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African Humid Period - How Can Humans Help End It?

The major, extensive landscape change that took place on the African continent throughout the Holocene was brought on by the end of the African Humid Period (AHP).

Most of northern Africa saw a big drop in rainfall, and plant and animal groups changed into new ecological niches, even though the changes happened at different times and in different places.

People also changed where they lived and how they made a living, switching from hunting and gathering to farming.

People say that after the AHP died out, complex social institutions with hierarchical leadership over irrigation systems, agricultural resources, and networks for redistributing food grew up in northern Africa and southern Asia.

The end of the AHP, which happened between 8000 and 4500 years ago in different parts of northern Africa, is usually thought to have been caused by an orbitally-induced weakening of monsoon advection over the Sahara. This increased albedo (the amount of light reflected by the earth's surface), sped up dust entrainment, and created a terrestrial-atmospheric feedback loop that made it even less likely that it would rain.

What Is The African Humid Period?

Between 6,000 and 9,000 years ago, the climate of North Africa had a humid period. The African Humid Period was during this time. Paintings from that era showing a radically different African scene, full of elephants, antelopes, giraffes, and other wildlife being followed by hunters, were found by the German explorer Heinrich Barth.

The explorer was attracted by the paintings' dissonance with the Sahara's lifeless nature since it contrasted with the vibrant creatures and landscapes that were shown in the paintings.

What Caused The African Humid Periods?

Orbital Changes

African Humid Periods may be explained by variations in the Earth's axial rotation brought on by the planet's various gravitational interactions with the moon, sun, and other planets.

The Milankovitch Cycles, which consist of three orbital cycles, have a significant influence on the climate of the Earth and, more specifically, on the climate of Africa. They relate to the Earth's orbital obliquity, eccentricity, and precession.

Precession is the one of these three that has the biggest effect on the climate in Africa. The Earth's upper axis wobbles due to precession. It has a big effect on the weather in North Africa because it determines how strong the monsoon will be and how much it will rain there.

The wobble boosts the amount of solar energy absorbed by the North African mainland by up to 8% when it brings the Earth closest to the sun, as it did between 6,000 and 9,000 years ago. As a result of the increase in land temperature and the resulting low-pressure area over the African mainland, there is an abundance of summer monsoonal rainstorms.

This rise in rainfall nourishes the soil, promotes vegetation development, and thus increases animal populations. The Sahara would return to being the dry desert it was throughout the winter, when this effect would be reversed.

Albedo Feedbacks

During the African Humid Periods, there are big changes in temperature and rainfall that can't just be explained by the Earth's orbit. The Albedo feedback is another frequently given justification. Albedo is a property of a landmass that shows how much solar radiation it reflects.

Lower temperatures would result from more light reflecting, while higher temperatures would result from less solar energy reflecting. The increasing vegetation on the landmass brought about by the orbital alterations would have an impact on Northern Africa's albedo levels. As more solar energy is retained and less is reflected back into the atmosphere, more vegetation results in decreased Albedo feedback. The monsoon season can therefore use more electricity.

Thus, a feedback loop is created. Albedo levels fall even further when more vegetation grows as monsoon levels rise as a result of lower albedo. In addition, a longer monsoon season and more vegetation considerably reduce dust production, thus lowering albedo. Thus, the effect of orbital changes is further amplified by this feedback loop.

Intertropical Convergence Zone Changes

The convergence of the northeast and southeast trade winds is known as the Intertropical Convergence Zone. This also features a feature called the Monsoon trough, which is in charge of creating many jungles all over the planet. Because the summers are getting warmer, the ITCZ may have moved north into Northern Africa, making the climate even better for plants to grow.

Change In Precipitation Levels

The diminished seasonality of the precipitation cycle is another potential explanation. This shortens the dry season and increases the amount of moisture, humidity, and rain over Northern Africa, making it even more likely that the African Humid Period will continue.

Tipping Points And Regime Shifts

When the biotic environment changes from one state to another, an ecological system reaches a tipping point, also known as a threshold.

More specifically, regime shifts are changes in trophic levels caused by internal or external factors, such as overgrazing, invasive species, or climate change. Other examples of regime shifts include overpopulation, biochemical, and geomorphological changes.

Once a regime shift has taken place, it is impossible to go back to the previous situation since there are too many variables involved in replicating the pre-regime conditions. Regime changes happen because of nonlinear dynamics. Often, several stresses on the landscape come together at the same time to change the ecology of a place.

In modern practice, finding regime transitions often requires putting together multivariate, long-term ecological research (LTER) datasets and statistically evaluating independent variables (for example, using Principal Component Analysis) to see if a sudden change has happened.

