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Dust (Climatological Implications) - Summary
Atmospheric dust is known to perturb earth's radiation balance; but its climatic effects are extremely poorly quantified.  Why is that?  It is because, in the words of Sokolik (1999), climate models "rely heavily on oversimplified parameterizations" of many important dust-related phenomena, "while ignoring others," with the result that "the magnitude and even the sign [our italics] of dust net direct radiative forcing of climate remains unclear."

Why is this so?  According to Sokolik, there are a number of unanswered questions about airborne dust, including: (1) How does one quantify dust emission rates from both natural and anthropogenic (disturbed) sources with required levels of temporal and spatial resolution?  (2) How does one accurately determine the composition, size and shape of dust particles from ground-based and aircraft measurements?  (3) How does one adequately measure and model light absorption by mineral particles?  (4) How does one link the ever-evolving optical, chemical and physical properties of dust to its life cycle in the air?  (5) How does one model complex multi-layered aerosol stratification in the dust-laden atmosphere?  (6) How does one quantify airborne dust properties from satellite observations?

In discussing these questions, Sokolik makes some interesting observations, noting that: (1) what is currently known (or believed to be known) about dust emissions "is largely from micro-scale experiments and theoretical studies," (2) new global data sets are needed to provide "missing information" on input parameters (such as soil type, surface roughness and soil moisture) required to model dust emission rates, (3) improvements in methods used to determine some of these parameters are also "sorely needed," (4) how to adequately measure light absorption by mineral particles is still an "outstanding problem," and (5) it "remains unknown how well these measurements represent the light absorption by aerosol particles suspended in the atmosphere."

Considering these many problems, it is easy to understand why Sokolik says that "a challenge remains in relating dust climatology and the processes controlling the evolution of dust at all relevant spatial/temporal scales needed for chemistry and climate models," for until this challenge is met, we will but "see through a glass, darkly," especially when it comes to trying to discern the effects of airborne dust on earth's climate.

In consequence of - or perhaps in spite of - the murky status of the subject, the work goes on; and in tackling one of the chief challenges set forth by Sokolik, Vogelmann et al. (2003) reiterate the fact that "mineral aerosols have complex, highly varied optical properties that, for equal loadings, can cause differences in the surface IR flux [of] between 7 and 25 Wm-2 (Sokolik et al., 1998)," while at the same time acknowledging that "only a few large-scale climate models currently consider aerosol IR effects (e.g., Tegen et al., 1996; Jacobson, 2001) despite their potentially large forcing."

In an attempt to rectify this situation, Vogelmann et al. "use[d] high-resolution spectra to obtain the IR radiative forcing at the surface for aerosols encountered in the outflow from northeastern Asia," based on measurements made by the Marine-Atmospheric Emitted Radiance Interferometer aboard the NOAA Ship Ronald H. Brown during the Aerosol Characterization Experiment-Asia."  This work led them to conclude that "daytime surface IR forcings are often a few Wm-2 and can reach almost 10 Wm-2 for large aerosol loadings," which values, in their words, "are comparable to or larger than the 1 to 2 Wm-2 change in the globally averaged surface IR forcing caused by greenhouse gas increases since pre-industrial times."  And in a massive understatement of fact, Vogelmann et al. say that these results "highlight the importance of aerosol IR forcing which should be included in climate model simulations."

Another aspect of the dust-climate connection centers on the African Sahel, which has figured prominently in discussions of climate change ever since it began to experience extended drought conditions in the late 1960s and early 70s.  Initial studies of the drought attributed it to anthropogenic factors such as overgrazing of the region's fragile grasses, which tends to increase surface albedo, which was envisioned to reduce precipitation, resulting in a further reduction in the region's vegetative cover, and so on (Otterman, 1974; Charney, 1975).  This scenario, however, was 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 sea surface temperature.

Building on the insights provided by these latter investigations, Giannini et al. (2003) presented 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 suggested 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 concluded 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."

In a companion article, 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.  With respect to this subject, they state 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," and their findings demonstrate why: the IPCC assumptions are simply wrong, which is pretty much par for the course that the highly politicized organization has pursued from the beginning of its operations.

Clearly, much remains to be learned about the climatic impacts of dust on both a regional and global scale before anyone can place any confidence in the climatic projections of the IPCC.

Charney, J.G.  1975.  Dynamics of desert and drought in the Sahel.  Quarterly Journal of the Royal Meteorological Society 101: 193-202.

Folland, C.K., Palmer, T.N. and Parker, D.E.  1986.  Sahel rainfall and worldwide sea temperatures, 1901-85.  Nature 320: 602-607.

Giannini, A., Saravanan, R. and Chang, P.  2003.  Oceanic forcing of Sahel rainfall on interannual to interdecadal time scales.  Science 302: 1027-1030.

Houghton, J.T., Ding, Y., Griggs, D.J., Noguer, M., van der Linden, P.J., Xiaosu, D., Maskell, K. and Johnson, C.A.  (Eds.).  2001.  Climate Change 2001: The Scientific Basis.  Cambridge University Press, Cambridge, UK.  (Contribution of Working Group 1 to the Third Assessment Report of the Intergovernmental Panel on Climate Change.)

Idso, S.B.  1977.  A note on some recently proposed mechanisms of genesis of deserts.  Quarterly Journal of the Royal Meteorological Society 103: 369-370.

Jackson, R.D. and Idso, S.B.  1975.  Surface albedo and desertification.  Science 189: 1012-1013.

Jacobson, M.Z.  2001.  Global direct radiative forcing due to multicomponent anthropogenic and natural aerosols.  Journal of Geophysical Research 106: 1551-1568.

Lamb, P.J.  1978.  Large-scale tropical Atlantic surface circulation patterns associated with sub-Saharan weather anomalies.  Tellus 30: 240-251.

Otterman, J.  1974.  Baring high-albedo soils by overgrazing: a hypothesized desertification mechanism.  Science 186: 531-533.

Prospero, J.M. and Lamb, P.J.  2003.  African droughts and dust transport to the Caribbean: climate change implications.  Science 302: 1024-1027.

Sokolik, I.N.  1999.  Challenges add up in quantifying radiative impact of mineral dust.  EOS: Transactions, American Geophysical Union 80: 578.

Sokolik, I.N., Toon, O.B. and Bergstrom, R.W.  1998.  Modeling the radiative characteristics of airborne mineral aerosols at infrared wavelengths.  Journal of Geophysical Research 103: 8813-8826.

Tegen, I., Lacis, A.A. and Fung, I.  1996.  The influence on climate forcing of mineral aerosols from disturbed soils.  Nature 380: 419-422.

Vogelmann, A.M., Flatau, P.J., Szczodrak, M., Markowicz, K.M. and Minnett, P.J.  2003.  Geophysical Research Letters 30: 10.1029/2002GL016829.