Classic Paper: Climate Change the Motor of Africa's Evolution

In this post I will explore a classic paper by Kuper and Kröpelin published in Science (2006). Entitled ‘Climate-Controlled Holocene Occupation in the Sahara: Motor of Africa’s Evolution’, the research article exains the linkages between climatic variation and prehistyoric occupation of the Sahara over the last 12,000 years.

With contemporary climate reconstruction approaches permitting the increasingly presice exhibition of environmental fluctuations, it is now clear that the Holocene represents an era of marked environmental changes. The addition of archaelogical, anecdotal and geological evidence can provide a remarkably fine resolution of environmental change.

Kuper and Kröpelin state that because of the hyperaridity and lack of natural oases and wells, the Eastern Sahara has been absent of human occupation in recent millennia.  As such, they describe it as ‘a unique natural laboratory for the reconstruction of the links between changing climate and environments, and human occupation and adaptation, with prehistoric humans as sensitive indicators of past climate and living conditions’. Evidence of previous occupation through archaeological remains and settlement sites provides a powerful argument for shifting climatic zones and innovative adaptive strategies.

During the Alleröd interstadial the Eastern Sahara was about as dry as it was during the LGM 20,000 B.C.E. However, carbonate lake formations in Sudan, radiocarbon dated to roughly 8,500 B.C.E., provide evidence of a changing climate and pluvial conditions between latitudes 16⁰N and 24⁰N. This indicates a northward shift of tropical rain belts over as much as 800km, largely attributed to migration of the palaeomonsoon system. A semi humid climate was prevalent over larger portions of the Sahara. Radiocarbon dates from occupation sites through the Eastern Sahara reveals the cesation of settlement at about 5,300 B.C.E. (bar favourable refugia).

Phases of human occupation can be mapped through time to illustrate the major stages of early and mid-Holocen occupation.

The early Holocene reoccupation is depicted by the blossoming rainfall across previously hyperarid regions. Savannah-like environments saw the migration of already adapted societies to this ecology in a northwards direction. Archaeozoological evidence suggests these settlers may have been hunter-gatherers. Sparseness of settlement along the Nile valley reflects conditions far too hazardous for dwelling. By 7,000 B.C.E, settlement was firmly founded and populations were maturing in turn with the development of the first farming communities and domestic livestock.

An abrupt end in occupation is described at 5,300 B.C.E. Monsoonal rainfall became irregular an infrequent. Settlement became more sporadic and fragmented . By 3,500 B.C.E., rainfall ceased even in ecological refugia such as Gilf Kebir. The return of hyperarid conditions to much of the Sahara tested the adaptive capabilities of prehistoric civilisation to the limit. Only the most advanced communites could survive and adapt and aclimatise to the changing environement. The expansion and contraction of the Sahara desert is described as ‘the motor of Africa’s evolution up to modern times’. 

Desertification in the News: Linking Human Activities and Climate Change.

Sudan – battling the twin forces of civil war and climate change:

Water stress and a food security crisis looms in Sudan, where millions of hectares of semi-desert has turned into desert. This great piece in the Guardian nicely shows the two way relationship between anthropogenic activities and environmental change.

Human Occupation of Deserts and Desertification Issues: An Introduction

I would like to now spend some time exploring humans in deserts and issues of desertification. Human occupation of arid regions across the globe has been both sporadic and highly variable in terms of populations and longevity. Rapid colonisation and recession of human settlement is a prominent feature in the history of desert populations, largely controlled by opportunity and marginalisation. Climatic change and human modification of the environment are two of the major factors responsible for the management of the extent and scope of desert civilisation. The next few blogs will highlight changing patterns of populations and their causes.

Closely intertwined with human occupation of the desert is the concept of desertification, the spreading of desert-like conditions. Many relate this exclusively to human activities; others argue climate change playing a significant role. Desertification is associated with the reduction of crop yields, reduction of biomass, river flow and groundwater depletion, encroachment of sand sheets over settlements or productive lands, and significant social disruption. Furthermore, existing species may be preferentially replaced by less desirable species leading to reduced livestock materials etc. Desertification is a critical contemporary phenomenon and proceeding posts will cover the monitoring of desertification, the causes of desertification and potential solutions to this severe environmental problem.

For now, here’s a couple of interesting introductory links:

Desertification

Temporary Drought or Permanent Desert?

Desert Dust, Aerobiology and Human Implications

 In recent years, the natural environment and human health have become increasingly associated. De Longueville et al (2010) reveal that around 30% of global diseases can be attributed to environmental dynamics. As we have discussed already, aeolian activity in deserts because of the extreme erosivity and erodibility can result in extensive entrainment of fine particles, and in some cases, long-distance transport on various trajectories. Transoceanic and transcontinental (in the case of Asian deserts) dust loadings are now known to facilitate the dispersal of pathogens. The biogeographical range of pathogens is consequently increased in significant dust events and the considerable health implications of allergens and pathogens means such occurrences are being increasingly investigated (Kellogg and Griffin, 2006).  

