There is ample evidence from palaeobotanic and palaeoclimatic reconstructions that during early and mid-Holocene between some 11,700 years (in some regions, a few thousand years earlier) and some 4200 years ago, subtropical North Africa was much more humid and greener than today. This African Humid Period (AHP) was triggered by changes in the orbital forcing, with the climatic precession as the dominant pacemaker. Climate system modeling in the 1990s revealed that orbital forcing alone cannot explain the large changes in the North African summer monsoon and subsequent ecosystem changes in the Sahara. Feedbacks between atmosphere, land surface, and ocean were shown to strongly amplify monsoon and vegetation changes. Forcing and feedbacks have caused changes far larger in amplitude and extent than experienced today in the Sahara and Sahel. Most, if not all, climate system models, however, tend to underestimate the amplitude of past African monsoon changes and the extent of the land-surface changes in the Sahara. Hence, it seems plausible that some feedback processes are not properly described, or are even missing, in the climate system models. Perhaps even more challenging than explaining the existence of the AHP and the Green Sahara is the interpretation of data that reveal an abrupt termination of the last AHP. Based on climate system modeling and theoretical considerations in the late 1990s, it was proposed that the AHP could have ended, and the Sahara could have expanded, within just a few centuries—that is, much faster than orbital forcing. In 2000, paleo records of terrestrial dust deposition off Mauritania seemingly corroborated the prediction of an abrupt termination. However, with the uncovering of more paleo data, considerable controversy has arisen over the geological evidence of abrupt climate and ecosystem changes. Some records clearly show abrupt changes in some climate and terrestrial parameters, while others do not. Also, climate system modeling provides an ambiguous picture. The prediction of abrupt climate and ecosystem changes at the end of the AHP is hampered by limitations implicit in the climate system. Because of the ubiquitous climate variability, it is extremely unlikely that individual paleo records and model simulations completely match. They could do so in a statistical sense, that is, if the statistics of a large ensemble of paleo data and of model simulations converge. Likewise, the interpretation regarding the strength of terrestrial feedback from individual records is elusive. Plant diversity, rarely captured in climate system models, can obliterate any abrupt shift between green and desert state. Hence, the strength of climate—vegetation feedback is probably not a universal property of a certain region but depends on the vegetation composition, which can change with time. Because of spatial heterogeneity of the African landscape and the African monsoon circulation, abrupt changes can occur in several, but not all, regions at different times during the transition from the humid mid-Holocene climate to the present-day more arid climate. Abrupt changes in one region can be induced by abrupt changes in other regions, a process sometimes referred to as “induced tipping.” The African monsoon system seems to be prone to fast and potentially abrupt changes, which to understand and to predict remains one of the grand challenges in African climate science.
Martin Claussen, Anne Dallmeyer, and Jürgen Bader
Timothy M. Shanahan
West Africa is among the most populated regions of the world, and it is predicted to continue to have one of the fastest growing populations in the first half of the 21st century. More than 35% of its GDP comes from agricultural production, and a large fraction of the population faces chronic hunger and malnutrition. Its dependence on rainfed agriculture is compounded by extreme variations in rainfall, including both droughts and floods, which appear to have become more frequent. As a result, it is considered a region highly vulnerable to future climate changes. At the same time, CMIP5 model projections for the next century show a large spread in precipitation estimates for West Africa, making it impossible to predict even the direction of future precipitation changes for this region. To improve predictions of future changes in the climate of West Africa, a better understanding of past changes, and their causes, is needed. Long climate and vegetation reconstructions, extending back to 5−8 Ma, demonstrate that changes in the climate of West Africa are paced by variations in the Earth’s orbit, and point to a direct influence of changes in low-latitude seasonal insolation on monsoon strength. However, the controls on West African precipitation reflect the influence of a complex set of forcing mechanisms, which can differ regionally in their importance, especially when insolation forcing is weak. During glacial intervals, when insolation changes are muted, millennial-scale dry events occur across North Africa in response to reorganizations of the Atlantic circulation associated with high-latitude climate changes. On centennial timescales, a similar response is evident, with cold conditions during the Little Ice Age associated with a weaker monsoon, and warm conditions during the Medieval Climate Anomaly associated with wetter conditions. Land surface properties play an important role in enhancing changes in the monsoon through positive feedback. In some cases, such as the mid-Holocene, the feedback led to abrupt changes in the monsoon, but the response is complex and spatially heterogeneous. Despite advances made in recent years, our understanding of West African monsoon variability remains limited by the dearth of continuous, high- resolution, and quantitative proxy reconstructions, particularly from terrestrial sites.
