Dust II

There is an increasing awareness in the importance of atmospheric dust loadings, and in particular the significant role they may play in climate change. The world’s deserts are major source regions of dust since the arid conditions and lack of vegetation enables deflation and entrainment of silt-sized sediment from a variety of materials (Middleton and Goudie, 2001). Human activities of grazing and vehicular use, for example, can increase the range of susceptible surfaces from which dust can be entrained.

Dust can influence climate in a number of ways. Goudie and Middleton (2001) advise that air temperatures may be affected through the absorption and scattering of solar radiation, marine primary productivity may be affected since dust may provide considerable quantities of iron, and changes in concentrations of condensation nuclei may influence cloud formation and in turn precipitation. Furthermore dust loadings may affect soil formation, calcrete formation, ocean sediment fluxes, and human health.

The Sahara provides the most substantial quantities of aeolian dust with emission estimates between 500 and 1000 Tg yr^-1 which makes up around 50% of the global total (Goudie, 2009). The Bodélé depression is the widely considered the most important source region in the Sahara and contributes a considerable proportion of the Saharan emissions. Saharan dust is comprised of a number of a number of different particles but is made up predominantly of SiO₂ and Al₂O₃. The dust flux generated in the Saharan region emphasises just how important geomorphologic processes are in the desert biome.

Entrained dust off the western coast of Africa, courtesy of NASA

Saharan dust has three main trajectories – over the North Atlantic to North and South America, northwards to southern Europe, and eastwards to the Middle East (Goudie and Middleton, 2001). The North Atlantic flux is the largest and large outbreaks can see dust transported to the Caribbean, US and Brazil. Dust concentrations have been recorded in Amazonia and corral reefs off Barbados, particularly noteworthy considering dust’s influence on nutrient dynamics and biogeochemical cycling of terrestrial and marine ecosystems.

So how does dust influence terrestrial and marine ecosystems? How will dust fluxes change in the face of climate change? Do humans significantly modify the extent of dust entrainment? How important are ‘dust hotspots’ such as the Bodélé depression? What are the other major source regions? Read on to find out………..!

Dust

A gentle introduction to the power of dust!

Distribution of Deserts

My area of focus over the next couple of weeks will be away from desert hydrology and on to geomorphology, starting more specifically with desert dust. Desert dust is a critical component of the climate system at both a local and global scale.

As a prelude to a number of posts on the subject, it is worth highlighting where the various desert regions are found, and hence where dust may come from.

The US Geological Survey uses the following map to describe the locations of the worlds major desert regions.

It is highly apparent that the majority of the deserts are found at 30 degrees latitude north and south of the equator. This coincides with the falling limb of the Hadley Circulation and the subtropical high-pressure belt. Persistent thermodynamic stability results in a suppression of vertical motion and hence minimal precipitation. Compartmentalisation of anticyclonic cells breaks up subtropical subsidence explaining why hyperaridity is not prevalent across the latitudes (Ahrens, 2009). Georgia Southwestern State University provide useful diagrams that help to illustrate how global circulatory patterns are responsible for the distribution of deserts.

There are exceptions to this general rule. The great deserts of Central Asia, for example, can be attributed to the shear distance from the sea and hence pronounced sources of water. Greater seasonal changes in the desert environment may be associated with these continental interiors (Goudie, 2002). Furthermore, polar deserts are found largely because of the freezing temperatures and minimal precipitation.

There can be a number of regional factors that influence the precise formation and nature of particular deserts, but these will be examined later through specific case studies related to individual deserts.



References

Ahrens, C. D (2009) Essentials of Meteorology, Brooks/Cole: Belmont

Goudie, A.S. (2002) Great Warm Deserts of the World, Oxford: Oxford University Press.

Desiccation of The Aral Sea

The Aral Sea is a closed basin situated in Uzbekistan and Kazakhstan with an extremely large catchment area. The desert landscape receives less than 90mm of rainfall per annum and exhibits a strong continental climate characterised by extreme temperatures in the summer and winter. The sea has no outflow but two rivers the Amu Dar’ya and Syr Dar’ya feed the basin with waters of snowfield and glacial origin (Laity 2008).

The Aral Sea once extended 66 100 km² but problems developed with the diversions of its premiere inputs in the 1960s and 70s. By 1987, 60/70% of the Aral Sea’s volume had been lost, with a water level reduction of 14m (Glantz, 2007). The Aral Sea became split into two bodies of water, the Large Aral Sea and the Small Aral Sea (Sorrel et al, 2006). Water levels had been known to fluctuate over the Holocene with marine fossils and relict shore terraces providing evidence of 20-40m oscillations in response to changes in the climate system and the subsequent implications for river discharge. Current changes are far from natural.

     

The affects of hydrological change in the Aral Sea are wide ranging. Diversion for the purpose of cotton and rice field irrigation has seen huge increases in the Aral Sea’s salt concentration, decimating the local fishing industry. Sorrel et al (2006) report surface water salinity rose from 10.4 g kg^-1 in 1960 to more than 80 g kg^-1 in 2003. In addition, strong winds transported toxic dust onto farms several hundred kilometers downwind from sediments that had once been covered under water. Life expectancies of approximately 3.5 million people have been cut significantly as a result of exposure to toxic chemicals. Rates of disease among children is increasing with intoxication of heavy metals causing renal tubular dysfunction, in addition to increased number of cases of tuberculosis, malignancies and psychiatric disease  (Kaneko et al, 2011; Matsapaeva et al, 2010). Desiccation of the Aral Sea has also had extensive climatic impacts. Small et al (2001) illustrate the changes in surface air temperature since the disruption of the Aral Sea’s inputs. Mean, maximum and minimum temperatures near the Aral Sea have changed by up to 6^o C. The magnitude of change decreases with distance from the 1960s shoreline.

