Land Use and Geo-Engineering - Introduction to Carbon Capture and Sequestration - page 529

Section 3
Geo-engineering: carbon
dioxide removal
To geo-engineer the CO 2 currently in the atmosphere, two types of tech-
nologies are possible. We can either enhance the uptake of CO 2 by driv-
ing natural sequestration processes, or capture CO 2 directly from the air
with subsequent sequestration. It is important to note that for either of
these two strategies to have any impact, the scale needs to be compa-
rable to that of present-day annual CO 2 emissions. Of the current pro-
posed geo-engineering ideas, we will discuss the three we feel are most
worthy of consideration: enhancing the weathering process, enhancing
the uptake by the oceans, and direct capture from air.
Enhanced biomass
Changing the land use, which we discussed in the previous section, is in
essence a form of geo-engineering in which we restore nature's capacity
to uptake larger amounts of CO 2 . In this section, we discuss some alter-
native biological routes to sequester CO 2 that have often been investi-
gated in this context.
One may argue that use of bioenergy and biofuels would be an obvi-
ous way to restore nature's uptake of CO 2 . However, in this chapter we
focus on technologies that allow us to reduce CO 2 levels. The conversion
of biomass to fuels is a very important development in renewable fuels;
each year one harvests exactly the same amount of carbon as is emitted.
The utilization of biofuels therefore contributes to a CO 2 -neutral econ-
omy, but does not reduce CO 2 levels. A simple way to achieve a net
reduction in CO 2 levels is, for example, to use biomass to make hydrogen
and subsequently react the hydrogen with CO 2 .
Storage of CO 2 as biomass is not equivalent to geological storage. If
done correctly, the ultimate fate of geologically stored CO 2 is mineraliza-
tion, i.e., formation of limestone as in the Cliffs of Dover. Biomass, how-
ever, will eventually decompose and become CO 2 . For example, if a tree
dies the time scale to decompose and emit CO 2 can be tens of years in