Greening the Sahara
by Alfred Burdett
To vegetate the Sahara one need only add water. The difficulty is to add enough water over a large enough area in a way that makes economic sense.
A number of approaches have been considered. One idea, originally proposed as the basis for a hydro-electric scheme, is to flood the Qattara Depression, an area of about 19,500 square kilometers in North Western Egypt that lies at depths to 133 meters below sea level. Water would be delivered by canal and tunnel from the Mediterranean Sea or the Nile River, the flow of water to match the rate of evaporation from the lake thus created.
Evaporation from this large body of water would generate sufficient precipitation downwind to support a band of vegetation comparable to that along Africa’s Mediterranean coast. The effect, however, would be regional, impacting only a small portion of the Sahara’s ten-million-square-kilometers.
A recent proposal for sequestering billions of tons of atmospheric carbon dioxide envisages desalinating seawater by reverse osmosis and pumping the fresh water inland to irrigate desert-spanning plantations of Eucalyptus and other heat tolerant tree species. Climate simulations indicate that the forest would lower the desert temperature by around 8C and increase rainfall by 700 to 1200 millimeters.
The drawback to such a scheme is the astronomical cost. To support even a sparse arid forest, a yearly supply of at least 750 mm of water would be required, or a minimum of 750,000 cubic meters per square kilometer (m3 km-2). With a current cost of US$0.80 m-3 the water alone would cost $2 trillion per year to irrigate the entire Sahara.
The energy for seawater desalination, at 1.5 kwh per m-3, would be over 120 TW or the output of more than 4800 nuclear reactors comparable in size to the four destroyed last year by a tsunami at the Fukushima Daichi plant in Japan.
Energy to lift the water from sea level to a mean height of approximately 200 m would approach that required for desalination, creating a total power requirement equal to three-quarters of present-day global generating capacity. At US$0.05 per kwh, the annual cost would be approximately $0.4 trillion. In addition would be the capital cost of the aquaducts, the pipes and the pumps to deliver the water to the place of use.
The Sahara Forest Project, though visually and architecturally exciting, is an even more hugely capital intensive approach to desert afforestation. Reliant on a greenhouse solar distillation system. A pilot project at an undisclosed cost is planned for a two square kilometer location in Jordan. Although the project may make sense as an intensive horticultural production system with some local environmental benefit, it does not, despite the name, offer an economically feasible approach to greening the world’s largest desert.
Here I propose a new approach to vegetating the Sahara, powered entirely by the sun, suitable for incremental implementation within the confines of any of the desert nations bordering either the Mediterranean Sea or the Atlantic Ocean, and capable of providing an acceptable return on the capital invested.
Each year, nature transports approximately 105 km3; of water from the ocean to the land by way of the atmosphere, the process entirely powered by the energy of the sun. Might it not then be possible to control the movement of packets of water-laden air and cause them to deliver their load of moisture where required?
To this end, I propose the use of rigid hot air balloons with a spherical geodesic frame, what Buckminster Fuller called a tensegrity spheres, as cloud containment structures (CCS). Fuller believed that the mass of a mile-wide geodesic sphere would be so slight compared to the mass of the air trapped within it that heating the air by as little as one degree above ambient would sufficient to make the structure buoyant in air.
A CCS would be floated on the sea adjacent to either the Atlantic or Mediterranean coast of the Sahara. At dawn, as temperature within the CCS rose relative to ambient air temperature due to the greenhouse effect, it would be ballasted with seawater to prevent it becoming airborne. At the same time, seawater would be sprayed into the sphere to humidify the air.
Mean, year-round solar radiation in the dry tropics averages 0.35 kw m2 on a horizontal surface. On a surface at right angles to the solar beam, which a sphere always presents, the mean year-round radiation intercepted per unit area will be 30 to 50% larger, or about 0.5 kw. With a diameter of 1.6 km, a CCS will have a cross section of almost exactly 8 km2 and will thus intercept 8 TW, or 96 TWh per day.
With a volume of just over 2 km3 the CCS contains 2.5 million tons (Mg) of air at 25 C. To raise the temperature of this mass of air by 1 C requires approximately, 0.6 TWh, so that the sun will warm the air within the CCS 20 C in about 1.5 hours, allowing for some convective heat loss at the surface.
At 45 C, the water content of air at saturation is about 65 g kg-1, or 105,000 Mg in 2 km3. If the initial water content of air in the CCS is 23 g kg-1 (i.e., saturated at 25 C), the energy required to saturate the air at 45 C is 66 Twh, or about 8 hours of sunshine.
Once fully charged with water vapor, the ballast water would be pumped out and the CCS allowed to rise into the air where it will drift with the breeze. Normally, during daylight hours there is a sea breeze, caused by a low pressure over the warm land surface, which causes an inflow of the denser and cooler air over the water. Thus the CCS would travel inland, until nightfall when it would be tethered as the air within cooled and the water vapor would condense to form mist, which could be seeded to produced rain, which would be delivered by sprinklers to the ground below. Having delivered its load, the CCS would be towed back to the coast, where vents would release the still relatively warm air within, thus allowing the sphere to settle once again on the ocean surface.