There are a number of ways in which dust particles can affect humans and ecological patterns and processes. At the most basic of levels, mineral dust can have a significant effect on horizontal visibility and air quality. More worryingly, perhaps, evidence has been presented of the long distance transfer of aerosolised microbiota around the globe. Moreover, the concentrations of such microorganisms have been confirmed as higher during dust events, although the magnitudes of concentrations are still to be verified (Kellogg and Griffin, 2006). Aspergillus fumigatus, Aspergillus niger, Staphylococcus gallinarum and Gordonia terrae have all been found in African dust. There are various hypotheses linking African dust transport to episodes of coral reef morbidity and Caribbean basin mortality with the discovery of the fungus Aspergillus sydowii in Sahelian soil (Rypien, 2008). Many of the bacteria found are spore-formers, making them more resistant to desiccation, increasing the potential of successful long distance transport.

Global  transport of microorganisms and toxic compounds in clouds of desert dust (USGS)

With humans no doubt adding to the production of dust by breaking down crusts, removing vegetation, overgrazing, vehicular use and construction activity, the potential for dust-borne pathogens to be transported to regions of high population density or ecological significance is ever increasing. Study is this discipline is in its relative infancy but will surely receive more attention because of the threat posed to human societies. 

The Role of Dust in Biogeochemical Cycles

Here is an exploration of three interesting papers on dust and biogeochemical cycling. I will attempt to describe dust’s involvement in major biogeochemical cycles and then focus in on a specific example.  We have already discovered the powerful impact dust can have on the global climate system and the consideration of their impacts on biogeochemical cycling is imperative to enhance our understanding of earth systems science.

Harrison et al (2000) suggests that changing dust fluxes and depositional rates through time can have a significant consequence for productivity of marine and terrestrial ecosystems because of their nutrient carrying capacities. In turn, changes in biospheric productivity have a direct impact on atmospheric composition (e.g. CO₂ and N₂O) potentially resulting in the initiation of widespread and various feedback mechanisms.

Dust also makes up a god proportion of oceanic sediments downwind of major desert regions. In the North Pacific, for example, it is estimated that around 75-98% of sediments have aeolian origin. Marine ecosystems can thus be significantly affected by the mineral composition of dust loadings, most notably silicon, iron and phosphorus.

Like sediments, soils can also be strongly influenced by desert-originated dust flows. Once again, key nutrients are sourced from this aeolian activity and, as outlined in a previous post on Amazonian dust deposition, can considerably alter ecosystem equilibrium. Saharan dust is attributable to the addition of key trace species such as K, NH₄ and NO₂. Mineral dust is also particularly important where leaching predominates to maintain a healthy nutrient balance.

Whilst we can recognise the importance of aeolian dust in affecting global biogeochemical cycles, Lawrence and Neff (2009) show that it is also important to consider past changes in the global dust cycle. Ice-core and sediment data show dust deposition was greatest during glacial maxima. This evidence only adds to the complexity of changing feedbacks and resultant environmental changes. Geochemical fluxes resulting from dust deposition are most potent in close proximity to source regions, but have these source regions changed over time? Hsu et al (2009) reinforce this important aspect of changing geochemical flux by highlighting the reduction in the quantity of dust deposited with distance from the source. 

Journey to the dustiest place on Earth

A short report on a study by former UCL academics.

Bodélé study

The Bodélé Depression

The Bodélé Depression is a palaeolake basin situated in northern Chad. Digital elevation models have highlighted a series of palaeoshorlines once prominent as palaeolake Megachad’s borders. Megachad was formerly the largest lake in Africa covering in excess of 350,000 km² but has since dried up exposing a largely diatomite surface (Bristow et al, 2009). TOMS (Total Ozone Mapping Spectrometer) Aerosol Index data illustrates the Bodélé Depression to be a critical mineral aerosol source throughout the year (Washington and Todd, 2005). Entrainment of dust into the atmosphere from this major source region forms an important component of the Earth’s climate system because of its potential to influence, for example, the radiation budget. Extreme erodibility and strong surface winds account for the magnitude of the dust plumes over the Bodélé. Engelstaedter et al (2006) suggest that anthropogenic activity is also responsible for the great entrainment of dust. Firstly through land use changes such as through agriculture, mining, water management or vehicular activity, and secondly through our contributions to climate change which in turn modify dust emissions.

      African aerosol indices and Bodele dust (From UCL and NASA)

Dust from the Bodélé Depression is also known to influence biogeochemical cycling. Koren et al (2006) advise that 40 million tons of dust are transported to the Amazon from the Sahara each year. With a considerable proportion of Saharan dust accounted for from the Bodélé, it is clear a significant proportion of Amazonian dust is from this region. Furthermore, Koren et al state that the Amazon basin has a dependence on the mineral fertilisation from Saharan dust sources. Washout of nutrients in heavy rainstorms makes a steady supply of mineral essential to maintain rainforest equilibrium.