How extensive are the movements of windblown dust around the world? What are some of their biological ramifications? And how are these impacts likely to be affected by the ongoing rise in the air's CO2 concentration?
Some insight into the first and second of these questions is provided by Shinn et al. (2004), who review what is known about the long-range transport of windborne dust throughout the world and the biological characters that hitch a ride on it. In the first instance, they report 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 large Asian dust storms, for example, they say that "an estimated 4,000 metric tons of soil per hour can affect the Arctic environment (Rahn et al., 1977)," and they state that "Perry et al. (1997) found African dust as far north as Maine and as far west as Carlsbad, New Mexico," adding that approximately half of the dust collected there had come from Africa.
Typically riding on this dust are a number of biological entities, including, in Shinn et al.'s words, "many species of fungi (commonly known as molds) and bacteria - including some that are human pathogens." In the case of African dust collected in the Caribbean, they report that Griffin et al. (2001) found that "approximately 30% of the microbes cultured and identified thus far are capable of causing disease in plants and animals, and 10% are opportunistic human pathogens." Hence, they rightly conclude that "atmospheric exposure to mold-carrying desert dust may affect human health directly through allergic induction of respiratory stress," and that "mold spores within these dust clouds may seed downwind ecosystems in both outdoor and indoor environments.
Many other scientists have come to the same conclusion. Griffin et al. (2002), for example, state that by some estimates, "as much as two billion metric tons of dust are lifted into the earth's atmosphere every year," and that riding along on these particles are "pollutants such as herbicides and pesticides and a significant number of microorganisms - bacteria, viruses and fungi." In fact, the scientists calculate there are easily enough bacteria thus moved about the planet each year "to form a microbial bridge between Earth and Jupiter."
But does dust from Africa and Asia really go that far? Well, it may not traverse interplanetary space; but it does cross both the Atlantic and Pacific Oceans. Griffin et al. report 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." Likewise, Prospero (2001) tells us that everyone in the United States living east of the Mississippi River is affected by dust of African origin.
Asian dust also travels immense distances. In April of 2001, for example, Griffin et al. report that 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."
Many of the biological entities associated with the dust particles that are thus dispersed about the planet have serious consequences for plants, animals and humans. Airborne fungi from Africa that frequently make their way to the Americas, for example, cause sugar cane rust, coffee rust and banana leaf spot. Griffin et al. describe how the scourge of Caribbean sea fans - Aspergillus sydowii - "is also found in the Caribbean atmosphere during African dust events," noting that the region's "sea fans and other coral reef organisms have experienced a steady decline since the late 1970s," when worsening drought in Africa predisposed increasing amounts of soil there to wind erosion (Prospero, 2001). The scientists also say they expect "future research will show that many other coral diseases are spread by dust from both Africa and Asia."
With respect to human health, Griffin et al. note that "African dust is reported to be a vector for the meningococcal meningitis pathogen Neisseria meningitis in sub-Saharan Africa," and that outbreaks of the disease "often follow localized or regional dust events, and these typically result in many fatalities." They also report there has been a 17-fold increase in the incidence of asthma on the island of Barbados since 1973, "which corresponds to the period when the quantities of African dust in the region started to increase."
Because the dust clouds that reach the Americas from Africa and Asia have traveled such long distances, most of the larger particles they originally contained have generally fallen out of them along the way. The particles that remain, therefore, are typically very small, so small, in fact, that Griffin et al. report that "once they are inhaled into the lungs they cannot be exhaled." What makes this situation especially serious is that the tiny dust particles typically are heavily coated with iron; and a substantial fraction of that iron is released to the lung tissue when the particles are deposited there. And iron, as Prospero notes, is "particularly efficient in producing an inflammatory response in the lungs."
Grousset et al. (2003) also describe the vast distances sometimes traversed by windblown dust. Working with two sets of dust samples collected in the French Alps, they (1) analyzed the mineralogical and geochemical composition of the dust particles, including the isotopic composition of the neodymium contained in the minerals, (2) reconstructed airmass backward trajectories from archived meteorological data, including corroboration by satellite imagery, and (3) 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."
Also focusing upon the transport of dust from Eurasia across the Pacific Ocean to North America, Wilkening et al. (2000) bluntly state that "the once-pristine air above the North Pacific Ocean is polluted," and they go on list several potential 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 both population and industrialization over the past century or so.
One possible consequence of this phenomenon is that the weakened ability of corals to withstand episodes of intense regional warming may be linked to the enhanced delivery of various substances that impair their natural abilities to withstand such environmental stresses. This subject was treated in detail by Shinn et al. (2000), who recount the general history of coral reef decline throughout the Caribbean over the past quarter century. They note, in this regard, that "coincidental with the decline of Caribbean coral reefs over the past 25 years there has been a sharp increase in the transport of African dust to the western Atlantic." Of particular note is their statement that "Caribbean-wide mortalities of acroporid corals and … coral bleaching beginning in 1987, correlate with the years of maximum dust flux into the Caribbean." They thus put forward the hypothesis that the influx of dust has been responsible for much of the declining health of Caribbean corals, including that manifest as coral bleaching, over the past quarter century.
