How, where and when is desert dust formed and where does it travel to?

Deserts generate dust, much of which travels great distances into non-desert areas, with diverse and often unexpected effects. Far-travelled dust particles are usually less than 2 micrometres (µm) in size, and are mostly made up of an aluminosilicate minerals. The major desert dust production mechanism is "saltation", a process triggered when larger wind-blown particles bounce on the desert soil's surface, thus releasing smaller dust particles from the surface. Dust is emitted from the Sahara, Arabian, Gobi, Taklimakan, Australian and South American deserts; but, quantitatively, most dust in the global atmosphere is emitted from the hyperarid northern African (50-70%) and Asian (10-25%) deserts. Frequent dust events are observed in enclosed depressions (Prospero and others 2002): from lake sediments deposited during wetter climate periods (like the Paleo-Lake Chad on the Saharan-Sahelian border, which contains the most active dust source on earth) or from the end-points of riverine transport of fine particles (like the Murray-Darling Basin in Australia). Global annual dust emissions are estimated to range from 1 000 to 3 000 million tonnes per year (IPCC 2001), less than 10 per cent of which is likely to result from human activities in the drylands (Tegen and others 2004).

Dust can be carried over thousands of kilometres by strong winds (Figures 3.1 and 3.2). Dust emitted in the Sahara can be carried across the North Atlantic to North and Central America, and even to the Amazon basin. Large amounts of Asian dust are carried over the North Pacific toward the mid- Pacific islands and North America. The lifetimes of atmospheric dust range from less than a few hours for particles larger than 10 µm, which are quickly removed by gravitational settling, to 10-15 days for submicron particles that are mostly removed by wet deposition (Jickells and others 2005).

Desert-generated dust affects productivity of land and ocean away from deserts

Dust generated in deserts adds essential nutrients to terrestrial and marine ecosystems away from deserts, such as phosphorus and silicon, which enhance growth in oceanic phytoplankton otherwise often limited by these minerals. Iron is a micronutrient whose shortage limits the uptake and assimilation of nitrogen, phosphorus and silicon. Enrichment by dust-carried iron can stimulate oceanic plankton growth, and therefore increase CO2 uptake in ocean regions, where iron is limiting. In nutrient-poor regions, dust-borne iron concentration in dust and are not easily soluble in water, the role of dust-borne iron in ocean productivity is not yet clear (Jickells and others 2005). Transported dust may also have negative oceanic effects: some authors argue that increased dust deposition in the western Atlantic over the past 25 years could have significantly contributed to coral reef decline by carrying bacterial or fungal spores (Shinn and others 2000). On the other hand, phosphate deposited by dust enhances forests of the south-eastern United States, and the Saharan dust deposited in the Amazon basin replenishes the phosphorus lost through the intense leaching caused by high rainfalls in this area (Okin and others 2004).

Desert dust affects atmospheric properties, rainfall, visibility, and health away from deserts

Depending on their size, distribution and refractive properties, dust particles in the atmosphere partly reflect and partly absorb incoming solar radiation (Sokolik and others 2001). Thus, dust blown away from deserts and over oceans increases the reflectance in an area in which the dark ocean surface would otherwise be absorbing radiation, and thus the atmosphere over the oceans is cooled. When desert dust reaches heights above 5 km, it absorbs and reflects back to space some of the solar radiation, and so warms the mid-troposphere (Kishcha and others 2003) at the expense of cooling the lowest levels. This generates a downward airflow that exacerbates desert conditions. The added dryness can lead to more desert dust, thus amplifying the initial effect. Desert dust particles can impair precipitation from potential rain clouds, and keep the desert drier, dustier and even less favourable to precipitation in a reinforcing feedback loop, which further increases dust generation by deserts and the likelihood of its transport to non-deserts. Far away from deserts, the transported dust may suppress precipitation from convective clouds by inhibiting the formation of raindrops (Rosenfeld and others 2001). Finally, desert-generated dust may reduce visibility to the point of seriously interfering with ground and air traffic away from deserts. Persistent dust storms also increase the incidence of respiratory diseases (Gyan and others 2005).

Desert dust and global climate change

In general, both climate change-induced increasing aridity of deserts and increasing wind speeds will increase overall dust emissions from deserts. In deserts where rainfall is predicted to decrease, concurrent loss of vegetation cover will allow more dust emissions to non-desert areas. In deserts where rainfall is predicted to increase, desert dust flux will be reduced, sustaining, in turn, wet conditions away from deserts (Lioubimtseva and Adams 2004). Yet, due to uncertainties, projections of dust emissions for the next 100 years range between a 60 per cent decrease to a 50 per cent increase in dust emissions (Mahowald and Luo 2003).