Mineral dust is the most important natural aerosol type by mass, with northern Africa the most prominent source region worldwide. Dust particles are lifted into the atmosphere by strong winds over arid or semiarid soils through a range of emission mechanisms, the most important of which is saltation. Dust particles are mixed vertically by turbulent eddies in the desert boundary layer (up to 6km) or even higher by convective and frontal circulations. The meteorological systems that generate winds strong enough for dust mobilization cover scales from dust devils (~100m) to large dust outbreaks related to low- and high-pressure systems over subtropical northern Africa (thousands of kilometers) and include prominent atmospheric features such as the morning breakdown of low-level jets forming in the stable nighttime boundary layer and cold pools emanating from deep convective systems (so-called haboobs). Dust particles are transported in considerable amounts from northern Africa to remote regions such as the Americas and Europe. The removal of dust particles from the atmosphere occurs through gravitational settling, molecular and turbulent diffusion (dry deposition), as well as in-cloud and sub-cloud scavenging (wet deposition). Advances in satellite technology and numerical dust models (including operational weather prediction systems) have led to considerable progress in quantifying the temporal and spatial variability of dust from Africa, but large uncertainties remain for practically all stages of the dust cycle. The annual cycle of dustiness is dominated by the seasonal shift of rains associated with the West African monsoon and the Mediterranean storm track. In summer, maximum dust loadings are observed over Mauritania and Mali, and the main export is directed toward the Caribbean Sea, creating the so-called elevated Saharan Air Layer. In winter the northeasterly harmattan winds transport dust to the tropical Atlantic and across to southern America, usually in a shallower layer. Mineral dust has a multitude of impacts on climate and weather systems but also on humans (air pollution, visibility, erosion). Nutrients contained in dust fertilize marine and terrestrial ecosystems and therefore impact the global carbon cycle. Dust affects the energy budget directly through interactions with short- and long-wave radiation, with details depending crucially on particle size, shape, and chemical composition. Mineral dust particles are the most important ice-nuclei worldwide and can also serve as condensation nuclei in liquid clouds, but details are not well understood. The resulting modifications to cloud characteristics and precipitation can again affect the energy (and water) budget. Complicated responses and feedbacks on atmospheric dynamics are known, including impacts on regional-scale circulations, sea-surface temperatures, surface fluxes and boundary layer mixing, vertical stability, near-surface winds, soil moisture, and vegetation (and therefore again dust emission). A prominent example of such complex interactions is the anti-correlation between African dust and Atlantic hurricane activity from weekly to decadal timescales, the causes of which remain difficult to disentangle. Particularly in the early 21st century, research on African dust intensified substantially and became more interdisciplinary, leading to some significant advances in our understanding of this fascinating and multifaceted element of the Earth system.
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.
Martin Claussen, Anne Dallmeyer, and Jürgen Bader
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.