Ocean fertilization field experiments show no consensus on the effi-
cacy of iron fertilization (Boyd et al., 2007; Smetacek et al., 2012).
Modelling studies estimate between 15 ppm and less than 100 ppm
drawdown of CO2 from the atmosphere over 100 years (Zeebe and
Archer, 2005; Cao and Caldeira, 2010) while simulations of mechanical
upwelling suggest 0.9 Gt / yr (Oschlies et al., 2010). The latter technique
has not been field tested. There are a number of possible risks including
downstream decrease in productivity, expanded regions of low-oxygen
concentration, and increased N 2O emissions (See WG I Section 6.5.3.2)
(low confidence). Given the uncertainties surrounding effectiveness
and impacts, this CDR technique is at a research phase with no active
commercial ventures.