Saharan dust storms: nature and consequences

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  • .Earth-Science Reviews 56 2001 179204www.elsevier.comrlocaterearscirev

    Saharan dust storms: nature and consequencesA.S. Goudie), N.J. Middleton

    School of Geography and the Enironment, Uniersity of Oxford, Mansfield Road, Oxford OX1 3TB, UKReceived 14 December 2000; accepted 14 May 2001

    Abstract

    This paper reviews recent work on the role of Saharan dust in environmental change, the location and strength of sourceareas, the transport paths of material away from the desert, the rates of Saharan dust deposition, the nature of that material .including PeriSaharan loess and the changing rates of dust activity in response to long and short-term climatic changes.The Sahara produces more aeolian soil dust than any other world desert, and Saharan dust has an important impact onclimatic processes, nutrient cycles, soil formation and sediment cycles. These influences spread far beyond Africa, thanks tothe great distances over which Saharan dust is transported. The precise locations of Saharan dust source areas are not well

    .known, but data from the Total Ozone Mapping Spectrometer TOMS suggest two major source areas: the Bodele depression and an area covering eastern Mauritania, western Mali and southern Algeria. Trajectories of long-distancetransport are relatively well documented, but the links between source areas and seasonal Saharan dust pathways are not.However, it is possible that Harmattan dust from the Bodele depression may not be the source of the prominent winter plume over the tropical North Atlantic, as is often suggested in the literature. Few of the data on particle size characteristics ofSaharan dust are derived from major source areas or from Africa itself. Saharan dusts sampled from the Harmattan plumeand over Europe are dominated by SiO and Al O , a characteristic they share with North American and Chinese dusts. The2 2 3concentrations of these two major elements are similar to those found in world rocks. PeriSaharan loess is conspicuous by itsrelative absence, considering the Saharas dominance of the global desert dust cycle both in the contemporary era andthrough the geological past. In recent decades, the frequency of Saharan dust events has varied markedly in response toclimatic factors such as drought and anthropogenic disturbance of desert marginal surfaces. Nonetheless, the Saharas twomajor dust sources are little affected by human activities and are in fact located in areas that receive very low rainfall totals.Hence, the Sahara does not fit the postulated global picture of a peak in dust storm activity in the 100200-mm mean annualrainfall zone. q 2001 Elsevier Science B.V. All rights reserved.

    Keywords: dust; Sahara; desert; geomorphology; atmospheric transport; TOMS

    1. Introduction

    The Sahara is the worlds largest source of aeo-lian soil dust Schutz et al., 1981; DAlmeida, 1987;

    ) Corresponding author. Tel.: q44-1865-271-921; fax: q44-1865-271-940.

    .E-mail address: andrew.goudie @geog.ox.ac.uk A.S. Goudie .

    .Swap et al., 1996 and probably accounts for almosthalf of all the aeolian material supplied to the worldsoceans. This large quantity of dust generation indi-cates the importance of the geomorphological pro-cesses of aeolian deflation and abrasion in mouldingthe landscape of parts of the Sahara. In this paper,we review recent work that has investigated the roleof Saharan dust in environmental change, the source

    0012-8252r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. .PII: S0012-8252 01 00067-8

  • ( )A.S. Goudie, N.J. MiddletonrEarth-Science Reiews 56 2001 179204180

    areas from which the dust is derived, the strength ofthe Saharan dust source, the transport paths of mate-rial away from the desert, the rates at which Saharandust is being deposited, the nature of that material .including PeriSaharan loess and the changing ratesof dust provision in response to long and short-termclimatic changes.

    2. Dust and environmental change

    One environmental consequence of atmosphericdust loadings is their significance for climate througha range of possible influences and mechanisms. Theymay affect air temperatures through the absorption

    and scattering of solar radiation Li et al., 1996;Moulin et al., 1997; Alpert et al., 1998; Miller and

    .Tegen, 1998 . Saharan dust modifies short-wave so-lar radiation transmitted through to the earths sur-face and long-wave infra-red radiation emitted tospace. However, the balance between these two ten-dencies determines whether this creates cooling orwarming, and this in turn, depends in part upon suchvariables as the size distribution of dust particles and

    .their chemical composition. Fouquart et al. 1987found that either warming or cooling could takeplace in a Saharan dust event largely dependent uponthe numberrsize distribution of the mineral particlepopulation. Other important factors in this equationare cloud cover and the albedo of the underlying

    .surface Nicholson, 2000 . In the case of clouds,their altitude and optical depth are important deter-

    minants of the direct radiative impact of dust Quijano.et al., 2000 .

