Royal Society - David Keith

would have to be mined and ground up, transported, and
then spread over fields. It is estimated that a volume of
about 7 km 3 per year (approximately twice the current rate
of coal mining) of such ground silicate minerals, reacting
each year with CO 2 , would remove as much CO 2 as we are
currently emitting. It is conjectured that the CO 2 could be
immobilised partly as carbonate minerals and partly as
bicarbonate ion in solution, but the consequences for soil
processes are currently not known.
Alternatively, it has been suggested that carbonate
rock could be processed and ground, and reacted with
CO 2 in chemical engineering plants (most likely with
concentrated CO 2 captured from power plants, for
example). The resulting bicarbonate solutions would be
released into the sea (Rau & Caldeira 1999; Rau 2008).
An alternative approach would be to release the
carbonate minerals to the sea directly (Harvey 2008).
They would however not dissolve until they reached
under-saturated deep water, so making the process very
slow to have any effect. In a variant (‘liming the ocean’),
which would operate faster, limestone carbonate rocks
would be heated to drive off pure CO 2 , (which must be
captured and sequestered) to form lime Ca(OH) 2 .
This would be added to the oceans to increase their
alkalinity, resulting in additional uptake of CO 2 from
the atmosphere (Kheshgi 1995; see also Submission:
CQuestrate). While this process is energy and therefore
cost intensive it would sequester roughly twice the
amount of CO 2 per unit of carbonate mined.
Alternatively, the rate of the reaction of CO 2 with basic
minerals such as basalts and olivine could be enhanced
in situ in the Earth’s crust (Kelemen & Matter 2008;
Submission: Sigurðardóttir & Gislason). This idea would
also require elevated CO 2 concentrations in the reactant
gas, and might be better thought of as a CO 2 sequestration
technique rather than remedial geoengineering, as the end
result of the method would be the creation of carbonates
in situ. Kelemen and Matter suggest there is the potential
to sequester more than 1 GtC/yr of carbon in Oman alone
by this method. Again, much further research is required
to know if it is in fact feasible at these scales.
It has also been proposed (House et al. 2007) to accelerate
silicate weathering using electrolysis to divide sea salt into
strong bases and strong acids. When strong bases are
dissolved in seawater they cause CO 2 to be stored in the
ocean as HCO 3 - as noted above. House et al. (2007)
propose to use the strong acid to weather silicate rocks.
The weathering of silicate rocks can neutralise the acid and
form a relatively benign salt that could also be added to the
ocean. This approach is energy intensive and requires a
large amount of mass handling, and thus is likely to be
more expensive than conventional CCS approaches.
Environmental impact of enhanced weathering methods
Enhanced weathering methods clearly have the capacity
to reduce climate risk, by reducing CO 2 emissions or
removing CO 2 from the atmosphere. However, before they
are deployed their side effects, lifecycle costs and
environmental effects must be better understood and
taken into consideration. For example, the final result of
nearly all of these methods would be to increase the
bicarbonate (anion) and calcium or magnesium (cation)
concentration (and hence the alkalinity) of sea water. Even
if the weathering reaction initially took place distributed
in soils (as with olivine above, for example), the resultant
chemicals would eventually be washed to the oceans.
Sea water contains substantial concentrations of these
ions already, and it would be possible to take up all the
excess CO 2 in the atmosphere without greatly increasing
those concentrations. Such an increase in bicarbonate
concentrations and alkalinity would reduce rather than
increase the acidity of sea water, helping to slow the
progress of ocean acidification (see Section 2.4), and
might therefore be beneficial to those organisms and
ecosystems otherwise threatened by rising atmospheric
CO 2 . It is not yet known, however, whether all the
combined effects on ocean chemistry or biology
would be negligible or benign.
Furthermore, to be quantitatively important, most of these
proposals require large mining and transportation activities.
These activities would likely damage the environment
locally (and ‘local’ here would mean over large areas,
comparable to or greater than those of present-day
cement production and coal mining). Some options require
large amounts of water. Others require additional energy
(for electrolysis or lime production), which would need to
come from carbon-free sources. In the case of solid
mineral production, there are also issues of disposal
(or use) of large amounts of solid material.
In summary, all enhanced weathering methods utilise
naturally occurring minerals and reactions, and produce
stable natural products, which are already present in large
quantities in soils and the oceans, and they may therefore
be regarded as benign in principle. They operate by making
soils or the ocean somewhat more alkaline, which on
a global scale reduces CO 2 induced acidification of the
terrestrial and marine environments. However, the
products are generated in large quantities in a more-or-less
localised way, and may therefore have substantial impacts
that would need to be managed. There are substantial
questions concerning desirable particle sizes and the rates
of dispersion, dilution and dissolution required. The pH
of soils and ocean surface waters would be increased
locally, with possible effects (not necessarily adverse) on
vegetation and marine biota, and potential for increased
precipitation of carbonate minerals that would reduce
their effectiveness. Moreover, because these chemical
approaches require that each CO 2 molecule react with
dissolved minerals, mass requirements for mineral
inputs and outputs will greatly exceed the mass of CO 2
sequestered. These approaches require major mining and
processing operations and are likely to be more expensive
to operate than conventional CCS (IPCC 2005), for example,
unless they are able to utilise cheap (‘stranded’) sources of
energy, or are undertaken where labour and other costs are
low (see Tables 2.5 and 2.6).
14 I September 2009 I Geoengineering the Climate The Royal Society