Can ocean iron fertilization mitigate ocean acidification?
Long Cao* and Ken Caldeira
Department of Global Ecology, Carnegie Institution, Stanford, CA, USA
*
Supporting online material
Methods:
1 Model
We use the Lawrence Livermore National Laboratory (LLNL) ocean general circulation model(Caldeira and Wickett, 2003) to study the effect of ocean iron fertilization on ocean acidification.The model divides the ocean into 24 vertical levels and has a horizontal resolution of 4º longitude by 2º latitude. An abiotic carbon cycle component based on the protocol of the Ocean Carbon Model Intercomparison Project(OCMIP-2) ( is coupled to the ocean circulation model. The representation of marine biology follows the OCMIP-2 protocolin which biological production is parameterized by restoring modeled phosphate concentrations in the euphotic zone (upper 75 m in the model) toward observational-based monthly mean values with a timescale of 30 days.
2. Simulation setup
The LLNL model was forced with pre-industrial atmospheric CO2 to reach a near-stationary state. After the model spinup, the model was integrated from the pre-industrial time (year 1765) to present-day (year 2007 here) forced with observed atmospheric CO2 concentrations. Then, the model was integrated into the future under three different scenarios as listed in Table 1. 1) Simulation A2_emis is the baseline in which the model was integrated underprescribed fossil-fuel CO2 emissions from IPCC SRES A2 scenario(Nakićenović, et al., 2000); 2) Simulation A2_emis+OIF in which the model was integrated under the same CO2emission scenario as A2_emis but with iron fertilization appliedover the global ocean from year 2008 to 2100. This simulation represents the scenario in which ocean iron fertilization (OIF) is used to mitigate atmospheric CO2; 3) Simulation A2_conc+OIF is the same as A2_emis+OIF, except that the model was forced with prescribed CO2 concentrations calculated from the A2_emissimulation. (Since the model does not include a representation of the terrestrial biosphere, the implicit assumption underlying modeled CO2 concentrations is a neutral terrestrial carbon uptake). In this scenario, we assume that the increase in net carbon storage in the deep ocean as a result of iron fertilization OIF can produce an equivalent amount of emission credits. Therefore, iron fertilization leads to greater CO2 emissions, but does not mitigate atmospheric CO2.
Following previous studies (Sarmiento et al., 1991; Gnanadesikan et al., 2003), here we simulate iron fertilization implicitly by restoring near-surface phosphate concentrations toward zero to represent the maximum possible macronutrient depletion driven by iron fertilization. The depletion of surface phosphate concentration is assumed to occur at all time over the global ocean. We emphasize that we do not intend to simulate the real-world effect of iron fertilization, which requires a comprehensive carbon cycle model including the iron cycle. Instead, these idealized experiments represent an upper bound of the extreme and idealized end-member cases for ocean iron fertilization, which is used to demonstrate basic logic and principles relating iron fertilization and ocean acidification.
3 Ocean chemistry analysis
Ocean chemistry is analyzed in terms of pH and the saturation state of calcium carbonate. The saturation state of seawater with respect to aragonite (Ωaragonite) and calcite (Ωcalcite), two major forms of calcium carbonate, is defined as the ion product of the concentrations of calcium and carbonate ions divided by the stoichiometric solubility product(Feely et al., 2004). These chemistry fields were calculated using the model-simulated dissolved inorganic carbon (DIC), alkalinity (ALK), temperature, and salinity fields and the chemistry routine from the OCMIP-3 project (
References:
Caldeira, K., and M. E. Wickett (2003), Anthropogenic carbon and ocean pH, Nature, 425, 365– 365.
Feely, R. A., C. L. Sabine, K. Lee, W. Berelson, J. Kleypas, V. J. Fabry, and F. J. Millero (2004), Impact of anthropogenic CO2 on the CaCO3 system in the oceans, Science, 305, 362–366.
Gnanadesikan A, Sarmiento J. L., Slater R. D. (2003), Effects of patchy ocean fertilization on atmospheric carbon dioxideand biological production.Global Biogeochem. Cycles 17 (2): 1050, doi:10.1029/2002GB001940.
Nakićenović, N., et al. (2000), Special Report on Emissions Scenarios, 570 pp., Camb. Univ. Press, Cambridge, MA.
Sarmiento, J. L., and J. C. Orr (1991), Three-dimensional simulations of the impact of Southern Ocean nutrient depletion on atmospheric CO2 and ocean chemistry, Limnol, Oceanogr., 36, 1928-1950.
Fig. S1. Distribution of aragonite saturation horizon (the depth below which the ocean is undersaturated with respect to aragonite) from different simulations. Pre-industrial (black dashed line); present-day (black solid line); A2_emis (red line); A2_emis+OIF (blue line), A2_conc+OIF(green line).Refer to Table 1 for simulation setup. It shows that the effect of iron fertilization on the shoaling of aragonite horizon is most pronounced in the Southern Ocean.