Ferrer-Gonzalez, M.

Ilyina, T.

English

MPG.PuRe

NOVA ACTA LEOPOLDINANeue Folge | Band 121 | Nummer 408Deutsche Akademie der Naturforscher Leopoldina – Nationale Akademie der Wissenschaften, Halle (Saale) 2015Wissenschaftliche Verlagsgesellschaft StuttgartDeglacial Changes in Ocean Dynamicsand Atmospheric CO2Michael Sarnthein and Gerald H. Haug (Eds.)Nova Acta Leopoldina NF 121, Nr. 408, 275 –278 (2015)275Mitigation Potential, Risks, and Side-Effects of Ocean Alkalinity Enhancement Miriam Ferrer-Gonzalez and Tatiana Ilyina (Hamburg) With 2 FiguresIncreasing atmospheric carbon dioxide (CO2) levels due to human activities leads to climate change and ocean acidi!cation (decline in ocean pH). In order to tackle these issues and due to the lack of effective mitigation actions, a great variety of geo-engineering techniques have been suggested, however, knowledge about their effectiveness and collateral effects remains sparse (Royal Society 2009). In 1995, Kheshgi proposed the carbon dioxide removal (CDR) method of arti!cial ocean alkalinization (AOA) (Kheshgi 1995). This is one of the ocean-based CDR methods that aims at enhancing the natural and slow (geological timescales of tens to hundreds of thousands of years) process of weathering by which CO2 is taken out of the atmosphere. Alkalinity is the charge balance of ions in water and it determines the CO2 oceanic uptake and storage as well as the buffering capacity of the seawater to inhibit changes in pH. It has been claimed that ocean alkalinity might have played a key role in gla-cial-interglacial cycles. For instance, Broecker and Peng (1989) proposed an increase in the alkalinity of polar surface waters as driver of the last glacial to interglacial atmospheric CO2 changes. Ocean alkalinity might be arti!cially increased by injecting the dissolution prod-ucts of alkaline minerals into the seawater (e. g. calcium hydroxide). This geo-engineering technique would not only tackle climate change, but also ocean acidi!cation, which has been found to accelerate climate change (Six et al. 2013) and it possess a serious risk for marine organisms (Doney et al. 2009). Ocean acidi!cation not only leads to changes in physiology and reproduction of living organisms, it also threats ocean biota due to the decrease of the mineral saturation state of calcium carbonate (CaCO3), commonly known as omega.Few studies exist that address the method of alkalinity enhancement. Kheshgi (1995) pro-posed this geo-engineering technique and carried out a preliminary analysis of its potential and limitations. He concluded that ocean alkalinization might enhance marine storage of atmospher-ic CO2 but its implementation would be an energy-intensive process. Using the carbon-cycle model LOSCAR, Paquay and Zeebe (2013) studied the impact of this method on ocean pH and atmospheric pCO2 during the time span 2020 to 2400. From this study, they came to the conclusion that large scale implementation of ocean alkalinization might stabilize ocean acid-i!cation but CO2 atmospheric levels would remain higher than pre-industrial levels. Ilyina and colleagues also carried out research on this matter using HAMOCC (Ilyina et al. 2013), they implemented and examined different alkalinity enhancement scenarios. In these scenarios, alkalinity was added to the ocean in proportional amounts to CO2 emissions. They show that this method, under the scenario where two moles of alkalinity per mol of emitted CO2 are dis-Miriam Ferrer-Gonzalez and Tatiana Ilyina276 Nova Acta Leopoldina NF 121, Nr. 408, 275 –278 (2015)tributed widely into the ocean, does have the potential to stabilize (at current levels) ocean pH and the saturation state of carbonate minerals. Regarding the atmospheric CO2 concentration, this method under the same scenario would only lead to a reduction of 300 ppm with respect to the unmitigated scenario by 2100. The novelty of our research relies on the fact that none of previous studies have addressed this topic with a fully coupled Earth system model of such a level of complexity. Fully coupled set-ups (versus box-models or forced subsystems) hold the potential of revealing new features within the Earth system dynamics. Coupled models pro-vide a more complete approach because box models are limited by simpli!ed formulations of the underlying processes. Besides, since climate engineering research is a new scienti!c !eld, many aspects of ocean-based carbon dioxide removal methods remain still unanswered. Our research might bring new insights into the link of deglacial changes with ocean dynamics and atmospheric CO2 through a better understanding of the underlying mechanisms. This project belongs to the German Research Foundation (DFG) priority program (SPP) 1689 that examines the risks and side-effects of different climate engineering technologies from a multidisciplinary perspective. Thus, it will be part of a comparative assessment of potential impacts, side-effects and uncertainties of different climate engineering measures.We use the Max Planck Institute Earth System Model (MPI-ESM) based on the Coupled Model Intercomparison Project Phase 5 version with low-resolution (MPIOM: 1.5. horizontal resolution and 40 vertical layers; ECHAM: 1.9. horizontal resolution and 47 levels) (Gior-getta et al. 2013). Fossil fuel CO2 emissions follow the pathway of scenario RCP8.5. Model scenarios of alkalinity enhancement (from 2006 until 2100) are designed to keep the atmos-pheric CO2 concentrations similar to values of the stabilization scenario