Side effects and accounting aspects of hypothetical large-scale Southern Ocean iron fertilization

Abstract

Abstract. Recent suggestions to slow down the increase in atmospheric carbon dioxide have included ocean fertilization by addition of the micronutrient iron to Southern Ocean surface waters, where a number of natural and artificial iron fertilization experiments have shown that low ambient iron concentrations limit phytoplankton growth. Using a coupled carbon-climate model with the marine biology's response to iron addition calibrated against data from natural iron fertilization experiments, we examine biogeochemical side effects of a hypothetical large-scale Southern Ocean Iron Fertilization (OIF) that need to be considered when attempting to account for possible OIF-induced carbon offsets. In agreement with earlier studies our model simulates an OIF-induced increase in local air-sea CO 2 fluxes by about 73 GtC over a 100-year period, which amounts to about 48% of the OIFinduced increase in organic carbon export out of the fertilized area. Offsetting CO 2 return fluxes outside the region and after stopping the fertilization at 1, 7, 10, 50, and 100 years are quantified for a typical accounting period of 100 years. For continuous Southern Ocean iron fertilization, the CO 2 return flux outside the fertilized area cancels about 20% of the fertilization-induced CO 2 air-sea flux within the fertilized area on a 100-yr timescale. This "leakage" effect has a radiative impact more than twice as large as the simulated enhancement of marine N 2 O emissions. Other side effects not yet discussed in terms of accounting schemes include a decrease in Southern Ocean oxygen levels and a simultaneous shrinking of tropical suboxic areas, and accelerated ocean acidification in the entire water column in the Southern Ocean at the expense of reduced globally-averaged surfaceCorrespondence to: A. Oschlies (aoschlies@ifm-geomar.de) water acidification. A prudent approach to account for the OIF-induced carbon sequestration would account for global air-sea CO 2 fluxes rather than for local fluxes into the fertilized area only. However, according to our model, this would underestimate the potential for offsetting CO 2 emissions by about 20% on a 100 year accounting timescale. We suggest that a fair accounting scheme applicable to both terrestrial and marine carbon sequestration has to be based on emission offsets rather than on changes in individual carbon pools.

Potential climate engineering effectiveness and side effects during a high carbon dioxide-emission scenario

“…However, these reductions are small compared with expected business-as-usual anthropogenic emissions, and atmospheric CO 2 continues to increase rapidly and still reaches more than twice the current level by the end of the century in all simulations. Those methods that have been proposed to reduce atmospheric CO 2 as a means of preventing warming, that is, carbon dioxide reduction (CDR) methods (Table 1), are thus, as expected from other studies3524252627, unable to prevent a 2.7–3.9 °C mean temperature increase (temperature increases by 3.8 °C with no climate engineering) in our model simulations under the RCP 8.5 emission scenario by the year 2100 (Table 2, Figs 2c and 3a,c,e,g). Indeed, the albedo change caused by simulated AF (Table 3, Supplementary Fig.…”

Developing a test-bed for robust research governance of geoengineering: the contribution of ocean iron biogeochemistry

“…For example, the latter has a series of historical precedents [37][38][39], has used the findings from 12 mesoscale (i.e. 1000 km 2 ) scientific studies into the links between changing ocean iron supply and the ocean carbon cycle [40], and also gained insights into issues such as artefacts and deleterious side effects [28,41,42].…”

“…These publications span both the science of our understanding of how the iron biogeochemical cycle influences the ocean carbon cycle-from the geological past-to present-day ocean productivity [40,46,47], but also extrapolate these findings, via modelling simulations [42,[48][49][50], in response to claims that iron enrichment could significantly mitigate atmospheric anthropogenic CO 2 loading [51,52]. This in-depth discussion on the role that iron could play in climate mitigation commenced as early as 1991 [53] and has resulted in a vigorous debate thereafter [ The policy and legislative issues surrounding ocean iron fertilization have been informed more indirectly; a series of other events have been running in parallel with this debate.…”

“…First, through a retrospective examination of some of the datasets from the 12 mesoscale ocean iron-enrichments [28] and naturally fertilized blooms [56][57][58]; second, via targeted CDR modelling experiments [42,50,59] and third, from insights gained on the fate of surface ocean carbon at depth during naturally occurring bloom events (often iron-mediated, [60]) and their impact on the oceans' biological pump [60][61][62]. Comparison of the efficiency with which phytoplankton carbon is exported from surface waters (surface export efficiency, as a proportion of primary production, after [65]), and then through subsurface waters (as a proportion of surface export, after [65]) for: (a) iron-enrichments (A, Southern Ocean EiFex study [64]); B, North Pacific SERIES study [63]; 'A and B low iron' = export under ambient low iron conditions at each site).…”

Research for assessment, not deployment, of Climate Engineering: The German Research Foundation's Priority Program SPP 1689

“…This happened while the ship was already on its way from Cape Town into the Southern Ocean. The concentrated and intensive effort of writing these assessments within a few days brought together scientists from different disciplines to assess the justification and possible implications of the planned experiment and stimulated further work, leading to several interdisciplinary scientific publications regarding iron fertilization [Güssow et al, 2010;Oschlies et al, 2010;Rickels et al, 2010Rickels et al, , 2012.…”