Page 85 - Managing Global Warming

56 Managing Global Warming

Plausibility range for years to maximum GGR

90
80
Years to reach maximum GGR 60
70

50

40
30
20

10
0
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
-1
Maximum GGR (Gt(C) year )

Fig. 2.6 Assumed linear relationship between eventual required annual amount of GGR
(Gt(C)/yr) and the minimum time taken to reach it. (This graph determines the period over
which GGR is scaled. The annual amounts of GGR within that period are dependent variables.)

than current industrial scale. The global economy would take some decades to reach
these levels of activity.
NETs access large-scale mass airflow, broadly by one of two routes: the wind or
powered fans. But each also requires other resources in different degrees, including
land, water, minerals, fertilizer, labor, transport, environmental management, and
energy. Novel technologies, such as solar towers, have the potential to industrialize
the acquisition of air on a small footprint and with a minimal call on other resources
but at the expense of high initial capital cost. By way of comparison, to capture
2
2
10Gt(C) by photosynthesis would require at least 10 10 12 m (10Mkm ) of land,
which represents 60% of the available land (Fig. 2.7), whereas to capture the same
amount using solar towers, assuming a 50% capture fraction, would require less than
2
2
0.2 10 12 m (0.2Mkm ).
The resource implications of the necessary scale cannot be underestimated. Bio-
mass is typically 50% carbon, so even assuming 100% efficiency, every gigatonne
of sequestered carbon requires 2Gt of dry biomass. If the process whereby the
carbon is extracted from the biomass and sequestered is, say, 50% efficient, the
amount of biomass feedstock doubles again to 4Gt. Moreover, many processes
involve multiple handling. Biochar requires biomass to be harvested, dried, chipped,
transported to biochar plants, and then the biochar and the residues to be distributed