This example demonstrates the enormous control humans can have on the environment, potentially causing irreversible damage. Not only do we have the power to change the environment, but also threaten the well being of the contemporary society, ecological communities, and even regional-scale climatic outputs. The scale of the damage caused in the Aral region is testified by the 300 projects proposed to alleviate crisis. Furthermore, responses are aimed towards minimising the damage, not restoring to former characteristics. Small et al reinforce the recklessness of many contributions by suggesting, “If every expert brought a bucket of water, the Aral Sea would be filled again”…  

References

Laity, J. (2008) Deserts and Desert Environments, Chichester: Willey-Blackwell

Alternative Uses of the Desert Environment!

My next blog will draw together many of the components involving climate and water resources in deserts mentioned beneath by focusing on a case study of the Aral Sea. But until then here's what else goes on in deserts!

Branson launches desert spaceport...

Humans and Water Resources

It has previously been noted that climatic shifts can have a significant impact on desert hydrology and in turn human occupation of the landscape. In addition to the climatic component of hydrological change there is an increasing anthropogenic component. There are four major ways in which humans are interfering with desert water resources (Laity, 2008):

1: Groundwater withdrawal. Groundwater is increasingly used for the maintenance of human settlements and plant and animal habitats. Its withdrawal by far exceeds natural rates of replenishment.

2: River flow alteration. Rivers are being diverted away from their natural cause for the purpose of land irrigation and the construction of dams to regulate flow.

3: Salinisation. Contemporary irrigation technologies have resulted in increased salinity as extremely high evaporation rates cause mineral salt precipitation. The land is severely degraded as a result and fertility is much reduced. In turn, crop yields reduce.

4: Lake disturbance. Pollution e.g. disturbances of geochemical balances through processes including eutrophication or toxic dust absorption can have long-standing impacts on closed or semi-closed basins.

References

Laity, J. (2008) Deserts and Desert Environments, Chichester: Willey-Blackwell

Hydrological Change and Human Occupation in Deserts

Viewing the diagrams incorporated within the mentioned sources enhances this post.

Hydrological shifts in desert regions have had significant impacts on the viability of the landscape for inhabitation. As a consequence, populations of deserts have fluctuated dramatically. One major controlling factor of the availability of water in deserts is climate. I would like to provide two examples of the influence of climatic change on desert hydrological regimes.

The western coast arid zone of South America includes the Peruvian and Atacama deserts. Initiated by uplift of the Andes and the development of the Antarctic bottom waters and the Peruvian-Humboldt current, the Atacama is generally accepted as being very old (Goudie, 2002). Hyper-aridity has predominated since the middle to late Miocene. Betancourt et al (2000) present evidence of hydrological changes over the last 22,000 years from the Atacama. Vegetational and groundwater change were reconstructed from radiocarbon dated fossil rodent middens and wetland deposits. Rodent middens provide high taxonomic resolution of past vegetation and are used in this study in terms of rainfall seasonality. Diatomaceous wetland deposits provide a more continuous record. Data reveals increasing summer precipitation, grass cover and groundwater from 16.2-10.5ka. Summer-flowering grasses spread to regions now lacking water and vegetation. In addition, another pluvial period occurred from 8-3ka. Betancourt et al suggest teleconnections or insolation forcing to be attributable.

Moving from South America to Africa, Damnati (2000) investigated lake fluctuations as a measure of palaeohydrological and palaeoclimatic change in the Sahara and Sahel. Lake status can be derived from a variety of data including stratigraphical, geochemical and palaeoecology, but also archaeological data since lakes are an important source of food and water in arid and semi-arid regions. It is clear from studying Damnati’s maps of lake status that there was a major pluvial phase 10 to 7ka. At 7 to 6ka many of the lakes shifted to an intermediate or low water balance. From 5ka to the present, lake levels in the Sahara and Sahel have continuously deteriorated to a point where over 95% of the sample have low or intermediate lake levels.

Climatic shifts determining water resources inherently influence occupation of arid and semi-arid environments. Kuper and Kropelin (2006) describe human occupation of the Sahara during the Holocene. Notable occupation events seem to correlate with Damnati’s findings. A major reoccupation of the Sahara occurred at 8.5 to 7ka as monsoonal rains transformed the landscape into a savannah-like environment. The Formation phase 7 to 5.3ka ended abruptly in congruence with diminishing lake levels. During this phase, however, there was the introduction of domestic animals including sheep and goats The Regionalisation phase saw retreat to highland refugia with greater precipitation or temporary lakes. Refugia included the Gilf Kebir and the Sudanese plains. Finally, 3.5 to 1.5 ka, human activities were restricted to northern Sudan. This is known as the Marginalisation phase where rains ceased even in the ecological niches. 

References

Goudie, A.S. (2002) Great Warm Deserts of the World, Oxford: Oxford University Press.