Shinn et al. (2004) review what is known about the long-range transport of windborne soil particulates, focusing on dust of African and Asian origin.  They begin by noting that "dust from the African desert can affect air quality in Africa, Europe, the Middle East, and the Americas," and that "Asian desert dust can affect air quality in Asia, the Arctic, North America, and Europe."  With respect to dust originating in Asia, they say that "an estimated 4,000 metric tons of soil per hour can affect the Arctic environment (Rahn et al., 1977)."  As for dust originating in Africa, they say that "Perry et al. (1997) found African dust as far north as Maine and as far west as Carlsbad, New Mexico" in the United States, adding that approximately half of the dust collected at the latter location had come from Africa.

More information about the planet's burden of atmospheric dust is provided by Griffin et al. (2002), who write that "as much as two billion metric tons of dust are lifted into the earth's atmosphere every year," and that soil particulates from Africa and Asia cross both the Atlantic and Pacific Oceans.  As a result of the trans-Pacific transport, Wilkening et al. (2000) state that "the once-pristine air above the North Pacific Ocean is polluted," and they go on list several implications for a number of terrestrial and oceanic ecosystems.  They also note that we can expect these impacts to increase with economic expansion around the world; and, by analogy, we can infer that such impacts have likely grown in tandem with population and industrialization over the past century or so.  With respect to trans-Atlantic transport, Griffin et al. remark that dust storms originating in North Africa "routinely affect the air quality in Europe and the Middle East," and that millions of tons of African sediment "fall on the North Amazon Basin of South America every year."

Also writing on the subject of trans-Atlantic dust transport, Prospero (2001) suggests that nearly everyone in the United States living east of the Mississippi River is affected by dust of African origin.  Likewise, Prospero and Lamb (2003) report that measurements made from 1965 to 1998 in the Barbados trade winds show large interannual changes in the concentration of dust of African origin that are highly anticorrelated with the prior year's rainfall in the Soudano-Sahel, and they note that the IPCC report of Houghton et al. (2001) "assumes that natural dust sources have been effectively constant over the past several hundred years and that all variability is attributable to human land-use impacts."  Of this statement, however, they say "there is little firm evidence to support either of these assumptions."

As for some specific examples of long-range Asian dust transport, Griffin et al. report that in April of 2001 a large dust cloud that originated over the Gobi Desert of China "moved eastward across the globe, crossing Korea, Japan, the Pacific (in five days), North America (causing sporadic reports of poor air quality in the United States), the Atlantic Ocean and then Europe."  Working with dust samples subsequently collected in the French Alps, Grousset et al. (2003) analyzed their mineralogical and geochemical composition, including the isotopic composition of the neodymium contained in the minerals, after which they (1) reconstructed airmass backward trajectories from archived meteorological data, including corroboration by satellite imagery, and (2) used a global transport model driven by assimilated meteorology to simulate dust deflation and long-range transport.  These efforts revealed that one of the sets of dust samples came from North Africa, while the second set originated in the Takla-Makan desert of China.  Their work additionally suggested that the latter set of dust particles had traveled "more than 20,000 km in about two weeks, and along their journey, crossed China, the North Pacific, North America and then the North Atlantic Ocean," which knowledge, in their words, "is important from the viewpoint of understanding the dust itself," as well as "the heavy metal, fungal, bacterial and viral pollution that may be associated with it."

In the search for dust storm trigger mechanisms, Liu et al. (2004) analyzed trends in spring dust storm frequency for western and southwestern China-Mongolia for the period 1952-2003, finding both interannual and interdecadal trends throughout the 52-year period.  By decade, the number of spring dust storms varied from 21 in the 1950s, to 44 in the 1960s, to a high of 60 in the 1970s, then back down to 35 in the 1980s, and to a low of 25 in the 1990s.  In addition, they determined that strong and cold Siberian air masses enhance dust storm numbers, while weaker and warmer Siberian air masses lower them.  Hence, if further warming of the globe increases temperatures in the northern part of China and Mongolia, Liu et al. say that "the China-Mongolia ridge will continue to rise and suppress Mongolian cyclones and dust storm activities in Western China-Mongolia."

