Royal Society - David Keith

It might be argued that one easy way to implement P
fertilisation of the oceans is to allow and even encourage
agricultural fertiliser runoff, which eventually reaches rivers
and the oceans. Such runoff is however one of the
principal causes of the substantial damage to freshwater,
estuary and coastal ecosystems by eutrophication that has
already occurred over recent decades. Increasing still
further this pathway for addition of P to the oceans is not
an option that society is likely to find acceptable.
2.3.2 Oceanic upwelling or downwelling
modification methods
A second group of ocean-based methods is based on the
principle that the rate at which atmospheric carbon is
transferred to the deep sea may be enhanced by increasing
the supply of nutrients by the upwelling or overturning
circulation of the ocean (Submission: Duke). It has been
proposed both to enhance upwelling rates locally using
vertical pipes to pump water from several hundred metres
depth to the surface (eg, Lovelock & Rapley 2007;
Submission: Atmocean Inc.) and to promote downwelling
of dense water in the subpolar oceans (Zhou & Flynn
2005). Most of the CO 2 in the deep sea is transported there
by the overturning circulation (the ‘solubility pump’) and
not by biologically-driven sedimentation (Sarmiento &
Gruber 2006), so there is some expectation that increasing
this circulation will lead to more rapid sequestration.
However, once again the calculation of the efficiency
of sequestration must take account of non-local effects:
increasing ocean downwelling (or upwelling) must be
compensated by increased upwelling (or downwelling)
at another location, which may in general be on the
other side of the world and which also will affect the
carbon balance.
Zhou and Flynn estimate that increasing downwelling
water by 1 million m 3 /s, which would be a very substantial
engineering challenge, would increase ocean uptake
of carbon by only ~0.01 GtC/yr. The amount of carbon
sequestered by the ocean pipes proposal will depend
critically on location and may well be negative, for example
leading to release, rather than uptake, of carbon from the
ocean (Yool et al. 2009). Making optimistic assumptions,
it is estimated that enhancing upwelling by 1 million m 3 /s
would lead to sequestration of only ~0.02 GtC/yr (Lenton &
Vaughan 2009).
2.4 Discussion
On the basis of the available literature, indications are
provided in Table 2.9 of maximum effects of the respective
technologies on CO 2 concentrations in the next century.
Figures are informed by the literature cited, and by Lenton
& Vaughan’s (2009) strong mitigation scenario table II, in
which atmospheric CO 2 concentrations rise to 450 ppm in
2050 and stabilise at 500 ppm in 2100. Deliberately wide
ranges are given, intended only to show the approximate
potential of these technologies if deployed to the
maximum, regardless of cost or possible side effects.
Costs are assessed as ‘low’ if generally less than $20 per
tonne of carbon sequestered, medium if between $20
and $80, otherwise ‘high’. Risk is assessed as high for
those technologies that involve manipulating the ocean
or relatively undisturbed natural land ecosystems at a
large scale, and medium for agricultural and biomass
technologies, on the rationale that agricultural impacts
are relatively well understood and would not directly
affect undisturbed terrestrial ecosystems.
It is clearly technically possible to remove CO 2 from the
atmosphere using many different technologies, ranging
from ecosystem manipulation to ‘hard’ engineering. Plans
to begin removal using some methodologies are in place
now, and if societies put a realistic value on carbon
removed (for example, more than $30 per tonne of carbon),
it would start to happen with existing technologies.
All other points being equal, methods that are (not in any
order of preference): (1) cheaper, (2) have fewer possibly
unintended side effects, (3) have large potential to remove
CO 2 , and (4) do not involve manipulation or interference
with natural or near-natural ecosystems are likely to be
preferred. Methods which: (5) are likely to be easily
accepted by society and (6) do not raise difficult issues
of governance are also likely to be favoured. Since none
of the proposed methods meets all of these criteria it is
necessary to balance these different properties against
one another, and this is bound to raise differences
of opinion.
The ocean fertilisation proposals are virtually the only ones
that have had anything amounting to sustained research
activity by the scientific community. This is an historical
accident, because relevant experiments were undertaken
to address fundamental research questions in marine
science, and not because of their possible geoengineering
applications. In the geoengineering context, the sole
attraction of these methodologies is that iron (and
possibly phosphate) fertilisation are potentially relatively
inexpensive. They do however have only a relatively small
capacity to sequester carbon, and verification of their
carbon sequestration benefit is difficult. Furthermore,
there are likely to be unintended and probably deleterious
ecological consequences. With these drawbacks societal
and political acceptance is likely to be low. Ocean
circulation methodologies have the same issues, but
also appear to have effects on atmospheric CO 2 that
are too small to be worthwhile.
Methods such as BECS, biomass burial and biochar, which
use biomass to sequester carbon, appear to have relatively
low cost, with moderate and predictable environmental
impacts and low-to-medium risk of unanticipated effects.
However, unless deployed on a very large scale, the carbon
sequestration potential is moderate, and there would be
competition with biofuels and agriculture for use of
available land. However the carbon sequestered by
biomass burial and biochar has value as fuel, and it could
be preferable to use this and displace fossil fuels such as
coal, at least until abundant low-carbon energy becomes
available. Land use management (afforestation and
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Geoengineering the Climate I September 2009 I 19