As mentioned in Section 1, dust blowing west from the Sahara provides iron, a trace micronutrient for phytoplankton in the N. Atlantic and Southern Ocean and phosphorous, an important nutrient for the Amazon. When the loading of dust is high, toxic “red tide” outbreaks can occur in the Gulf of Mexico due to increased growth of the food supply for a bacterium that supports growth of the red tide algae (163).
In general, desert dust from sand storms contributes to cloud condensation nuclei, thereby enhancing rainfall potential (116). It also contributes to reflecting some 1 Wm-2 of sunlight globally and 25 Wm-2 over the Arabian Gulf (123). However, the overall impact of all mineral dust including that due to anthropogenic and natural sources like deserts is still poorly understood (55).
So, while dust from sand storms plays a vital role in the global ecology as well as its weather, can enough land be covered so that the local transport is shut down, but enough dust covered land remains to produce the useful effects, i.e., the dust that is made airborne passes over the covered areas at high altitude, on its way to fertilize the oceans and make the rain without depositing on the covered areas? Similarly, can the coverage be accomplished without altering wind flow patterns that send the dust out over the ocean? Answers to these questions must be found before large-scale surface coverage is attempted.
4.4.6.2.3 Robotic Vacuum Cleaners
Experience with mirrored surfaces exposed to natural weathering shows that the concentration of small particles <5 microns tends to increase more rapidly than that of large particles, with those between 0.3-1 microns the most significant source of solar spectrum scattering and reflectivity reduction. The relative rate of accumulation of particles <5 microns increases with decreasing wind velocity (164).
Because washing of the plastic cover will be too labor intensive and costly, resistance to accumulation of dust should also be an important consideration. For example, in investigating high albedo roof coatings, it was found that the average reduction in the albedo of roofing materials after 1 year of soiling was 0.15, with little additional through 6 years (165). However, roofs are distinct structures from which accumulated soil and dust are periodically blown or washed away. Thus, the analogy with the GAEP many not apply.
Clearly it will not be possible for humans to keep thousands of square miles of covered surface clean of these small particles as well as large ones manually, i.e., maintaining an albedo of 0.8 in the face of periodic sand and dust storms. It is also unlikely that a material can be developed that will repel the dust. However, a material that reduces the electrostatic attraction of small particles might facilitate their removal. The best way to address the cleaning problem is to use automated or robotic vacuum cleaners or “geovacs.”
The ideal geovac would have the following features: self-recharging via solar cells; self-emptying, possibly into white plastic bags so as not to reduce albedo or release soil collected; equipped with reflectivity or albedo monitors which report to a control station when the albedo has degraded below an acceptable level or returned to that level; equipped with balloon tires to minimize damage to surface cover and tracking of soil; large enough to collect all surface contamination in a given area; positioning controlled by GPS technology and possibly tasking by distributed programming of otherwise autonomous units (166); and cost effective design.
