In the Carbyon analysis, it became clear that the physical/chemical solution for harvesting CO2 from air is too energy intensive to achieve the goal in a reasonable time. The main problem is the availability of electricity; there simply isn’t enough capacity at present, and increasing it enough would take too much time. There are probably other capacity problems, but these weren’t investigated.
Perhaps, there is a faster alternative. The big deserts may support plants if it is possible, somehow, to get water there. This would probably not work in the areas of shifting sand dunes, but the Sahara desert has large flat surfaces that can support plants, as is demonstrated after rare rain episodes.
But, although it may be possible to change the local climate to get more rains, this seems very uncertain, and a more controllable way to supply water might be preferred.
At Reden, we like to see numbers to support ideas. In this case, we bring forward an idea and check to see if it is promising. Sometimes it is the best to kill your own idea before spending any more effort. So let's take a look at it:
Is it possible to desalinate and pump seawater to the Sahara (preferably using solar energy), sell the salt, or put it back into the sea? And most importantly, is it possible to plant fast growing plants, either trees to store CO2 for a long time, or sugar cane to make renewable fuels (ethanol in particular, as is done in Brasil)?
The big advantage of plants over solar cells is that they grow autonomously and do not require expensive materials or costly processing. Moreover, they do not produce waste that is not biodegradable.
The first problem with getting water to the Sahara is obvious: it is (mostly) above sea level, as is shown on the map below.
I do not know how much water is needed. This is probably not too difficult to find out, but for the order of magnitude we could use the annual rainfall in a country where agriculture is not vulnerable. The annual rainfall in the Netherlands is 847 mm, but this is probably more than needed, as much rain water is discarded into the sea. The map below shows the annual rainfall for the globe, and it suggests that 25 cm (250 mm) is a reasonable estimate for the amount of water needed. This is 0.25 m3/m2. At first, the desert may require more water, as it has been dry for thousands of years.
The plants can be used in two ways, either for supplying green fuel, or for keeping carbon out of the atmosphere after it has been absorbed by the plants.
Planting to produce green fuel
The excellent book 'Sustainable Energy- without the hot air' by David J.C. MacKay gives numbers for the energy production by sugarcane in Brazil: 1.25 W/m2. For the Sahara desert (which is not tropical, after all), this number could be lower, say 0.8 W/m2 or (0.8x8766 hr/yr =) 7.0 kWh/(m2year). Pumping water 350 m up from sea level at 0.25 m3/m2 would cost at least (3.5*106x0.25 = 875 kJ/m2 =) 0.24 kWh/m2. This is only when we account for the pressure head, hence ignoring pipe pressure losses and pump efficiency. Desalination of water using reverse osmosis costs 5.5kWh/m3 (Wikipedia), so this would add 1.375 kWh/m2. Desalination would be done at sea level, of course. It is also necessary to drain the irrigated area to prevent accumulation of salt. Draining should be achievable by gravity.
So far, the balance is still positive: 7.0 – 0.24 – 1.375 = 5.4 kWh/m2 per year! If we round this off to 5 kWh/(m2year), and plant a surface area of 500,000 km2 (roughly the size of Spain), then the net energy harvest would be 2.5*1012 kWh/year, whereas the total energy consumption is currently 1.6*1014 kWh/year . This is 64 times more. The whole Sahara covers an area of 9,200,000 km2 (Wikipedia), or 18.4 times more than our Spain-sized example. Even if the whole Sahara would be planted, it would be able to supply only 28% of the current energy consumption. Remember that the rough calculation is just an estimate, so it could be more, or less.
If we also plant other large deserts, let's say the nr. 3 to nr. 10 of the list of largest deserts (Wikipedia), the area would be just under twice the area of the Sahara. This would contribute very significantly to the global energy demand.
At present, this is as far as the quick analysis brings us. The numbers show that the idea may be a good one. The numbers used are open to discussion, so the idea may work better than the quick analysis suggests, or worse. A closer look would be good!
Planting to store CO2
What about CO2-storage? We started by wanting to extract CO2 from the atmosphere. A quick glance at the Wikipedia page on Carbon Sequestration shows that this is a complex area, but we need a number to get the order of magnitude right. The figure is from a study on a tropical location (12° 41′3.15″ S, 69° 36′ 47.76″ W) at an altitude of 266 m above sea level). This is not directly comparable to the Sahara, so the numbers may be off a little. The part of the trees above ground would hold an amount of roughly 60 ton of carbon per hectare after 27 years, corresponding to 44/12*60= 220 ton of CO2 per 0.01 km2, or 22000 ton CO2 per km2. For the whole Sahara (9,200,000 km2 ) this would mean a total of 202 Gton. The Carbyon analysis showed that there was an excess amount of 92 ppm, and an increase of 2.6 ppm per year (70.2 ppm in 27 years). It also showed that 1 ppm corresponds to 7.3 Gton. To compensate for the yearly increase would mean storing 70.2*7.3 = 513 Gton. This would require 2.5 Sahara deserts.
Again, the numbers may be off a little, but the picture is clear: planting the Sahara with sugar cane or trees would have a big impact on the CO2 levels in the atmosphere. It may even be a very good scheme, better than many others. However we cannot solve the CO2 problem completely by making the Sahara green, we would still need other (drastic) measures.