Ensemble modeling of the coupled carbon cycle-climate system

Frölicher, Thomas L. (2009). Ensemble modeling of the coupled carbon cycle-climate system (Unpublished). (Dissertation, Universität Bern, Philosophisch–naturwissenschaftliche Fakultät, Physikalisches Institut, Abteilung für Klima– und Umweltphysik)

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The overarching theme of this thesis is the impact of anthropogenic carbon emissions on the coupled carbon cycle-climate system. This topic is connected to three specific research questions: (i) the role of internal unforced variability, natural forced variability by explosive volcanism and anthropogenic changes, (ii) the understanding of the long-term consequences of anthropogenic carbon and greenhouse gas emissions, and (iii) the impact of ongoing oceanic carbon uptake and climate change on ocean acidification and dissolved oxygen (O2) depletion.
Anthropogenic emissions of carbon dioxide (CO2) and other forcing agents force the atmospheric composition, Earth’s climate, and the chemical state of the ocean towards conditions that have probably not occurred over the past 20 million years (Blackford and Gilbert, 2007) and at a speed that is unprecedented at least during the last 22’000 years (Joos and Spahni, 2008). Today’s atmospheric CO2 concentration of 386 ppm is above the natural range (172 to 300 ppm) of the last 800’000 years (Lüthi et al., 2008). Climate impacts of future anthropogenic CO2 release will persist for millennia (Archer et al., 2009) and poses a significant risk to the human society. This raises questions of adaption to and mitigation of climate change. Besides global warming, anthropogenic carbon emissions cause ocean acidification and oxygen depletion (Meehl et al., 2007; Denman et al., 2007). These processes are less discussed than climate change but potentially with equally far-reaching consequences.
A major challenge in climate research is the uncertainty associated with the high level of natural variability in the coupled carbon cycle-climate system. It is still ambitious to identify a current anthropogenic trend in the marine carbon cycle. Understanding and quantifying mechanisms of the carbon cycle-climate system variability is therefore crucial for interpreting observed changes in the context of natural and anthropogenic changes.
The publications presented in this study are based on ensemble simulations with the National Center for Atmospheric Research CSM1.4-carbon model. The comprehensive fully coupled carbon cycle-climate model allows us to investigate current and future climate change and variability in a spatially and temporally resolved setting. The use of ensemble simulations, each forced by identical boundary conditions but started at different initial conditions is key to extract the climate signal in response to a certain external forcing from the natural internal variability of the climate system. Ensemble simulations with complex climate models prove to be a powerful approach to tackle problems which cannot be answered with intermediate-complexity models or observation-based estimates.
The first chapter gives an overview of natural variability in the coupled carbon cycle-climate system and its perturbation through human activities followed by a brief description of the marine carbon cycle. In a second part possible consequences of ongoing anthropogenic carbon emissions - ocean deoxygenation and acidification - are highlighted. In the framework of climate modeling, fully coupled carbon cycle-climate models and their application are briefly introduced.
Chapter 2 presents the NCAR CSM1.4-carbon model, the experimental design and the applied forcings. The model drift of the control simulation is discussed.
In chapter 3, the impact of natural variability on the detection of anthropogenic trends in oceanic oxygen is investigated (Fr¨olicher et al., 2009). Dissolved oxygen concentration is a key quantity for ocean biogeochemistry and ecology, because it is a particularly sensitive indicator of change in ocean transport and biology (Joos et al., 2003). A six-member ensemble simulation with the NCAR CSM1.4-carbon model was performed over the period 1820 to 2100. The results show that simulated anthropogenically forced O2 decrease is partly compensated by volcanic eruptions, which cause considerable interannual to decadal variability. While well identified on global scales, the detection and attribution of local O2 changes to volcanic forcing is difficult because of unforced variability. Oxygen variability in the North Atlantic and North Pacific are associated with changes in the North Atlantic Oscillation and Pacific Decadal Oscillation indices. The future part of the global warming simulations shows that the oceanic O2 inventory is projected to significantly decrease over the next century, in agreement with earlier work (e.g. Plattner et al. (2002)). It is concluded that under the large interannual to decadal variations in oceanic oxygen and the limited data availability the detection of human-induced O2 changes is still challenging.
Chapter 4 presents a study examining reversible and irreversible impacts of anthropogenic carbon and greenhouse gas emissions on human timescales (Frölicher and Joos, 2009). Three illustrative commitment scenarios are performed to analyze the climate response and associated regional changes in ocean biogeochemistry to an abrupt cessation of anthropogenic emissions. In contrast to earlier studies, the changes are compared with natural, unforced variability on a regional scale. The study shows that carbon emissions of the 21st century have irreversible impacts on centennial to millennial timescales on temperature, precipitation, sea ice, and carbonate chemistry. System time lags and physical-biogeochemical coupling often cause largest impacts to accrue after emissions have been stopped. However, regional perturbations in temperature and precipitation by historical emissions and by non-CO2 agents are largely reversible within centuries. In the lights of these results, it is concluded that emission trading schemes, related to the Kyoto Process, should not permit trading between emissions of relatively short-lived agents and CO2 given the irreversible impacts of anthropogenic carbon emissions.
Chapter 5 features a study, which examines ocean acidification and the increase in the volume of undersaturated water with respect to aragonite and calcite over the industrial period and over this century (Steinacher et al., 2009). It is shown that water saturated by more than 300%, considered suitable for coral growth, vanishes by year 2070 (CO2 is about 630 ppm) and the ocean volume fraction with saturated water decreases. Largest simulated pH changes worldwide occur in Arctic surface waters, where projected climate change amplifies the decrease in mean saturation and pH by more that 20%, mainly due to freshening and increase carbon uptake in response to sea ice retreat. Results of this study emphasize that surface waters in the Arctic Ocean will become corrosive to aragonite, with potentially large implications for the marine ecosystem, if anthropogenic carbon emissions are not reduced and atmospheric CO2 not kept below 450 ppm.
Chapter 6 includes the study of Schneider et al. (2008), where the present-day spatial and temporal variability in net primary production and export production of particulate organic carbon are investigated. Results from the CSM1.4-carbon model are compared to observation-based data and results form inverse modeling, and to results from two other comprehensive 3D coupled carbon cycle-climate models. The model comparison shows that the temporal variability of primary production on the global scale is largely dominated by the permanently stratified, low-latitude ocean. However, an adequate representation of iron and macronutrient colimitation of phytoplankton growth in the tropical ocean is crucial in determining the capability of the models to reproduce observed interactions between climate and primary production.
An outlook regarding future research tasks in the framework of coupled carbon cycle-climate modeling is given in chapter 7. Finally, the appendix provides some useful technical aspects of running the CSM1.4-carbon and its newer version on different supercomputing mainframes. Benchmark tests and the implementation of different external forcings into the model code are presented.
In summary, this thesis contributes to the understanding of the coupled carbon cycle-climate system and reveals important insights into the variability of the marine biogeochemical cycle. However, the complexity of the carbon cycle-climate system and the multiple interactions that determine its behavior impose still limitations on our ability to understand it. Nevertheless, preventing ocean acidification and oxygen holes provide another good reason, besides climate change, to quickly reduce carbon emissions.

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