The development of coherent ecological resilience models using combinations of terrestrial, aquatic, and atmospheric datasets, along with quantitative reporting of as many independent variables as possible, has led to multifactorial and holistic indicator sets of where thresholds exist in current landscapes and how long and how much pressure it would take to push them over.

The main thing that LTER studies have shown is that regime shifts that hurt biodiversity are never caused by a single forcing event. Instead, they always have more than one cause, happens after a long period of stress on the ecosystem, and the final tipping point doesn't have to be big.

The Scenario

Three requirements must be satisfied for human agents to be regarded as potential triggers of landscape change significant enough to cause the crossing of an ecological threshold. The first is that the pre-threshold crossing condition of the landscape was close to tipping because of things that made the system less resilient as a whole.

The second is that a triggering event might have credibly produced conditions bad enough to prompt a regime change. Given the size of the shift both chronologically and spatially—the Sahara and Sahel contain 12.5 million km2 of extremely different geographies, especially during the AHP—internal dynamics of the system in the case of the AHP's termination are implausible. Although localized cascading effects have been theorized to affect enormous geographic regions, it is implausible that they could have simultaneously (in geological periods) caused a series of regime shifts across such a vast range.

This presentation will analyze how landscapes that had never been subject to grazing by domesticated animals have been reported as crossing ecological thresholds quickly after new grazing pressures were introduced in order to support the hypothesis that human agency played a role in the change. The third requirement is that there must be evidence that a change in the ecological regimes of a certain landscape happened and that people may have had something to do with it.

Human agency in the AHP's demise would have caused a localized, erratic transition in scale. Also, changes in local ecologies caused by humans would have caused a feedback loop that would have affected changes on a continental scale.

The Precipice Of Landscape Change

Regime changes in northern and eastern Africa during the AHP were neither randomly distributed nor single-cause events. The Sahara and the Sahel experienced their highest maximum summer insolation values between 10,000 and 9000 years ago, which is generally recognized as the AHP's apex.

Although there are currently no full-scale vegetation maps of the entire region based on proxy data, it is challenging to generalize the vegetation for such a vast and topographically diverse area. However, rough approximations can be created.

Although Hély et al. give a latitudinal reconstruction of vegetation based on proxy data, time-transgressive longitudinal investigations have been hindered by the lack of sample locations. However, the overall consensus of both models and proxy evidence shows that the present-day Sahara was substantially wetter and had a much higher plant diversity than is found in the region today. Paleoclimatic models frequently disagree on the specifics of the regional distribution.

A number of zebras and antelopes grazing on a large field with trees
A number of zebras and antelopes grazing on a large field with trees

External Forcing Causes Threshold Crossing

It has been suggested that diversified plant communities in habitats that supported the AHP termination lowered the sensitivity of the Saharan ecosystem to climate impacts. The progressive effects of steadily declining precipitation on the floristic composition of the site in a study of vegetation response to AHP termination from Lake Mbalang in northern Cameroon are attributed to the "stability of vegetation" that existed before the event and predated climatic change.

Pollen records from Lake Tilla, Lake Mega-Chad, parts of the eastern Sahara, and an offshore sediment core near Senegal have revealed similar landscape dynamics. In these places, plants took a long time to adjust to changes in the way it rained until clear signs of human influence were found in the proxy record.

Threshold Crossing And The Arrival Of Domesticated Livestock In Africa

Humans are potential amplifying agents in diminishing NPP when seen through the lens of anthropogenically-induced regime change, and local landscape change frequently happens at large scales in regions where archeological and palynological data are contemporaneous.

A large-scale study found that when farming and animal pastoralism got better during the early termination phase of the AHP (around 8000 years ago), sediment flow in the Nile River went up.

The ways in which large-scale alterations could have been induced are illustrated by more localized investigations. When domesticated taxa arrived in the western Mediterranean at Ifri Oudadane some 7,300 years ago, native grasses and arboreal pollen (AP) significantly decreased, while maquis species significantly increased. During the Neolithic period, when people moved into the Mediterranean area, burning and grazing animals are thought to have led to the growth of maquis.

During the growth of the maquis, evergreen holm oaks (Quercus ilex), strawberry trees (Arbutus unedo), and basswood (Phillyrea latifolia) and elms (Ulmus sp.) did better than deciduous oaks, manna trees, terebinths, wild service trees, and elms.

Between 7,400 and 6,800 years ago, the amount of AP at the site of Ifri n'Etsedda in Morocco slowly went down, while the amount of NAP and cereal cultivars went up.