    It is also possible that dust may affect climatethrough its influence on marine primary productivity .Jickells et al., 1998 , and there is some evidence

    that dust may cause ocean cooling Schollaert and.Merrill, 1998 . Changes in atmospheric temperatures

    and in concentrations of potential condensation nu-clei may affect convectional activity and cloud for-

    mation, thereby modifying rainfall amounts Bryson.and Barreis, 1967; Maley, 1982 . Dust particles in

    storms reaching Israel from the Hoggar, for example,are typically coated with sea salts after their longjourney across the eastern Mediterranean, as well as

    .anthropogenic sulphates Levin et al., 1996 . Theseparticles are thought to act as ice and cloud conden-

    sation nuclei, hence playing a role in the formationof rainfall.

    Secondly, dust deposition provides considerablequantities of nutrients to ocean surface waters andthe seabed, particularly in the North Atlantic Ocean.

    .Talbot et al. 1986 have determined the percentageof a number of nutritive species in western African

    . .aerosols Table 1A , while Swap et al. 1996 havecalculated the westward mass flux of these same

    .nutrients Table 1B . In addition to these four species,aeolian dust contains appreciable quantities of iron .Zhu et al., 1997 , the addition of which to oceanwaters may increase plankton productivity by stimu-lating N2-fixation and thus acting to reduce nitrogenlimitation stresses on the phytoplankton community .Gruber and Sarmiento, 1997 . Saharan dust aerosolsinfluence the nutrient dynamics and biogeochemicalcycling of both terrestrial and oceanic ecosystems.Moreover, because of the thousands of kilometersover which the dust is transported, its influence

    extends as far afield as Northern Europe Franzen et. .al., 1994 , Amazonia Swap et al., 1992 , and the

    coral reefs of the Caribbean. In the case of Caribbeancoral reefs, Saharan dust has also been implicated by

    .Shinn et al. 2000 as an efficient substrate fortransporting disease-spreading spores, which on oc-casion, can cause Caribbean-wide epidemics thatdiminish coral reef vitality, a good match having

    Table 1Nutrient contributions from the Sahara to the North Africanaerosol .A % Contribution

    Mean Minimum MaximumqK 0.17 0.1 0.33qNH 0.18 0.01 0.274yNO 0.51 0.21 1.533yPO 0.037 0.017 0.0714

    .after Talbot et al., 1986; Swap et al., 1996

    . .B Westward mass flux mean values for 1989199211 .10 gryear

    Mean Minimum MaximumqK 4.9 2.9 9.5qNH 5.2 0.3 7.84yNO 14.7 6.1 43.133yPO 1.1 0.5 2.14

    .modified from Table 4B in Swap et al., 1996

  • ( )A.S. Goudie, N.J. MiddletonrEarth-Science Reiews 56 2001 179204 181

    been found between times of coral-reef die-off andpeak dust deposition.

    Thirdly, dust loadings may change substantially inresponse to climatic changes, such as the North

    Atlantic Oscillation and to drought phases Moulin et.al., 1997; Middleton, 1985; Littmann, 1991a and in

    response to land cover changes Tegen and Fung,.1995 .

    Fourthly, additions of dust to land surfaces mayaffect soil formation. This has been proposed in the

    .context of West Africa by Vine 1987 and for theformation of terra rossa soils in southern Europe and

    the Levant Yaalon and Ganor, 1973; MacLeod,.1980; Rapp, 1984 . It may also contribute to the

    character of soils on Caribbean and West Atlantic .Islands Muhs et al., 1990 . Dust that has a high

    carbonate content may be a factor in the formation ofcalcretes.

    Fifthly, dust additions play a major role in thedelivery of sediments to the oceans. For example,

    .Guerzoni et al. 1999, p. 147 have suggested thatABoth the magnitude and the mineralogical composi-tion of atmospheric dust inputs indicate that eolian

    .deposition is an important 50% or even dominant .)80% contribution to sediments in the offshorewaters of the entire Mediterranean basin.B

    3. Source areas of Saharan dust

    One of the most important advances required tounderstand the production of Saharan dust is to beable to identify the major source areas. Herrmann

    .et al. 1999 attempted to survey previous studies .of Saharan dust sources, but remarked p. 142 ,

    AAstonishingly, the results with regard to sourceareas are totally different. No overlapping can bedetected which could serve as a confirmation ofresults.B One explanation for this unsatisfactory situ-ation is the range of source identification methodsthat have been used by different workers: remotesensing, analysis of surface dust observations, backtrajectory analysis of isobar data, and the use ofmineral tracers.

    However, in recent years, some progress has beenmade in identifying source areas by measurements ofinfra-red radiances such as those acquired by ME-TEOSAT. These can be used to produce the Infra-red

    . Difference Dust Index IDDI Brooks and Legrand,.2000 . This method has highlighted the Bodele De-

    pression between Tibesti and Lake Chad as an im-portant source region, together with a large swathe ofcountry covering portions of Mauritania, Mali andsouthern Algeria. It also suggests that the Horn ofAfrica and the Nubian Desert in southern Egypt andNorthern Sudan are important sources. The impor-tance of the Bodele region was also shown by Kalu . .1979 and Herrmann et al. 1999 , but the status ofthe other regions is less clear.