David McGee and Peter B. deMenocal
The expansion and intensification of summer monsoon precipitation in North and East Africa during the African Humid Period (AHP; c. 15,000–5,000 years before present) is recorded by a wide range of natural archives, including lake and marine sediments, animal and plant remains, and human archaeological remnants. Collectively this diverse proxy evidence provides a detailed portrait of environmental changes during the AHP, illuminating the mechanisms, temporal and spatial evolution, and cultural impacts of this remarkable period of monsoon expansion across the vast expanse of North and East Africa. The AHP corresponds to a period of high local summer insolation due to orbital precession that peaked at ~11–10 ka, and it is the most recent of many such precessionally paced pluvial periods over the last several million years. Low-latitude sites in the North African tropics and Sahel record an intensification of summer monsoon precipitation at ~15 ka, associated with both rising summer insolation and an abrupt warming of the high northern latitudes at this time. Following a weakening of monsoon strength during the Younger Dryas cold period (12.9–11.7 ka), proxy data point to peak intensification of the West African monsoon between 10–8 ka. These data document lake and wetland expansions throughout almost all of North Africa, expansion of grasslands, shrubs and even some tropical trees throughout much of the Sahara, increases in Nile and Niger River runoff, and proliferation of human settlements across the modern Sahara. The AHP was also marked by a pronounced reduction in windblown mineral dust emissions from the Sahara. Proxy data suggest a time-transgressive end of the AHP, as sites in the northern and eastern Sahara become arid after 8–7 ka, while sites closer to the equator became arid later, between 5–3 ka. Locally abrupt drops in precipitation or monsoon strength appear to have been superimposed on this gradual, insolation-paced decline, with several sites to the north and east of the modern arid/semi-arid boundary showing evidence of century-scale shifts to drier conditions around 5 ka. This abrupt drying appears synchronous with rapid depopulation of the North African interior and an increase in settlement along the Nile River, suggesting a relationship between the end of the AHP and the establishment of proto-pharaonic culture. Proxy data from the AHP provide an important testing ground for model simulations of mid-Holocene climate. Comparisons with proxy-based precipitation estimates have long indicated that mid-Holocene simulations by general circulation models substantially underestimate the documented expansion of the West African monsoon during the AHP. Proxy data point to potential feedbacks that may have played key roles in amplifying monsoon expansion during the AHP, including changes in vegetation cover, lake surface area, and mineral dust loading. This article also highlights key areas for future research. Among these are the role of land surface and mineral aerosol changes in amplifying West African monsoon variability; the nature and drivers of monsoon variability during the AHP; the response of human populations to the end of the AHP; and understanding locally abrupt drying at the end of the AHP.
An orbitally induced increase in summer insolation during the last glacial-interglacial transition enhanced the thermal contrast between land and sea, with land masses heating up compared to the adjacent ocean surface. In North Africa, warmer land surfaces created a low-pressure zone, driving the northward penetration of monsoonal rains originating from the Atlantic Ocean. As a consequence, regions today among the driest of the world were covered by permanent and deep freshwater lakes, some of them being exceptionally large, such as the “Mega” Lake Chad, which covered some 400 000 square kilometers. A dense network of rivers developed. What were the consequences of this climate change on plant distribution and biodiversity? Pollen grains that accumulated over time in lake sediments are useful tools to reconstruct past vegetation assemblages since they are extremely resistant to decay and are produced in great quantities. In addition, their morphological character allows the determination of most plant families and genera. In response to the postglacial humidity increase, tropical taxa that survived as strongly reduced populations during the last glacial period spread widely, shifting latitudes or elevations, expanding population size, or both. In the Saharan desert, pollen of tropical trees (e.g., Celtis) were found in sites located at up to 25°N in southern Libya. In the Equatorial mountains, trees (e.g., Olea and Podocarpus) migrated to higher elevations to form the present-day Afro-montane forests. Patterns of migration were individualistic, with the entire range of some taxa displaced to higher latitudes or shifted from one elevation belt to another. New combinations of climate/environmental conditions allowed the cooccurrences of taxa growing today in separate regions. Such migrational processes and species-overlapping ranges led to a tremendous increase in biodiversity, particularly in the Saharan desert, where more humid-adapted taxa expanded along water courses, lakes, and wetlands, whereas xerophytic populations persisted in drier areas. At the end of the Holocene era, some 2,500 to 4,500 years ago, the majority of sites in tropical Africa recorded a shift to drier conditions, with many lakes and wetlands drying out. The vegetation response to this shift was the overall disruption of the forests and the wide expansion of open landscapes (wooded grasslands, grasslands, and steppes). This environmental crisis created favorable conditions for further plant exploitation and cereal cultivation in the Congo Basin.