Oxygen and other biogeochemical variables in global warming projections

Cocco, Valentina (2012). Oxygen and other biogeochemical variables in global warming projections (Unpublished). (Dissertation, Universität Bern, Philosophisch–naturwissenschaftliche Fakultät, Physikalisches Institut, Abteilung für Klima– und Umweltphysik)

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Measurements and reconstructions of the past atmospheric carbon dioxide (CO2) concentrations reveal that mankind has emitted large amount of CO2 into the atmosphere, which have brought the atmospheric CO2 concentrations to values 40% higher since preindustrial times. The severe effects of human greenhouse gas emissions on climate affect many relevant socio-economical aspects, such as agriculture, public health, and water, food and energy supply. Since the marine biogeochemical cycle deeply influences the atmospheric abundance of radiative agents, the role of the ocean is crucial to understand the anthropogenic influence on the climate system. This thesis provides a contribution to the understanding of the coupled climate-carbon cycle system, with particular attention on ocean biogeochemistry. A main focus of this work lies on oceanic oxygen (O2) depletion as simulated for the present time and for the future by a suite of Earth System Models. Such models represent an important tool to understand the future evolution of the climate system and have been used here to investigate 21st-century changes under the high-emission scenario SRES A2.
Chapter 1 provides an introduction to the three peer reviewed studies presented in the thesis, giving some general information about the global carbon cycle as it naturally varied in the past and as it has been perturbed by human activities. In particular, the chapter focuses on the effects of rising atmospheric concentrations of carbon dioxide (CO2) and climate change on ocean biogeochemistry. Important concepts such as the oceanic CO2 uptake, the three main marine carbon pumps, ocean deoxigenation, and other effects of climate change on the oceans are illustrated.
In Chapter 2 an introduction to climate modeling and model intercomparisons is provided. Global climate models with atmosphere and ocean components coupled together (AOGCMs) have developed over the past few decades as computing power has increased. AOGCM intercomparison activities conducted through coordinated numerical simulations designed to investigate the two-way climate change-carbon cycle interaction have been receiving increasing attention in the last years and the work done during this thesis belongs to this framework. The CCSM3-carbon model (run at the Swiss Supercomputing Centre in Manno during this thesis) is described and details about the simulations, the simulation set up, the emission scenarios, and the forcing applied are given. Further, the calculation of the fugacity of O2 (fO2) is introduced. This variable represents the direct thermodynamically-based measure of the driving force for transferring O2 from the environment through the living tissues and plays a central role in the study presented in Chapter 3.
Chapter 3 presents a study investigating decadal-to-century scale trends in O2 and a range of other marine environmental variables in the thermocline. Results from seven comprehensive global coupled carbon cycle-climate models featuring representations of marine ecosystems of different complexity and structure and forced by a common high greenhouse gas emission scenario (SRES A2) are compared. The models as a class represent the observation-based distribution of fO2, the fugacity of carbon dioxide (fCO2), and the logarithm of their ratio, i.e. the Respiration Index (RI), even if mismatches between observation-based and simulated distributions remain for individual models. All models project a decrease in upper ocean pH and in the aragonite saturation state, and a decrease in the total ocean inventory of dissolved O2 by 2% to 4%. Projected fO2 changes in the thermocline show a complex pattern with both increasing and decreasing trends reflecting the subtle balance of different competing factors such as circulation, production, remineralisation, and temperature changes. While a widespread increase of fCO2 in the thermocline is projected, the projected changes in the total volume of hypoxic and suboxic waters remain relatively small in all models. It is concluded that current coupled carbon cycle-climate models project large uncertainties about the expected future O2 decrease in the ocean, and diverging scenarios about the next-decade fate of low-oxygen regions are simulated by the different models.
The highest column inventory of anthropogenic carbon is found in the North Atlantic. This basin alone is able to store 23% of the global ocean anthropogenic carbon content and is therefore of great importance for the global carbon cycle. Climate modes such as the North Atlantic Oscillation (NAO) represent the internal variability of the climate system and influence the oceanic carbon cycle, possibly masking trends in the anthropogenic carbon sink. A relationship between the North Atlantic carbon sink and the North Atlantic Oscillation (NAO) has been recently confirmed in the observations and in the modeling study presented in Chapter 4. Analyzing control runs from six fully coupled Earth System Models (the same model used in the study of Chapter 3), the response of the ocean carbon cycle to the NAO is quantified. The dominating response appears to be a seesaw pattern between the subtropical gyre and the subpolar Northern Atlantic, which is instantaneous and dynamically consistent with the observations, in all models and for a range of physical and biogeochemical variables. Wind-driven dynamics is indicated as the main driver of the response to the NAO. Vertical mixing, upwelling and the associated entrainment of dissolved inorganic carbon and nutrients leave an imprint on surface pCO2, air-sea CO2 flux, and on biological export production, pH and calcium carbonate saturation state.
Marine productivity plays a crucial role for climate as it influences the oceanic CO2 uptake and hence the abundance of atmospheric CO2, but its simulation in general circulation models remains challenging. In Chapter 5, the same simulations with four of the models considered in Chapter 3 and 4 are analyzed to investigate the ability of the models to represent the present spatio-temporal pattern of net primary production and to project its future changes under the high emission scenario SRES A2. All the four models, despite differences in magnitude, project a decrease in productivity over the 21st century. Two main mechanisms are identified for this decrease at different latitudes: in the North Atlantic and in the low- and mid-latitude the reduced nutrient supply to the productive zone in response to a shallower mixed layer depth and a slowed circulation is identified as the dominant mechanism. A productivity increase, on the other hand, is projected to take place in part of the Southern Ocean as a result of an alleviation of light and/or temperature limitation. Furthermore, a method based on regional model skill metrics is developed to generate weighted multi-model means of projected changes in productivity, giving a -8% decrease in net primary production by the end of the 21st century.
Finally, the outlook highlights research challenges and further topics for a future continuation of the work done during this thesis.

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