Taking a much longer look at the subject, Lim et al. (2005) examined the eolian quartz content (EQC) of a high-resolution sedimentary core taken from Cheju Island, Korea, from which they produced a proxy record of major Asian dust events that reached the region over the past 6500 years.  This analysis indicated that the EQC was relatively low from 6500-4000 years BP, high between 4000-2000 years BP, and low again from 2000 years BP to the present, with the most recent 1500 years BP being lower in EQC than any previous time in the record.  The Asian dust time series was also found to contain significant millennial- and centennial-scale periodicities.  Cross-spectral analysis between the EQC and proxy solar activity record showed significant coherent cycles at 700, 280, 210 and 137 years with nearly the same phase changes, leading the three researchers to conclude that the centennial-scale periodicities in the EQC can be ascribed primarily to short-term fluctuations in solar activity.

With the intent to determine what might help to reduce the particulate burdens of dust storms, Engelstaedter et al. (2003) used dust storm frequency (DSF) data from 2405 stations represented in the International Station Meteorological Climate Summary as a surrogate measure of dust emissions to test the assumption that vegetation is an important control of dust emission at the global scale.  To represent vegetation cover, they used two independent data sets: a satellite-derived distribution of actual vegetation types and a model-derived distribution of potential natural vegetation.  Employing these tools, they learned that "the highest DSFs are found in areas mapped by DeFries and Townshend (1994) as bare ground," while "moderate DSFs occur in regions with more vegetation, i.e., shrubs & bare ground, and lowest DSFs occur in grasslands, forests, and tundra," where ground cover is highest.  Hence, they concluded that "average DSF is inversely correlated with leaf area index (an index of vegetation density) and net primary productivity," which suggests that whatever increases the vegetative cover of the ground should reduce the severity of dust emissions from the soil beneath, as well as the dust's subsequent transport to various parts of the world.

What will rising atmospheric CO2 concentrations do in this regard?  First of all, the well-documented increase in plant water use efficiency that results from increases in the air's CO2 content (see Water Use Efficiency in our Subject Index) should allow more plants to grow in the arid source regions of earth's dust clouds, which should help to stabilize and shield the soil, decreasing its susceptibility to wind erosion and thereby reducing the amounts of dust made airborne and transported by globe-girdling winds.  Second, the propensity for elevated CO2 concentrations to increase soil moisture content as a consequence of CO2-induced reductions in plant transpiration (see Water Status of Soil (Field Studies) in our Subject Index) should do likewise.  Third, the ability of extra atmospheric CO2 to enhance the growth of cryptobiotic soil crusts (see Deserts (Algae and Lichens) in our Subject Index) should directly stabilize the surface of the soil, even in the absence of higher plants.

In view of these thoughts and observations, it is enlightening to conclude this summary with a review of the findings of Piao et al. (2005), who used a "time series data set of Normalized Difference Vegetation Index (NDVI) obtained from the Advanced Very High Resolution Radiometer available from 1982 to 1999 (Tucker et al., 2001; Zhou et al., 2001), and precipitation and temperature data sets, to investigate variations of desert area in China by identifying the climatic boundaries of arid area and semiarid area, and changes in NDVI in these areas."  In doing so, they found that "average rainy season NDVI in arid and semiarid regions both increased significantly during the period 1982-1999."  Specifically, they found that the NDVI increased for 72.3% of total arid regions and for 88.2% of total semiarid regions, such that the area of arid regions decreased by 6.9% and the area of semiarid regions decreased by 7.9%.  They also report that by analyzing Thematic Mapper satellite images, "Zhang et al. (2003) documented that the process of desertification in the Yulin area, Shannxi Province showed a decreased trend between 1987 and 1999," and that "according to the national monitoring data on desertification in western China (Shi, 2003), the annual desertification rate decreased from 1.2% in the 1950s to -0.2% at present."

Noting that "variations in the vegetation coverage of these regions partly affect the frequency of sand-dust storm occurrence (Zou and Zhai, 2004)," Piao et al. concluded that "increased vegetation coverage in these areas will likely fix soil, enhance its anti-wind-erosion ability, reduce the possibility of released dust, and consequently cause a mitigation of sand-dust storms."  Interestingly, in this regard, they report that "recent studies have suggested that the frequencies of strong and extremely strong sand-dust storms in northern China have significantly declined from the early 1980s to the end of the 1990s (Qian et al., 2002; Zhao et al., 2004)."  It would appear, therefore, that the dreaded climate changes claimed by climate alarmists to have been experienced by the globe over the latter part of the 20th century were either (1) not so dreaded after all or (2) totally dwarfed by opposing phenomena that significantly benefited China, as its lands grew ever greener during this period and its increased vegetative cover helped to stabilize its soils and throw feared desertification into reverse, resulting in a decrease in the particulate burden of their sand and dust storms.

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