Land Use and Geo-Engineering - Introduction to Carbon Capture and Sequestration - page 535

that same restriction. Nevertheless, compared to capture from fl ue gas-
ses, the costs of Direct Air Capture are signifi cantly higher [11.13].
Ocean fertilization
In Chapter 3, we saw that the oceans are responsible for a signifi cant
uptake of CO 2 . In most parts of the oceans the CO 2 uptake is limited by
nutrients [11.14]. If we were to artifi cially supply these nutrients, we
would be able to enhance the CO 2 uptake. The limiting nutrients that
have been studied include N, P, and Fe. To estimate the effects of these
nutrients on CO 2 uptake, we consider their relative amounts used by
algae in building their organic tissue. These are expressed by the char-
acteristic Redfi eld ratios of the nutrient elements, C:N:P:Fe. For algae,
the ratios are typically 106:16:1:0.001. This implies that for every addi-
tional P atom, one will able to sequester 106 carbon atoms. The Redfi eld
ratio shows that Fe has the largest impact, and therefore most research
has focused on the effects of increasing the Fe content of the ocean. Of
course, a key concern is that adding the amounts needed to impact the
CO 2 levels of the atmosphere could produce large, potentially undesira-
ble effects on ecosystems. These possible effects are poorly
understood.
Ocean up- or downwelling
The slow mixing of the surface layer of the ocean and the deep ocean is
responsible for the very long time (>200 years, see Chapter 3) required
before the CO 2 levels in the atmosphere will decrease. If we could
enhance the mixing time by up- or downwelling water from the deep
ocean reservoir to the surface, we would be able to shorten this time. The
amount of water that would lead to sequestration of ~0.02 Gt C/yr is
1 million m 3 /s. This fl ow rate exceeds the known volume of all the major
rivers of the world combined!