    Another recently developed method to detect dustsource regions is the Total Ozone Mapping Spec-

    .trometer TOMS , which is currently carried on boardthe Earth Probe Satellite Middleton and Goudie, in

    .press . TOMS can detect UV-absorbing aerosols inthe atmosphere from the spectral contrast betweenthe 340- and 380-nm channels, and an Aerosol Index .AI has been developed which is linearly propor-

    tional to the aerosol optical thickness Herrmann et.al., 1999; Chiapello et al., 1999; Hsu et al., 1999 .

    AI values indicate the intensity of dust content, nottotal dust flux.

    .The TOMS data Fig. 1 confirm that the Bodele Region is the most intense source region not only inthe Sahara, but also in the world, with AI values thatexceed 30. It also demonstrates the presence of a

    .large but less intense area AI values over 24 in theWest Sahara. This extends through to the Atlanticcoast of Mauritania. Relatively high AI values arealso observed in the interior of Libya.

    The importance of Bodele as a dust source relates to various factors. Firstly the region is very dryFaya Largeau receiving an average annual rainfall

    .of just 17 mm , but is fed with silty alluvium bystreams draining from the Tibesti Massif. There mayalso be susceptible silty materials that were laiddown in an expanded Lake Chad during earlyHolocene and Pleistocene pluvials, and Mainguet

    .and Chemin 1990 have argued that deflational ac-tivity downwind from Tibesti may be substantial andhelp to explain the excavation of Lake Chad itself.The reasons for the importance of the West Saharandust source in Mali, Mauritania and Algeria are lesswell-understood. However, it is an area of low reliefbounded on the north and east by uplands. Whilesuch upland areas are not themselves major dustsource regions, wadis draining from them will have

  • ( )A.S. Goudie, N.J. MiddletonrEarth-Science Reiews 56 2001 179204182

    Fig. 1. Annual mean Aerosol Index for the Sahara, derived from TOMS.

    transported silt-rich alluvium into the area. Likewise,in the past, the southern part of the region may havereceived alluvial inputs from the Niger river prior toits capture by southeast-trending drainage near

    .Tosaye Urvoy, 1942 . It also contains an enormousclosed depression some 900 km long and variousergs that could provide a dust source through win-nowing. The depression contains many ancientlakebeds that show signs of intense deflation in the

    . .Holocene Petit-Maire, 1991 . Dubief 1953 maps itas an area of high aeolian activity, and it is also

    Table 2Estimates of the source strength of the Sahara

    .Author s Annual quantity .millions of tonnesryear

    .Jaenicke 1979 260 .Schutz et al. 1981 260

    .Prospero 1996a,b 170 .Swap et al. 1996 130460 .dAlmeida 1986 630710

    .Marticorena and Bergametti 1996 586665 .Callot et al. 2000 760

    rather dry, with annual precipitation levels of be-tween 5 and 100 mm.

    4. The strength of the Saharan dust source

    Various attempts have been made to estimate andmodel the source strength of the Sahara using dataon mineral loadings in the atmosphere, surface mate-

    .rial characteristics Callot et al., 2000 and transport .models Table 2 . The estimates show a wide range

    of values that may reflect differences in modelling

    Table 3Estimates of dust emissions to the atmosphere at a global scale

    .Author s Annual quantity .millions of tonnes

    .Schutz 1980 up to 5000 .Peterson and Junge 1971 500

    .Andreae 1995 1500 .Duce 1995 10002000

    .dAlmeida 1986 18002000 .Tegen and Fung 1994 3000

  • ( )A.S. Goudie, N.J. MiddletonrEarth-Science Reiews 56 2001 179204 183

    Table 4Maximum mean AI values for major global dust sources deter-mined from TOMSLocation

    Bodele Depression of Central Sahara )30 West Sahara in Mali and Mauritania )24

    .Arabia Southern OmanrSaudi border )21 .Eastern Sahara Libya )15 .Southwest Asia Makran coast )12

    TaklamakanrTarim basin )11 .Etosha Pan Namibia )11

    Lake Eyre Basin )11 .Mkgadikgadi Basin Botswana )8

    .Salar de Uyuni Bolivia )7Great Basin of the USA )5

    procedures, differences in the time scales considered,and differences in the areal extent of the source.

    There are few data available which allow a com-parison with other major source areas. An exception

    .to this is provided by the work of Zhang et al. 1997on the Taklamakan Desert. For this region, they

    estimate an annual dust production of 800 milliontonnes. On this basis, they propose that this may bearound half of the global production of dust.

    Estimates of soil dust emissions to the atmosphere . on a global scale Table 3 show a large range see

    .the excellent review by Prospero, 1996a , largelybecause of differences in the assumptions made indifferent models with regard to such factors as thera...

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