About five and a half millenniums ago, North Africa underwent a dramatic transformation. The Sahara Desert expanded and the grasslands, forests and lakes loved by humans disappeared. People were forced to retreat to the mountains, the oases and the Nile valley and delta.
As a relatively large and scattered population was squeezed into smaller and more fertile areas, it had to invent new ways of producing food and organizing society. Shortly afterwards, one of the world’s first great civilizations emerged: ancient Egypt.
This transition from the most recent ‘African Humid Period’, which lasted 15,000 to 5,500 years ago, to the current dry conditions in North Africa is the clearest example of a climate tipping point in recent geological history. Climate tipping points are thresholds that, once crossed, result in dramatic climate change to a new stable climate.
Our new study, published in Nature Communications, shows that before North Africa dried out, its climate ‘flickered’ between two stable climate states before finally toppling over. This is the first time such flickers have been shown to have occurred in Earth’s past. And it suggests that places with highly variable cycles of changing climate are, in some cases, heading toward their own tipping points.
Whether we will get warnings of climate tipping points is one of the biggest concerns of climate scientists today. As we pass 1.5˚C of global warming, the most likely tipping points are the collapse of the ice caps in Greenland and Antarctica, the death of tropical coral reefs or the abrupt thawing of permafrost in the Arctic.
Some say there will be warning signs of these major climate shifts. However, these are highly dependent on the actual type of turning point and the interpretation of these signals is therefore difficult. One of the big questions is whether tipping points will be marked by flickers or whether the climate will initially appear more stable before toppling all at once.
620,000 years of environmental history
To investigate this further, we gathered an international team of scientists and went to the Chew Bahir basin in southern Ethiopia. There was a vast lake here during the last humid period in Africa, and deposits of sediment, several kilometers deep, beneath the lake floor, record very accurately the history of climate-induced fluctuations in lake level.
Today the lake has largely disappeared and the deposits can be drilled relatively cheaply without the need for a floating rig or drillship. We drilled 280 meters below the dry lake bed – almost as deep as the Eiffel Tower is tall – and extracted hundreds of tubes of mud about 10 centimeters in diameter.
By assembling these tubes in sequence, they form a so-called sediment core. That core contains essential chemical and biological information that records the past 620,000 years of East African climate and environmental history.
We now know that at the end of the African Humid Period there was a climate in which the climate regularly alternated between intensely dry and wet for about 1,000 years.
In total, we observed at least 14 dry phases, each lasting between 20 and 80 years and repeating at intervals of about 160 years. Later there were seven wet phases, of similar duration and frequency. Finally, about 5,500 years ago, a dry climate prevailed for good.
Climate flickers
These high-frequency, extreme wet-dry fluctuations represent a pronounced climate flicker. Such flickering can be simulated in climate model computer programs and has occurred during previous climate transitions at Chew Bahir.
We see the same types of flickers during an earlier change from humid to dry climate, about 379,000 years ago, in the same sediment core. It seems to be a perfect copy of the transition at the end of the African Humid Period.
This is important because this transition was natural, having occurred long before humans had any impact on the environment. Knowing that such a change can occur obviously refutes the argument of some academics that the introduction of livestock and new agricultural techniques may have hastened the end of Africa’s last humid period.
Conversely, people in the region were undoubtedly affected by the climate change. The flickering would have had a dramatic impact, easily noticed by one human, compared to the slow climate transition that spanned dozens of generations.
It might explain why the archaeological finds in the region are so different and even contradictory in times of transition. People retreated during the dry phases and some returned during the wet phases. Eventually, people retreated to places that were constantly wet, such as the Nile Valley.
Confirmation of climate flickers as precursors to a major climate change is important because it can also provide insight into possible early warning signals for major climate changes in the future.
It appears that highly variable climate conditions, such as rapid wet-dry cycles, could warn of a significant shift in the climate system. Identifying these precursors now could provide the warning we need that future warming will take us past one or more of the sixteen identified critical climate tipping points.
This is especially important for regions such as East Africa, where almost 500 million people are already highly vulnerable to the effects of climate change, such as drought.
This article is republished from The Conversation under a Creative Commons license. Read the original article.
Martin H. Trauth receives grants from DFG, ICDP, NERC and NSF. Martin H. Trauth is an active member of DGGV and EGU.
Asfawossen Asrat receives funding from ICDP, NSF, NERC
Mark Maslin is the UNFCCC designated point of contact for UCL. He is co-director of the London NERC Doctoral Training Partnership and a member of the Climate Crisis Advisory Group. He serves on the advisory boards of Sopra-Steria CSR, Sheep Included Ltd, Lansons and NetZeroNow. He has received grants from the NERC, EPSRC, ESRC, DFG, Royal Society, DIFD, BEIS, DECC, FCO, Innovate UK, Carbon Trust, UK Space Agency, European Space Agency, Research England, Wellcome Trust, Leverhulme Trust, CIFF, Sprint2020 and British Council. He has received funding from the BBC, Lancet, Laithwaites, Hearst, Seventh Generation, Channel 4, JLT Re, WWF, Hermes, CAFOD, HP and Royal Institute of Chartered Surveyors.