Royal Society - David Keith

Table 2.2. Land-use and afforestation summary evaluation table. The ratings given (refer Table 5.1) are according to the
criteria explained in Chapter 1
Land use and afforestation
Effectiveness Limited potential for carbon removal Low
Affordability Cheap to deploy Very high
Timeliness
Safety
Ready for immediate deployment and starts CO 2 reductions immediately
Slow to reduce global temperatures (CDR method)
Few undesirable side effects except for potential land use conflicts and biodiversity
implications
Medium
High
management regimes, often exceeding national
jurisdictions in order to gain the benefits of scale.
As summarised in Table 2.2 these methods are feasible and
are low risk, but are long-term and can achieve only small
to medium effects on atmospheric CO 2 concentrations
(see also Table 5.1). Several regional scale studies have
demonstrated that overall benefits to the economy and
to other ecosystem services such as water regulation,
amenities, biodiversity conservation and agriculture can
result from integrated land-use planning that would deliver
enhanced CO 2 draw-down and storage. However, carbon
stored in vegetation is not securely sequestered in the
long-term, as it can easily be released by fire, drought or
deliberate deforestation (Royal Society 2001).
2.2.2 Biochar and biomass-related methods
As terrestrial vegetation grows it removes large quantities
of carbon from the atmosphere during photosynthesis.
When the organisms die and decompose, most of the carbon
they stored is returned to the atmosphere. There are four
ways in which the growth of biomass may be harnessed
to slow the increase in atmospheric CO 2 (Keith 2001).
1. Land Carbon Sinks. Carbon may be sequestered in situ
in soil or as standing biomass, as discussed above in
Section 2.2.1.
2. Bioenergy & Biofuels. Biomass may be harvested and
used as fuel so that CO 2 emissions from the fuel’s use
are (roughly) balanced by CO 2 captured in growing
the energy crops. The use of bioenergy and biofuels
(Royal Society 2008a) is considered to be a means of
reducing emissions, rather than geoengineering and
is not considered further here.
3. Bioenergy with CO 2 capture and sequestration (BECS).
Biomass may be harvested and used as fuel, with
capture and sequestration of the resulting CO 2 ; for
example, one may use biomass to make hydrogen or
electricity and sequester the resulting CO 2 in geological
formations.
4. Biomass for sequestration. Biomass may be harvested
and sequestered as organic material, for example, by
burying trees or crop wastes, or as charcoal (biochar).
Bioenergy with CO 2 sequestration (BECS) builds directly
on existing technology for bioenergy/biofuels and for CCS,
and inherits the advantages and dis advantages of both of
these technologies. There is no doubt that it is technically
feasible, and there are already some small real-world
examples (Keith 2001; Obersteiner et al. 2001; IPCC 2005).
It is again not necessarily or normally regarded as
geoengineering, and has been reviewed in some detail
by the IPCC (2005). However, BECS has much in common
with some other methods considered here, and has
therefore been included for comparison purposes,
but is not reviewed in detail (see Table 2.3).
Sequestration of biomass and biochar have been
proposed as a method for intervening in the natural
cycle so that some or all of the carbon fixed by organic
matter can be stored in soils or elsewhere for hundreds
or thousands of years. For example, it has been proposed
to bury wood and agricultural waste both on land and
in the deep ocean to store the carbon rather than
allow decomposition to return it to the atmosphere
(Submission: Mark Capron; Submission: Newcastle
University; Submission: Ning Zeng; Strand & Benford
2009). In contrast to bioenergy with CO 2 sequestration,
there is relatively little peer-reviewed literature about
biomass for sequestration, though there appears to be
growing interest in the biochar process (discussed later
in this section).
Methods involving burying biomass in the land or deep
ocean will require additional energy consumption for
transport, burying and processing. Most seriously, the
processes involved may disrupt growth, nutrient cycling
and viability of the ecosystems involved. In the deep
ocean, for example, organic material would be
decomposed and the carbon and nutrients returned to
shallow waters, since oxygen is generally present (unless
sufficient material were deposited to create anoxic
conditions, which would constitute a major ecosystem
perturbation). Full assessments are not yet available to
assess the costs and benefits involved but it seems unlikely
that this will be a viable technique at any scale that could
usefully reduce atmospheric carbon.
Biochar (charcoal) is created when organic matter
decomposes, usually through heating, in a low- or zero
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Geoengineering the Climate I September 2009 I 11