Volume 7, Number 14: 7 April 2004
The African Sahel has figured prominently in discussions of climate change ever since it began to experience extended drought conditions in the late 1960s to early 1970s, which conditions have only intensified over the past two decades. Initial studies of the situation attributed the drought to anthropogenic factors such as overgrazing of the region's fragile grasses that tend to increase surface albedo, which was envisioned to reduce precipitation that, in turn, would further reduce the region's vegetative cover, and so forth (Otterman, 1974; Charney, 1975). This scenario was subsequently challenged by Jackson and Idso (1975) and Idso (1977) on the basis of empirical observations; while Lamb (1978) and Folland et al. (1986) attributed the drought to large-scale atmospheric circulation changes triggered by multidecadal variations in global sea surface temperature. With the subject again coming to the fore, new information is substantially tilting the scales in favor of the latter view.
Giannini et al. (2003) present evidence based on an ensemble of integrations with a general circulation model of the atmosphere -- forced only by the observed record of sea surface temperature -- which suggests that the "variability of rainfall in the Sahel results from the response of the African summer monsoon to oceanic forcing amplified by land-atmosphere interaction." The success of this analysis led them to conclude that "the recent drying trend in the semi-arid Sahel is attributed to warmer-than-average low-latitude waters around Africa, which, by favoring the establishment of deep convection over the ocean, weaken the continental convergence associated with the monsoon and engender widespread drought from Senegal to Ethiopia." Hence, they further conclude that "the secular change in Sahel rainfall during the past century was not a direct consequence of regional environmental change, anthropogenic in nature or otherwise."
With that subject tending towards a reasonable consensus settlement, we can now turn our attention to the physical nature and consequences of the most recent long siege of drought in the Sahel, which is what Prospero and Lamb (2003) do in a companion article, wherein they 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. With respect to this subject, they note that the Intergovernmental Panel on Climate Change (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 they say "there is little firm evidence to support either of these assumptions," and their findings demonstrate why: the IPCC assumptions are just plain wrong.
With respect to the consequences of African dust transport to the Americas, Prospero and Lamb say that "during intense drought phases, the concentration of respirable dust over the Caribbean probably exceeds the U.S. Environmental Protection Agency's 24-hour standard," but they immediately add "there is no evidence that exposure to dust across this region presents a health problem."
This comment is surprising, to say the least, in light of the prior paper of Prospero (2001), wherein he explicitly states that "reports from Caribbean islands show that emergency room visits for asthma and other respiratory illnesses increase markedly during African dust events." It also stands in stark contrast to the statement of Griffin et al. (2002) that there has been a 17-fold increase in the incidence of asthma on the island of Barbados since 1973, which they say "corresponds to the period when the quantities of African dust in the region started to increase."
Prospero (2001) also informs us that "the average iron content of all dust particles from Africa is 3 to 5 percent." Why is this important? Because, as he continues, "a substantial fraction of the iron on dust could be quickly released into the lungs once the particles are deposited on lung tissue," and iron, as he notes, is "particularly efficient in producing an inflammatory response in the lungs." Furthermore, because dust of African origin is carried such long distances, what remains when it reaches the Americas is usually of such small size that about half of the particles can be deeply inhaled.
Prospero (2001) additionally notes that substantial numbers of pathogenic fungi, bacteria and viruses capable of infecting humans hitch a ride on the African dust particles and survive the trans-Atlantic trip. And from the Caribbean islands, the dust goes on to visit -- and provide the potential to infect -- nearly everyone in the United States living east of the Mississippi River. Last of all, the study of Shinn et al. (2000), of which Prospero is a coauthor, suggests that the influx of African dust has been partially to blame for much of the declining health of Caribbean corals, including that manifest as coral bleaching, over the past quarter century.
We recount this additional information to indicate that the seriousness of the situation is much greater than what is implied in the final sentence of Prospero and Lamb's Science paper. We also do it to indicate a unique health benefit that the affected regions may reap from the ongoing rise in the air's CO2 content. Specifically, we note that with the CO2-conferred ability to produce more biomass with less water [see Water Use Efficiency in our Subject Index], plants of the future will be able to grow and reproduce over large areas of the earth where it has previously been too dry for them to even exist; and these areas are the very regions that serve as the planet's major sources of windblown dust. Hence, with greater vegetative coverage of these regions protecting their surfaces from the erosive power of the wind, there should be much less dust wafting about in the air with much fewer pathogenic fungi, bacteria and viruses being carried around the world on their surfaces.
| Sherwood, Keith and Craig Idso |
References
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