cbd-ts-66-en

Part I: Impacts of Climate-related Geoengineering on Biological Diversity
geoengineering technique. Whilst enhanced dimethyl sulphide (DMS) emissions from plankton might be considered
a “beneficial”, unintended outcome of ocean fertilization, due to albedo effects,297 the scale (and even sign) of this
response is uncertain, and the overall linkage between DMS and climate is now considered relatively weak.298
5.3 MODIFICATION OF UPWELLING AND DOWNWELLING
Artificial upwelling is an ocean fertilization technique that has been proposed to bring deep water (from 200–1000
m) naturally rich in a range of nutrients to the surface, through some type of pipe, to fertilize the phytoplankton299.
Limited field experiments have been carried out in the Pacific.300, 301
The intended effects are essentially the same as for externally-adding nutrients, as above and will therefore not be
repeated here. However, there is a major problem with the concept, as the nutrient-rich water brought up to the
surface also contains high concentrations of dissolved CO2 derived from the decomposition of organic material.
The release of this CO2 to the atmosphere302, 303 would counteract most (if not all) of the potential climatic benefits
from the fertilization of the plankton.
Upwelling in one area necessarily also involves downwelling elsewhere. Modifying downwelling currents to
carry increased carbon into the deep ocean by either increasing the carbon content of existing downwelling or
by increasing the volume of downwelling water has also been proposed as a possible geoengineering approach,
without necessarily involving enhanced biological production.
While the view of some authors304 is that “modifying downwelling currents is highly unlikely to ever be a costeffective
method of sequestering carbon in the deep ocean”, lower-cost structural approaches have recently been
proposed, with the claim that the downwelling would stimulate adjacent upwelling, increasing primary production
and carbon drawdown, and benefitting fisheries.305 In order to estimate the number of such structures necessary to
achieve global climate impact, the hydrodynamics and biogeochemistry of such systems would need further attention.
However, such an approach is likely to require coverage of a significant proportion of the ocean surface, since—
as for other CDR techniques—the geoengineering requirement is for long-term sequestration of anthropogenic
carbon, not stimulating carbon cycling per se. In particular: i) if the increased phytoplankton growth is stimulated
by nutrients from deeper water, such water will also contain higher CO2, thus net drawdown of atmospheric CO2 is
unlikely to be achieved; ii) there is considerable variability in the timescale of carbon re-cycling in the ocean interior,
determining the rate of return of additional, biologically-fixed CO2 to the atmosphere and iii) enhancement of
biological production at the scale required for climatic benefits is likely to significantly deplete mid-water oxygen,
resulting in increased CH4 and N2O release.
Because of these complications, the overall effectiveness of any upwelling/downwelling modification is questionable.
Furthermore, high costs are likely to be involved in achieving reliable data on any long term carbon removal (needed
for international recognition of the effectiveness of the intervention), and in quantifying potentially counter-active
negative impacts, over large areas (ocean basin scale) and long time periods (10–100 years).
297 Wingenter et al. (2007).
298 Quinn & Bates (2011).
299 Lovelock & Rapley (2007).
300 Maruyama et al. (2011).
301 White et al. (2010).
302 Oschlies et al. (2010).
303 Yool et al. (2009).
304 Zhou & Flynn (2005).
305 Salter (2009).
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