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

• Published in: Biogeosciences
• Main Authors: A. Oschlies, W. Koeve, W. Rickels, K. Rehdanz
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus Publications 2010
• Subjects: Ecology; QH540-549.5; Life; QH501-531; Geology; QE1-996.5; Southern Ocean; Ocean acidification
• Online Access: https://doi.org/10.5194/bg-7-4017-2010; https://doaj.org/article/ce54c6a9b2d54a83aff14a4867df1cd4

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 OIF-induced 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 surface-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 ...