Climate tipping points can either result from a slow but linear development, or can "flicker" between two stable climatic states that alternate before a final, permanent transition occurs, finds a new study featuring a UCL researcher.
The study, published in Nature Communications, confirms this alternating transition for the end of the African Humid Period, a time between about 14,000 and 5,000 years ago when northern Africa was much wetter, as it shifted to the pronounced aridity that is typical 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 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. The researchers hope that a better understanding of the various tipping points and their typical early warning signals could prove helpful for further climate change research and modelling.
The transition from the African Humid Period (AHP) to dry conditions in North Africa is the clearest example of climate tipping points in recent geological history. Tipping points occur 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 also the case in North Africa, where the grasslands, forests, and lakes favoured by humans disappeared, causing them to retreat to areas like the mountains, oases, and the Nile Delta.
This development is of particular relevance for researchers in part because it is an impressive example of how quickly and extensively climate change can affect human societies.
Co-author Professor Mark Maslin (UCL Geography) said: "Understanding the mechanisms underlying these environmental tipping points is more important than ever as our planet is facing unprecedented changes because of climate change. Learning how northern Africa shifted from a lush grassland to an arid desert could help us predict what other kind of environmental changes we might be in store for in the future."
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.
Lead author Professor Martin H. Trauth at the University of Potsdam said: "The two types of tipping points differ with regard to the early warning signals that can be used to recognise them. 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 researchers analysed lake sediments obtained by means of scientific deep drilling in the Chew Bahir Basin, a former freshwater lake in eastern Africa.
Co-author Dr Verena Förster-Indenhuck from the University of Cologne said: "For the current study, six shorter (9 to 17 metres) and two long (292 metres) drill cores were evaluated, which can be used to reconstruct the past 620,000 years of climate history in the region."
Professor Trauth added: "At the end of the AHP, we observed at least 14 dry events in the short cores from Chew Bahir, each of which lasted 20-80 years and recurred at intervals of 160±40 years. Later in the transitional phase, starting in 6,000 BC, 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."
The fact that very similar transitions can also be found in the older sections of the sediment cores also supports this. In particular, another 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.
Co-author Professor Stefanie Kaboth-Bahr of Freie Universität Berlin said: "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."
Some researchers have argued that the climatic shift in northern Africa may have been caused by human activity, but this research suggests otherwise. Conversely, people in the region were undoubtedly affected by the climate tipping: the traces of settlement in the Nile valley at the end of the African Humid Period attract millions of tourists to the region every year.
The project brought together researchers from UCL and the Universities of Cologne, Potsdam, Aberystwyth and Addis Ababa, and was supported by the German Research Foundation (DFG).
