I'm going to put a few numbers behind this statement:
With the right approach, IMO, we could start a process today that would probably result in the ability to draw down CO2 within a 5-10 year active time, using (bio-)technology that might mature within 20 years.Now, let's start with assuming a 100ppm (parts per million) draw-down, which would be equivalent to reducing our current level of ~400ppm to 300ppm, equivalent to a date prior to 1960. Just how much carbon would we have to remove from the atmosphere?
The density of the atmosphere at sea level is about 1Kg/M3, and the pressure is roughly 10 tons/M2. Now, the average molecular weight of air is about 29, while the average molecular weight of CO2 is about 44. Parts per million are calculated by volume, which corresponds to number of molecules.
So 100ppm CO2 would weigh (44/29)*(100/1,000,000)*10,000Kg/M2 = ~1.5Kg/M2. The carbon would be about 12/44 of that, or about 414g/M2, or 414 tons/Km2. The Earth's surface area is about 5.1*108Km2, which adds up to 211.17GTon carbon to be removed. Given the nuclear connotations of "gigaton", I'm going to call it Petagrams (Pg), which is common in carbon literature.
The Azolla Event
About 50 million years ago (MYA), there occurred an event of global importance called the Eocene Azolla event. Evidence suggests it began around 49.3MYA, lasting until around 48.1MYA, thus lasting about 1.2MY. In this event, it appears that fresh water from Arctic rivers formed a layer over the surface of the heavier salt water, and the entire Arctic (or large parts of it) experienced a massive bloom of a fern called Azolla.
Azolla is an intriguing plant, actually a symbiosis between a secondarily degenerate fern (Azolla sp.) and a blue green algae (Anabaena azollae). It's just a few inches in size, and floats entirely on water, without normally anchoring. It's one of the fastest growing plants known, capable of producing 25-90gm/day/M2. It normally expands vegetatively, although under appropriate circumstances it will reproduce sexually.
The speed with with the Eocene Azolla grew appears to have been such that it reduced the atmospheric level of CO2 from 3500ppm to 650ppm, probably within that small 1.2 million year stretch. Thus, it makes a great candidate for drawing down CO2.
Let's do some more numbers. The circumstances under which Wagner grew Azolla Nilotica were probably not as optimized as could be done with modern technology, and I'm going to assume that harvesting processes could keep the Azolla growing at maximum rate continuously. 90 gm/day/M2 is equivalent to the same number of tons/Km2/day, multiplied by 360 gives 32,400 tons of biomass/Km2/year. Carbon content for various European strains of Azolla ranged from 37-42%. I'm going to assume 40% (by weight) coming to 12,969, or roughly 13Kilotons Carbon/Km2/year.
Now comes the technology. I'm going to assume for the moment that within a decade or two we have the technology to float a layer of fresh water on top of salt for very large areas. Later I'll go into possible methods, but for the moment let's just assume one million square kilometers, less than 1/7th the area of Australia. This works out to 13 Pg (=gigatons)/year. Remember above we said that we have ~212Pg to remove in order to draw-down 100ppm? Dividing 212 by 13 gives about 16.3 years. Double this, and we're down to 8.2 years, triple it and we're down to about 5.5 years. And that's still less than half the area of Australia.
I'm not going to go into detail regarding methods. Fresh water is lighter than sea water at the same temperature, which explains how the fresh water managed to stay separate from underlying salt water during the Azolla Event. The difference is only about 2.5%, however, which is pretty small. The lower levels of the Arctic appear to have been anoxic, in the same way the Black Sea is today.
It's remotely possible that simply floating a layer of fresh water on top of salt water might work, but I'm going to assume not. Given this, the easiest way I see to handle it is with an intermediate layer of some viscoelastic material, with a density intermediate between fresh and sea water. More material intensive, but perhaps cheaper, might be a sort of "air mattress" with a lot of internal tensile stiffening. Another option would be to maintain a layer of pressurized air topped with a stiff but slightly flexible layer, with water above it.
Obviously, all these options would require a good deal of engineering and development. However, consider the difference between 1991 and today. No cell phones (except huge experimental clunkers that only worked in a few areas). The Internet was just getting set up, and mostly just existed in educational environments. When we look at the difference just 20 years has made, there's no good reason to suppose we couldn't do this.
The total amount of the world's good quality agricultural land is around 16.5 million Km2. I've discussed using 3 million Km2. The total amount of lower quality agricultural land in the world is around 43.7 million Km2. This has been described in the following terms:
If there is a choice, these soils must not be used for grain crop production, particularly soils belonging to Class IV. All three Classes require important inputs of conservation management. In fact, no grain crop production must be contemplated in the absence of a good conservation plan. Lack of plant nutrients is a major constraint and so a good fertilizer use plan must be adopted. Soil degradation must be continuously monitored. Productivity is not high and so low input farmers must receive considerable support to manage these soils or be discouraged from using them. Land can be set aside for national parks or as biodiversity zones. In the semi-arid areas, they can be managed for range. Risk for sustainable grain crop production is 40-60%.As our population expands, methods to manage, control, and maintain these lands will become increasingly expensive, relative to more basic types of agriculture. At the same time, technology to manage activities on the water will be coming down in cost. Very likely, they'll meet at some point, at which point it will be cheaper to build new prime agricultural land floating on the ocean than continue using poorly suited terrestrial land. Long before this happens, simple technologies like that necessary for the Azolla Alternative will have become cost effective.
I'm going to leave economic and political issues for another post.
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Wagner, G.M. (1997) Azolla: A Review of Its Biology and Utilization The Botanical Review 63(I): 1-26, January-March 1997
Zahran, H.H., Abo–Ellil, A.H., Al Sherif, E.A. (2007) Propagation, taxonomy and ecophysiological characteristics of the Azolla-Anabaena symbiosis in freshwater habitats of Beni-Suef Governorate (Egypt) Egyptian Journal of Biology, 2007, Vol. 9, pp 1-12 Read more!