Afghanistan Dust Storm 20th December 2011: NASAs Image of the Day!

Sorry this is a slight diversion from the current theme! Here is a picture I found on NASA's Earth Observatory Website under the image of the day section. Although its not relevant to what i've been talking about in recent weeks its a very interesting image and important for what I have previously discussed on dust.

The accompanying text goes as follows:

"A dense cloud of dust swept across southern Afghanistan and Pakistan on December 20, 2011. When the Moderate Resolution Imaging Spectroradiometer (MODIS) took this image from the Terra satellite at 10:45 a.m., the dust was largely hemmed in by the Makran and Sulaiman Ranges in Pakistan with only a few wisps reaching south over the Arabian Sea. By the time Aqua MODIS flew over just three hours later, the storm had reached the coast. The dust storm continued on December 21.

The storm is being propelled by strong winds from the north. The winds picked up dust from dry lakebeds in the Hamun wetlands, on the border between Afghanistan and Iran. Concentrated plumes of dust rise from the pale wetlands to become a more diffuse cloud in the south and east. Dry lakebeds and wetlands are among the most common sources of dust in the world.

Dust storms can happen any time of the year in Afghanistan. On average, Afghanistan experiences blowing dust one to two days per month in the winter and six days per month at the height of the summer. Zabon, an Iranian city located near the border in the Hamun wetlands, reports 81 dust storms per year.

Blowing dust poses a hazard to transportation, as low visibility closes roads and airports. This particular storm prevented British Prime Minister David Cameron from visiting a British military base because the runway was closed for low visibility."

The reason for pasting this is to draw your attentions to a couple of sentences in particular that reinforce some of what I have said on dust and some of the more abstract theories with a real world example. From the first paragraph it is clear that the dust storm has travelled a considerable range, increasing its potential to affect a whole host of systems, both environmental and human. The dust in this example was sourced from another dry lake bed and wetlands, not dissimilar from the type of source that contributed the most dust to the system in Africa that I touched upon. Finally, the last paragraph highlights just one of the ways in which can affect human lives and operations....

Apologies for the interjection, I just thought this was a neat example that ties up some of the dust posts.

United Nations Environment Program: Global Desert Outlook

I have found a great website from the United Nations Environment Program that provides a great range of interesting information on desert environments. Many of the topics I have discussed are included such as climate, evolution and global tele-connective properties including dust and biogeochemical cycling. Furthermore, many more features of the drylands are introduced. Biological adaptation, sustainable development and desert research are all mentioned.

In particular, I would like to draw your attention to this section of the website which expands on what I have been discussing recently in terms of humans in deserts, their effects and future change. Chapter 6, 'Scenarios of Change', talks about driving forces or change and scenarios of change for water and land degradation.

Well worth a read!

What future for dryland populations?

I have introduced concepts and provided evidence for climate change in exacerbating desertification and that anthropogenic activities may further enhance the associated environmental changes. With all this in mind, what is the future for desert occupation? Overcultivation, overgrazing, land use changes, unsustainable practices all induce a host of feedbacks including nutrient depletion, reduction of moisture-holding capabilities of soil, mobilisation of sediments etc etc (Mouat, 2008). In turn cultural and societal routines may be impinged through, for example, the exhaustion of food and water resources leading to malnourishment, famine, disease and so on. The effects of environmental change are both varied and extremely serious and may develop on an exponential basis according to those such as Charney, and recovery policies may be difficult to implement.

Critical to the future of dryland populations is our openness to adaptation. Without adaptation humans must migrate or risk death. By adaptation I mean a willingness to develop and utilise coping mechanisms (new technologies, methods etc) and perhaps most importantly, a preparedness to adopt an alternative lifestyle that may not be complicit with previously apparent traditional heritage or cultures. Mouat and Lancaster (2008) highlight the inextricable linkages between environmental security and human security.

One of the main concerns, however, is that not everybody is able to adapt or migrate. Meze-Hausken (2000) provides a table that explains the factors that may influence migration during times of drought:

The tables give great insight in to strategies that may be employed to help societies out of trouble in drylands. Policies should focus on varied and appropriate crop planting, family size and planning issues, water availability, civil unrest and war, and the number of survival strategies they themselves are aware of. It is highly apparent that there is a distinct lack of focused and directed education in these areas. Is education the most appropriate and sustainable dryland population management solution? 

Desertification Risk

I came across two very interesting maps from the United States Department of Agriculture Natural Resources Conservation Service (USDA NRCS). The first map shows desertification vulnerability. The second shows the risk of human-induced desertification. I felt it was particularly striking, perhaps a little obvious, but still extremely important to recognise how many of the regions most under threat were those of the largest populations! Does this exacerbate perception that humans may be the largest threat to desertification phenomena? The social and cultural 'sub-cycle' to Charney's hypothesis perhaps becomes even more important!   

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?

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.