africa experienced a dramatic change. The Sahara expanded, and grasslands, forests, and lakes, which were suitable for human habitation, disappeared. People were forced to retreat to the mountains, oases, and the Nile valley and delta.

As a relatively large and dispersed population squeezed into smaller and more fertile areas, they had to innovate new ways of food production and social organization. Soon after, one of the world's first great civilizations emerged - ancient Egypt.

This transition from the latest "African Humid Period," which lasted from 15,000 to 5,500 years ago, to the current arid conditions in North Africa is the clearest example of a climatic tipping point in recent geological history. Climatic tipping points are thresholds that, once crossed, result in a dramatic shift in climate to a new stable climate.

Our new study published in the journal Nature Communications reveals that before the drying of North Africa, the climate "flickered" between two stable climate states before permanently transitioning to another state. This is the first time such flickering has been recorded in Earth's past. This suggests that places with highly variable climate change cycles today may be on the path to their own tipping points.

One of the biggest questions for climate scientists today is whether we will have any warnings of climatic tipping points. As we approach global warming of 1.5 °C, the most likely tipping points include the collapse of ice sheets in Greenland or Antarctica, the extinction of tropical coral reefs, or the sudden thawing of Arctic permafrost.

Some believe there will be warning signs for these major climate changes. However, these signs greatly depend on the type of tipping point, and interpreting these signals is therefore challenging. One of the big questions is whether tipping points will be marked by flickering or if the climate will initially appear more stable before suddenly tipping.

To investigate further, we assembled an international team of scientists and went to the Chew Bahir basin in southern Ethiopia. During the last African Humid Period, there was an extensive lake here, and beneath the lake bed are sediments several kilometers deep that very precisely record the history of lake level changes influenced by climate.

Today, the lake is mostly gone, and the sediments can be relatively cheaply drilled without the need for a drilling platform on a floating platform or drilling ship. We drilled 280 meters below the dry lake bed – almost as tall as the Eiffel Tower – and extracted hundreds of mud tubes about 10 centimeters in diameter.

By connecting these tubes in sequence, we obtained a so-called sediment core. This core contains vital chemical and biological information that records the last 620,000 years of East African climate and environmental history.

We now know that at the end of the African Humid Period, there were about 1,000 years during which the climate regularly alternated between intensely dry and wet conditions.

We observed a total of at least 14 dry phases, each lasting between 20 and 80 years, repeating approximately every 160 years. Later, there were seven wet phases of similar duration and frequency. Finally, about 5,500 years ago, dry climate prevailed.

These high-frequency, extreme wet-dry fluctuations represent pronounced climatic flickering. Such flickering can be simulated in climate modeling computer programs and also occurred during earlier climate transitions in Chew Bahir.

We see the same flickering during the previous change from wet to dry climate about 379,000 years ago in the same sediment core. It looks like a perfect copy of the transition at the end of the African Humid Period.

This is important because this transitional phase was natural, occurring long before humans had any impact on the environment. Knowledge of such natural change refutes claims by some academics that the introduction of livestock and new agricultural techniques could have accelerated the end of the last African Humid Period.

However, people in the region were undoubtedly affected by the climatic tipping point. Flickering would have had a dramatic impact, easily noticeable to individuals, unlike a slow climate transition stretching over dozens of generations.

Perhaps this could explain why archaeological findings in the region are so diverse, even contradictory, at the time of transition. People retreated during dry phases, and then some returned during wet phases. Ultimately, people retreated to places that consistently remained wet, such as the Nile valley.

Confirming climatic flickering as a precursor to a major climatic tipping point is important because it could also provide insights into potential early warning signals for significant climate changes in the future.

It seems that highly variable climatic conditions like rapid wet-dry cycles can signal significant changes in the climate system. Identifying these precursors now can provide the warning we need that future warming could lead to one or more of the sixteen identified key climatic tipping points.

This is especially important for regions like East Africa, whose nearly 500 million people are already extremely vulnerable to the effects of climate change, such as droughts.

Original:
Martin H. Trauth
Professor, University of Potsdam
Asfawossen Asrat
Professor at Addis Ababa University
Mark Maslin
Professor of Natural Sciences, UCL