RCP4.5, whilst fossil Fig. 1  Time series of globally averaged annual means for scenarios: RCP8.5, AOA and RCP4.5. Coloured area is model internal variability based on 3 ensemble members. Solid lines show 5-year-running means.Mitigation Potential, Risks, and Side-Effects of Ocean Alkalinity EnhancementNova Acta Leopoldina NF 121, Nr. 408, 275 –278 (2015) 277fuel emissions follow the pathway of the scenario RCP8.5. Alkalinity is added globally into the upper 12 meters of the ocean in different seasons and years. Note that this CE method ad-dresses only atmospheric CO2 reduction, therefore, land use, air pollutants, and other green- house gases (GHGs) remain unchanged with the values of the RCP8.5 emission scenario.In order to maintain atmospheric CO2 at RCP4.5 levels under the high emission scenario RCP8.5 (Fig. 1B), alkalinity was added in the upper ocean (Fig. 1A). In total, approximately 105 Petamol would be needed until the year 2100. Compared to the unmitigated scenario (RCP8.5) this AOA scenario leads to a reduction in the annual global mean of air surface tem-perature of around 1.5 K (Fig. 1D), following more closely the RCP4.5. The slightly higher temperature (0.5 K) of AOA compared to RCP4.5 is due to the radiative forcing effect of other GHGs (e. g. N2O, CH4 and halogenated GHGs). AOA strongly mitigates ocean acidi!cation leading to higher pH and omega (calcite) values than those associated with the RCP4.5 sce-nario and preindustrial values over the whole period (Fig. 1C and E). Largest changes of pH occur in the Arctic ocean where values higher than 8.4 are reached (see Fig. 2). This AOA Fig. 2  Averages of 10-year periods of pH (left) and the saturation state of carbonate minerals (Ω, calcite) (right) for different time intervals.Miriam Ferrer-Gonzalez and Tatiana Ilyina278 Nova Acta Leopoldina NF 121, Nr. 408, 275 –278 (2015)scenario leads to changes in different properties of the climate system which are noticeable within centennial timescale. For instance, in global precipitation (Fig. 1F). Despite model internal variability, it is clear that state of the Earth system is similar to the RCP4.5 scenario. However, mitigating atmospheric CO2 alone does not lead to an identical climate state.Averages of a 10-year period of pH (left) and the saturation state of carbonate minerals (Ω, calcite) (right) for different time intervals are shown in Figure 2. By the end of this century, whilst global values of pH (see Fig. 1C) would be similar to preindustrial levels, regional values would differ and that has a strong impact on the ocean biogeochemistry. This AOA implementation scenario would prevent the extreme ocean acidi!cation projected for the un-mitigated scenario (RCP8.5); however, high pH and Ω values would be reached leading to a potential impact on marine biota (Cripps et al. 2013).ReferencesBroecker, W. S., and Peng, T.-H.: The cause of the glacial to interglacial atmospheric CO2 change: A polar alkalin-ity hypothesis. Global Biogeochem. Cycles 3/3, 215 –239 (1989)Cripps, G., Widdicombe, S., Spicer, J. I., and Findlay, H. S.: Biological impacts of enhanced alkalinity in Carcinus maenas. Marine Pollution Bull. 71/1–2, 190 –198 (2013)Doney, S. C., Fabry, V. J., Feely, R. A., and Kleypas, J. A.: Ocean acidi!cation: The other CO2 problem. Annu. Rev. Marine Sci. 1/1, 169 –192 (2009)Giorgetta, M. A., Jungclaus, J., Reick, C. H., Legutke, S., Bader, J., Böttinger, M., Brovkin, V., Crueger, T., Esch, M., Fieg, K., Glushak, K., Gayler, V., Haak, H., Hollweg, H. D., Ilyina, T., Kinne, S., Korn-blueh, L., Matei, D., Mauritsen, T., Mikolajewicz, U., Mueller, W., Notz, D., Pithan, F., Raddatz, T., Rast, S., Redler, R., Roeckner, E., Schmidt, H., Schnur, R., Segschneider, J., Six, K. D., Stockhause, M., Timmreck, C., Wegner, J., Widmann, H., Wieners, K. H., Claussen, M., Marotzke, J., and Stevens, B.: Climate and carbon cycle changes from 1850 to 2100 in MPI-ESM simulations for the coupled model intercom-parison project phase 5. J. Adv. Model. Earth Systems 5/3, 572–597 (2013)Ilyina, T., Wolf-Gladrow, D., Munhoven, G., and Heinze, C.: Assessing the potential of calcium-based arti!-cial ocean alkalinization to mitigate rising atmospheric CO2 and ocean acidi!cation. Geophys. Res. Lett. 40, 1– 6 (2013)Kheshgi, H. S.: Sequestering atmospheric carbon dioxide by increasing ocean alkalinity. Energy 20/9, 915 –922 (1995)Paquay, S., and Zeebe, R. E.: Assessing possible consequences of ocean liming on ocean pH, atmospheric CO2 concentration and associated costs. Int. J. Greenhouse Gas Control 17/0, 183 –188 (2013)Royal Society: Geoengineering the Climate: Science, Governance and Uncertainty. London: Royal Society 2009Six, K. D., Kloster, S., Ilyina, T., Archer, S. D., Zhang, K., and Maier-Reimer, E.: Global warming ampli!ed by reduced sulphur #uxes as a result of ocean acidi!cation. Nature Clim. Change 3/11, 975 –978 (2013) Miriam Ferrer-Gonzalez Dr. Tatiana Ilyina Max Planck Institute for Meteorology Max Planck Institute for Meteorology  Bundesstraße 53 Bundesstraße 53 B117  B218 20146 Hamburg 20146 Hamburg Germany Germany Phone: +49 40 41173193 Phone: +49 40 41173164 Fax: +49 40 41173298 Fax: +49 40 41173298 E-Mail: miriam.ferrer-gonzalez@mpimet.mpg.de E-Mail: tatiana.ilyina@mpimet.mpg.de

Mitigation potential, risks, and side-effects of ocean alkalinity enhancement

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Mitigation potential, risks, and side-effects of ocean alkalinity enhancement

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