In support of their hypothesis, Shinn et al. note that atmospheric dust "can serve as a substrate for numerous species of viable spores, especially the soil fungus … Aspergillus sydowii, the cause of an ongoing Caribbean-wide seafan disease." They further note that this fungus has been cultured from air samples taken during dustfalls in the Virgin Islands, but that spores of the fungus are absent when the air is clear. It is their contention that these observations and experiments "provide a reasonable explanation for the near synchronous widespread distribution of [coral disease] outbreaks around remote oceanic islands in the Caribbean," and they suggest that "a possible link between zooxanthellae expulsion, elevated water temperature, and dust delivery of pathogens and nutrients needs to be investigated," especially in light of the fact that "hundreds of millions of tons/year of soil dust … have been crossing the Atlantic during the last 25 years."
People, as noted earlier, are also negatively affected by the many dangerous entities that travel with intercontinental dust clouds, as has been demonstrated in numerous studies. Kwon et al. (2002), for example, explored the effects of 28 dust clouds originating from the arid deserts of Mongolia and China on daily mortality in Seoul, South Korea, over the period 1995-1998, evaluating the association between daily death counts and dust events using Poisson regression analysis adjusted for time trends, weather variables and day-of-week. Their work revealed that the estimated percentage increase in the rate of deaths due to Asian dust cloud exposure was 1.7% for all people due to all causes, 2.2% for deaths of persons aged 65 years and older due to all causes, and 4.1% among all people for deaths due to cardiovascular and respiratory causes. These findings led them to conclude that "persons with advanced cardiovascular and respiratory disease may be susceptible to the Asian dust events."
In another pertinent paper, Prospero (2001) says "reports from Caribbean islands show that emergency room visits for asthma and other respiratory illnesses increase markedly during African dust events," while Griffin et al. (2002), as noted earlier in this Summary, report there has been a 17-fold increase in the incidence of asthma on the island of Barbados since 1973 that "corresponds to the period when the quantities of African dust in the region started to increase."
Also highlighting the dangers of airborne dust were Wu et al. (2004), who collected and identified airborne fungal spores on a continuous basis between December 2000 and April 2001 in Tainan City, southern Taiwan, and then went on to compare the results for all days classified by the Central Weather Bureau of Taiwan as "yellow sandstorm" days (which derive their burdens of dust and assorted biological materials from the deserts of China) with the results for each day that preceded and followed the sandstorm days. They report that Cladosporium was the most predominant fungal spore collected, comprising more than 60% of total fungal spores; and they note that "many species within this genus are pathogens to a variety of plants and animals (Banerjee et al., 2002; de Wit and Joosten, 1999; Mariani et al., 2002; Weber, 2002)." Also frequently present were Basidiospores, which they say "are allergenic," citing Horner et al. (1993a,b), Lehrer et al. (1994) and O'Neil et al. (1998) as authorities for this statement.
Penicillium and Aspergillus spores were likewise regularly detected; and Wu et al. state that they "have long been considered to be associated with asthma and other allergic disorders in different regions of the world (Dill and Niggemann, 1996; Garrett et al., 1998; Horner et al., 1995." Most important of all, they say that during sandstorm days, atmospheric concentrations of these spores, together with many rare spores, such as Nigospora, Arthrinium, Curvularia, Rusts, Stemphylium, Cercospora and Pithomyces, "were 2-12 times higher than those observed in normal days." Hence, they conclude that their data "support strongly [the hypothesis] that Asian sandstorm events may alter fungal spore compositions in atmospheric environments, and the change might have potential impacts on human health and ecosystem[s]," which consequences, of course, would be detrimental across the board, i.e., to plants, animals and humans.
Having thus described the extent and seriousness of the windblown dust phenomenon for both nature and humanity, we come to the last of the three questions posed at the start of this Summary: How are the deleterious biological impacts of airborne dust likely to be affected by the ongoing rise in the atmosphere's CO2 concentration?
In broaching this subject, we begin with the study of Engelstaedter et al. (2003), who used dust storm frequency (DSF) data -- obtained from 2405 meteorological stations represented in the International Station Meteorological Climate Summary -- as a surrogate measure of dust emissions to explicitly test the assumption that vegetation is an important control of dust emission at the global scale. To represent vegetation cover, the researchers 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 tends to increase the vegetative cover of the ground will tend to reduce the severity of dust emissions therefrom and their subsequent transport to various parts of the world.
So what will increasing atmospheric CO2 concentrations do in this regard? First of all, the well-documented increase in plant water use efficiency that results from increases in atmospheric CO2 concentration [see Water Use Efficiency (Grass Species) in our Subject Index] should allow more plants to grow in the arid source regions of earth's dust clouds, which should help to both stabilize and shield the soil and decrease its susceptibility to wind erosion, thereby reducing the amounts of dust transported by these events. 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 CO2 in the atmosphere 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. Last of all, as noted by Zavaleta et al. (2003), global warming itself may increase soil moisture contents in water-limited regions by hastening plant senescence and thereby reducing the period of time over which transpiration-driven soil water losses occur.
All in all, therefore, if the air's CO2 content continues to rise, even in the face of further warming, we should see a gradual reduction in the intensity of global dust events, along with a muting of the multitude of adverse biological consequences that typically accompany them.
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