7.5.1.1 Dust

Estimates of the global source strength of bulk dust aerosols with diameters below 10 µm of between 1,000 and 3,000 Tg yr^–1 agree well with a wide range of observations (Duce, 1995; Textor et al., 2005; Cakmur et al., 2006). Seven to twenty percent of the dust emissions are less than 1 µm in diameter (Cakmur et al., 2006; Schulz et al., 1998). Zhang et al. (1997) estimated that about 800 Tg yr^–1 of Asian dust emissions are injected into the atmosphere annually, about 30% of which is re-deposited onto the deserts and 20% is transported over regional scales, while the remaining approximately 50% is subject to long-range transport to the Pacific Ocean and beyond. Asian dust appears to be a continuous source that dominates background dust aerosol concentrations on the west coast of the USA (Duce, 1995; Perry et al., 2004). Uncertainties in the estimates of global dust emissions are greater than a factor of two (Zender et al., 2004) due to problems in validating and modelling the global emissions. The representation of the high wind tail of the wind speed distribution alone, responsible for most of the dust flux, leads to differences in emissions of more then 30% (Timmreck and Schulz, 2004). Observations suggest that annual mean African dust may have varied by a factor of four during 1960 to 2000 (Prospero and Lamb, 2003), possibly due to rainfall variability in the Sahel zone. Likewise, simulations of dust emissions in 2100 are highly uncertain, ranging from a 60% decrease to a factor of 3.8 increase as compared to present-day dust emissions (Mahowald and Luo, 2003; Tegen et al., 2004; Woodward et al., 2005; Stier et al., 2006a). Reasons for these discrepancies include different treatments of climate-biosphere interactions and the climate model used to drive the vegetation and dust models. The potentially large impact of climate change on dust emissions shows up in particular when comparing present-day with LGM conditions for dust erosion (e.g., Werner et al., 2002).

The radiative effect of dust, which, for example, could intensify the African Easterly Waves, may be a feedback mechanism between climate and dust (Jones et al., 2004). It also alters the atmospheric circulation, which feeds back to dust emission from natural sources (see Section 7.5.4). Perlwitz et al. (2001) estimate that this feedback reduces the global dust load by roughly 15%, as dust radiative forcing reduces the downward mixing of momentum within the planetary boundary layer, the surface wind speed, and thus dust emission (Miller et al., 2004a). In addition to natural dust production, human activities have created another potential source for dust mobilisation through desertification. The contribution to global dust emission of desertification through human activities is uncertain: estimates vary from 50% (Tegen et al., 1996; Mahowald et al., 2004) to less than 10% (Tegen et al., 2004) to insignificant values (Ginoux et al., 2001; Prospero et al., 2002). A 43-year estimate of Asian dust emissions reveals that meteorology and climate have a greater influence on Asian dust emissions and associated Asian dust storm occurrences than does desertification (Figure 7.19; Zhang et al., 2003).

Figure 7.19. (a) Chinese desert distributions from 1960 to 1979 and desert plus desertification areas from 1980 to 1999. (b) Sources (S1 to S10) and typical depositional areas (D1 and D2) for Asian dust indicated by spring average dust emission flux (kg km^–2 per month) averaged over 1960 to 2002. The percentages with standard deviations in parentheses denote the average amount of dust production in each source region and the total amount of emissions between 1960 and 2002. The deserts in Mongolia (S2) and in western (S4) and northern (S6) China (mainly the Taklimakan and Badain Juran, respectively) can be considered the major sources of Asian dust emissions. Several areas with more expansions of deserts (S7, S8, S9 and S5) are not key sources. Adapted from Zhang et al. (2003).

In addition, aerosol deposition affects global ecosystems. Deposition of mineral dust plays an important role in the biogeochemical cycle of the oceans, by providing the nutrient iron, which affects ocean biogeochemistry with feedbacks to climate and dust production (Jickells et al., 2005; Section 7.3.4.4). Conversely, water-soluble particulate iron over the Pacific Ocean is linked to elemental carbon emissions resulting from anthropogenic activity in Asia (Chuang et al., 2005). The input of trace elements by dust deposition is also of importance to terrestrial ecosystems. For example, it has been proposed that the vegetation of the Amazon basin is highly dependent on Saharan dust deposition, which provides phosphorus, necessary for maintenance of long-term productivity (Okin et al., 2004; Section 7.3). The Hawaiian Islands also depend on phosphorus from Asian dust transport (Chadwick et al., 1999). Moreover, mineral dust can act as a sink for acidic trace gases, such as sulphur dioxide (SO₂) and HNO₃, and thereby interact with the sulphur and N cycles (e.g., Dentener et al., 1996; Umann et al., 2005). Coatings with soluble substances, such as sulphate or nitrate, will change the ability of mineral dust aerosols to nucleate cloud droplets (Levin et al., 1996; Section 7.5.2.1).