Connecting Humans, Vegetation and Desertification: Charney’s Hypothesis

 In 1975, Charney et al published an influential paper outlining a biogeophysical feedback mechanism that attempted to help explain the global advancement of deserts. This classic paper, not without its criticisms, speculated that an increase in albedo as a result of a decrease in plant cover (overgrazing, misuse of the environment etc) causes a decrease in rainfall because of the reduced temperature and hence convective potential in the atmosphere. Subsidence in the troposphere would initiate the feedback processes of reduced precipitaiton and reduced plant growth and hence develop a potenitaly devestating and never-ending cycle ending in continued desertification.

This cycle raises interesting questions. Yes, reduced vegetation can be a result of drought, but also as a direct consequence of human intervention and misuse of resources. The biogeophysical feedbacks (anthropogenically or naturally induced) leads to a host of other ‘societal’ feedbacks. Desertification leads to a decrease in the productivity of land, social marginalisation, population pressures, further overgrazing etc etc. So whatever your take on the causes of the biogeophysical cycle one thing is for sure, the consequences can be vast. The interaction of these processes is neatly depicted below. The next couple of posts will ask what can be done about the externalities associated with desertification as we move deeper into the anthropocene, and humans have an ever increasing power over the environment. 

Could the Desert Sun Power the World?!

I recently stumbled upon this article in the Guardian. In an environment of increasing resource stress and awareness of the various externalities associated with resource misuse, could the deserts of the world really be used generate mass 'clean' power.

"In just six hours, the world's deserts receive more energy from the sun than humans consume in a year. If even a tiny fraction of this energy could be harnessed – an area of Saharan desert the size of Wales could, in theory, power the whole of Europe".......

                                                   From Leverage Academy

So... What do you think? Can you identify any significant pitfalls in the argument?

The Normalised Difference Vegetation Index (NDVI) and the Microwave Polarisation Difference Index (MPDI) for vegetation

The effects of desertification on human societies, and also its potential to initiate a host of other environmental feedbacks, makes it an important process to monitor, learn about past episodes, and to predict the scale of spread or decline in the future under a number of inputs. A classic paper by Becker and Choudhury (1988) discusses how the Normalised Difference Vegetation Index (NDVI) and the Microwave Polarisation Difference Index (MPDI) for vegetation can help to analyse desertification processes.

As previously highlighted 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 the depletion of livestock materials etc. Monitoring global vegetation can hence provide a sensitive indicator of environmental changes.

Becker and Choudhury state that ‘several research efforts have been undertaken in order to find relevant indices characterizing these processes and which are observable from satellites. Among them, albedo (a), surface temperature (T), and Normalized Difference Vegetation Index (NDVI) have received much attention’ and that more recently ‘another index could be of great value… namely the normalized difference of brightness temperatures in horizontal and vertical polarization measured at 37 GHz by SMMR on board Nimbus 7’ (MPDI).

So how do they work as a tool in monitoring desertification? NDVI is correlated to leaf area index (LAI), defined by The Global Climate Observing System as ‘one half the total green leaf area per unit ground surface area’, and the vegetation cover fraction. The key mechanism is the absorptivity and reflectivity of biomass due to chlorophyll absorption. Hence chlorophyll concentrations received from satellite technology plays an important part in the NDVI calculation and hence the quantity of biomass at a given location. The MPDI method incorporates brightness temperatures in horizontal and vertical polarisation and is sensitive to the water content of plants rather than chlorophyll absorption. NDVI is generally considered more appropriate for monitoring vegetation. 

                                                         Global NDVI

Further advancement of these techniques will equip us with the tools to build high resolution, high accuracy models that help us to predict regions most susceptible to the threat of desertification and hence implement policies and practices designed to mitigate the ill effects it can bring. Remote sensing will no doubt be a key feature of desertification management and monitoring. 

Desertification Success Stories

This link to the United Nations Environment Program  illustrates wide ranging success stories in the control of desertification and how humans are trying to overcome its effects. Very much worth a look!

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?