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An insight into future ocean carbon uptake

Christoph Heinze, coordinator of CARBOCHANGE, details the project findings on the quantification of net ocean carbon uptake under different climate change scenarios.

Seas and oceans are a key contributor to the absorption of CO2 emissions to the atmosphere caused by human activities. But does this mean we can rely on this capacity to alter ongoing climate change? And, most importantly, where does the excess CO2 from the atmosphere go to? The CARBOCHANGE project considerably advanced scientific knowledge and predictions on the matter. With the help of a comprehensive network of buoys, floats and research vessels, the EU-funded CARBOCHANGE (Changes in carbon uptake and emissions by oceans in a changing climate) project aimed to provide the best possible quantification of net ocean carbon uptake now and in the future, under different climate change scenarios based on past and present ocean carbon cycle changes. This research is key to understanding the Earth’s future under changing climate conditions, as Earth’s oceans are thought to have absorbed about one quarter of the CO2 humans pumped into the atmosphere over the past 20 years. The flip side of this process is that, as they absorb CO2, oceans also become more acidic with dramatic consequences for sea life. Even worse, oceans might not be able to cope with a future increase in man-made CO2 emissions in the same way they have been doing this so far. The project is now complete and its results not only provide researchers with invaluable data and a global carbon model, but they also improve scientific understanding of key biochemical and physical processes, quantify them, shed light the vulnerability of oceans to increased CO2 intake, help predict future climate and are set to help policy makers in taking concrete actions. Christoph Heinze, Professor in chemical oceanography at the University of Bergen and coordinator of CARBOCHANGE, details some of the key findings of the project. What is your prediction model based on? Our work consisted in quantifying CO2 transfer between the ocean and the atmosphere as well as carbon fluxes within the ocean itself, by means of a combination of observational datasets and models. The observational datasets, which included sea surface data as well as water column data of the oceanic carbon system, were used in combination with predictive and diagnostic models. The Earth system models employed for projections based on emission scenariosare among the most complex and demanding computer programmes human brains ever created. Since the marine carbon cycle is influenced by physics, chemistry, and biological action, an interdisciplinary team was needed to tackle the challenge of quantifying changes in the carbon budget under a changing climate. What are the main findings of the project, in terms of factors strongly affecting oceans’ reduced carbon uptake? There is evidence for substantial regional and temporal variations of the air-sea CO2 fluxes on various time scales, up to an order of magnitude of plus or minus 50 % for certain oceanic domains. The same holds true for variations of oceanic uptake of human-produced CO2 from the atmosphere. However, a transient weakening in ocean carbon uptake in one ocean basin can be compensated by increased uptake at another location. All in all, the annual marine CO2 uptake rates have been able to more or less keep pace with the increase in atmospheric CO2 so far: The annual percentage of new human-caused CO2 emissions taken up by the oceans globally is rather constant. However future projections with Earth system models, either fully fledged complex model systems or so called Earth system models of intermediate complexity, reveal that this may change once emissions of CO2 further accumulate in the atmosphere and climate change accelerates in the coming decades. A more sluggish ocean circulation in combination with a decrease of seawater buffering ability at high sea surface CO2 concentrations will induce a weakening of the ocean CO2 uptake efficiency. Exciting new results include the effect of increasing bacterial decomposition of organic matter in the ocean water column and the decrease of biogenic aerosol emissions to the atmosphere under rising temperatures. Both feedback effects will accelerate global warming. More generally, what would you say are the most groundbreaking achievements of CARBOCHANGE? The project team has achieved a series of key results. I will name just a few. First of all, the team has contributed to Earth system model projections as a basis for the assessment reports of the Intergovernmental Panel on Climate Change (IPCC), to the annual updates of the Global Carbon Budget issued via the Global Carbon Project, and to the most complete and highest quality observational ocean carbon data collections ever collected (SOCAT for the surface ocean and GLODAP for the three-dimensional ocean). CARBOCHANGE has thus been key contributor to huge international research efforts, and this has been acknowledged by our colleagues worldwide. Other results worth mentioning include the discovery that, in order to limit global warming, ocean acidification, ocean de-oxygenation, and land biomass loss, stronger CO2 emission reductions are necessary in comparison to what it would take to tackle global warming only. We also found