Similar flora changes have been observed at the Algerian archaeological site of Tin-a-Hanakaten, along with evidence of significant eolian activity beginning around 7,200 years ago. This time period is linked to the first time that large numbers of cattle bones were found in archeological deposits.

Could African Neolithic Populations Have Induced Regime Changes?

In each of the above scenarios, the economy of domesticated animals, the decline of AP and/or grasslands, the rise of shrub species, and the change from wet to dry conditions are all linked in space and time.

Correlation does not prove causality, and domesticated animal economies may have moved into shrub-transitioning settings in reaction to ecological alterations. This is the most common explanation for how climate change caused the Neolithic to spread in sub-Saharan Africa.

Early African pastoral economies were diverse and co-evolved with their surroundings. Domesticated cattle from Nabta Playa may be among the world's earliest and may have been tamed 11,000 years ago. In southwestern Libya in the 9th millennium BP, Barbary sheep (Ammotragus lervia) were fenced in as part of a delayed return foraging strategy.

More secure domestic animal contexts in the Fayum Depression date to 7350 BP and are related to fish remains and mobility and settlement activities. At 7,300 BP, low-mobility pastoral economies occupied the Nile's alluvial plain.

Strontium isotopes from burials in the Acacus Mountains of southeastern Libya suggest an increase in movement as the climate dries. Domesticated animals were introduced in the Hodh depression of south-central Mauritania 4,500 years BP, along with Neolithic communities like Dhar Tichitt.

Given how complicated the relationships between people, animals, and landscapes were in northern and eastern Africa during the early to middle Holocene, there is no one-size-fits-all model for how ecological systems, which are themselves very different, adapt to human pressures.

People Also Ask

What Happened To End The African Humid Period?

The analysis's unambiguous result is that the end of the African Humid Period moved from north to south, closely matching what orbital forcing would predict. In particular, the monsoon rains were progressively delayed as latitude decreased, starting in the north.

What Is The Evidence For The African Humid Period?

The volume and variety of paleoclimate data are astounding, and the Early Holocene AHP is one of the most well-studied and precisely dated climate change events in the geologic record. These land and sea records help us figure out how long and how big the humid period was.

How Was The Vegetation At The Time Of The African Humid Period?

During the last glacial-interglacial transition, orbitally driven increases in summer insolation heated land masses relative to the ocean surface. Warmer land surfaces in North Africa caused a low-pressure zone, bringing Atlantic monsoonal rains northward. Because of this, some of the world's driest places used to be covered by permanent, deep freshwater lakes, like "Mega" Lake Chad, which is 400 000 square kilometers.

Conclusion

The geographical and temporal variety of African Humid Period termination suggests anthropogenic effects causing local biological threshold crossing events, which catalyzed devegetation and negative feedback between terrestrial and atmospheric domains. As humans steer livestock to new pastures, local regime shifts may spread.

Human population increase and natural (orbital-induced) temperature change drove Middle Holocene landscape pressures, but domesticated ungulates' grazing habits on fire-adapted, xeric landscapes were the final element in irreversibly lowering NPP in threshold habitats. Numerous historical examples from the European colonization of the Americas and Pacific Rim indicate the validity of this scenario—as humans alter ecosystems to create high ACP niches, they cause changes that harm NPP.

Deep-time histories of domesticated bovids in Eurasia and Africa make it hard to get a synchronic view of the beginning spread of livestock. On the other hand, diachronic evidence from the Sahara shows that the arrival of domesticated stock and the change of the landscape from high NPP to desert followed similar patterns.

About The Authors

Suleman Shah

Suleman Shah - Suleman Shah is a researcher and freelance writer. As a researcher, he has worked with MNS University of Agriculture, Multan (Pakistan) and Texas A & M University (USA). He regularly writes science articles and blogs for science news website immersse.com and open access publishers OA Publishing London and Scientific Times. He loves to keep himself updated on scientific developments and convert these developments into everyday language to update the readers about the developments in the scientific era. His primary research focus is Plant sciences, and he contributed to this field by publishing his research in scientific journals and presenting his work at many Conferences. Shah graduated from the University of Agriculture Faisalabad (Pakistan) and started his professional carrier with Jaffer Agro Services and later with the Agriculture Department of the Government of Pakistan. His research interest compelled and attracted him to proceed with his carrier in Plant sciences research. So, he started his Ph.D. in Soil Science at MNS University of Agriculture Multan (Pakistan). Later, he started working as a visiting scholar with Texas A&M University (USA). Shah’s experience with big Open Excess publishers like Springers, Frontiers, MDPI, etc., testified to his belief in Open Access as a barrier-removing mechanism between researchers and the readers of their research. Shah believes that Open Access is revolutionizing the publication process and benefitting research in all fields.

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