that progressing ocean acidification is unequivocally affecting also the deep ocean with potential biodiversity loss especially among vulnerable deep-sea organisms. Finally, we provided evidence that combined stressors for marine ecosystems will become increasingly critical towards the coming decades, whereby the evolving hot spots can be estimated from models. Did you face any difficulties during the project and how did you solve them? As scientists we are always operating at the boundaries of our abilities. This makes our profession both tiring and exciting at the same time. We had no major logistical difficulties during this project – the consortium worked extremely constructively. One emerging issue, however, was that the systematic combination of observational data sets with complex ocean models is even more difficult than we expected at the beginning of the project. We could achieve some important improvements of models and new insights into carbon uptake processes through such data assimilation procedures. But in order to use the observations of the present ocean to their full potential, one first would have to systematically calibrate Earth system models for a situation representing the unperturbed pre-industrial world. However, solving this problem will require repeated, highly expensive computer model runs. This is a challenge to be re-addressed in future work. How will seas and oceans be affected by the various climate change scenarios? Here it can clearly be said that smaller amounts of further emissions per year will result in better resilience of the oceans to anthropogenically-induced forcing. Ocean warming and CO2 uptake from the atmosphere develop over long timescales, from decades to tens of millennia. Even if we immediately stopped emitting further CO2 from fossil fuel burning, land use change and cement manufacturing to the atmosphere, the ocean would come back to a quasi-equilibrium both physically and chemically only after several tens of thousands of years. But it is also important to note that projections on marine CO2 uptake under so-called green emission scenarios (with strong, imminent reductions in human-caused CO2 emissions) show that in this case, the remarkable buffer capacity of the ocean for CO2 will come more effectively into play. If human societies could achieve emission levels foreseen in the RCP2.6 or at least the RCP4.5 scenario, damage to the Earth system could be limited. At present the world is still under the ‘business as usual’ RCP8.5 scenario — a development of serious concern. What do you hope will be the impact of your research? Human societies need to move quickly to a sustainable use of resources and a decarbonised energy production. Our research helps find optimal pathways for climate mitigation to tackle global climatic and environmental change. The quantification of the marine carbon sinks and sources with respect to the atmosphere is essential also for keeping track of the fate of human-produced CO2. Where is it ending up, what controls its cycling, have we overlooked key processes so far? We have developed methodologies to observe and simulate the ocean carbon cycle over the long term with a suite of methods which complement each other. These techniques will have to be extended and applied also in the future as CO2 emissions are still increasing strongly. Also with respect to the verification of national greenhouse gas budgets, an accurate knowledge of the ocean carbon sink is essential, as quantifications of air-sea CO2 fluxes of larger areas can be done more precisely than for land areas. Therefore, if we want to understand continental changes, knowledge about the marine realm is key. The quantification of ocean carbon state variables under CARBOCHANGE through models and observations is key to acidification impact studies. Relevant communities now have a wealth of data available to scale their impact experiments. Do you have any plans for follow up research now that the project has ended? We have concrete plans to continue with an extension of our research. This includes new and additional sensors on autonomous floats and gliders for CO2 partial pressure, pH, oxygen and other variables, and also an extension and continuous support of routine carbon measurements from commercial ships. We are already busy upgrading our ocean models with improved process representations for the next round of climate projections for international assessments. New versions of the international ocean carbon data syntheses are already on their way, and we see also great potential in developing new concepts for marine ecosystem modelling. A key issue for the coming years will be to improve annual carbon budgets both globally, but also basin wide and nationally. We are developing new approaches for optimally estimating the progressing CO2 invasion into the ocean and also how the corresponding air-sea CO2 surface fluxes vary. Bringing down uncertainties in these variables will contribute to a sound greenhouse gas budget verification system of high political relevance for the coming 10 years. Collaborative projects coordinated and carried out at EU level have proven an efficient means of pushing the limits in greenhouse gas research and to optimally exploit their results towards a hopefully sustainable future. For further information, please visit: CARBOCHANGE



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