Abstract:

Human-induced emissions of greenhouse gases such as carbon dioxide (CO₂) are the main drivers of global warming. Global warming poses a serious threat to the security of food, water resources, energy, economy, and other fields. Alleviating global warming is imperative. Not only does it require massive greenhouse gas emissions reduction, but also large-scale deployment of carbon dioxide removal (CDR) or negative emissions techniques to intentionally remove CO₂ from the air and sequestrate it for a long period so that to decrease global net CO₂ emissions to zero as soon as possible, and achieve "carbon neutrality". The ocean accounts for 70% of the earth's surface area and is the largest active carbon pool. It has a huge potential to absorb CO₂. Ocean-based CDR is necessary to achieve carbon neutrality. The research on the theory, method, and technology of ocean CDR has become a hot spot and frontier field. At present, the knowledge of ocean CDR is still relatively limited, and there is a large space for development. The urgent need to mitigate global warming is promoting the rapid development of the basic theory of marine carbon sinks and ocean CDR research, and original progress is emerging. This paper mainly summarizes the theoretical basis of the Iron-Aluminum Hypothesis and discusses the potential of ocean aluminum fertilization as a CDR strategy. The iron-aluminum hypothesis indicates that aluminum can enhance carbon fixation by phytoplankton in the upper ocean, reduce the decomposition rate of biogenic carbon, improve the efficiency of the biological pump, increase carbon export and sequestration to the deep sea, regulate marine carbon sinks, and affect the concentration of CO₂ in the atmosphere. Thereby, as well as iron, aluminum may be a key factor in influencing historical and modern climate changes. Aluminum improves the efficiency of iron use and carbon export to the deep ocean, which can make up for the shortage of artificial ocean iron fertilization, and endow ocean aluminum fertilization with the potential to become a new CDR method and technology based on natural carbon sinks. Despite its potential high efficiency, ocean aluminum fertilization as a CDR method is still nascent. We suggest further study on the mechanisms underlying the roles of aluminum in enhancing marine carbon sinks from the three aspects 1) carbon fixation by marine phytoplankton in the upper ocean, 2) biogenic carbon export to the deep ocean, and 3) long-term carbon sequestration, and thus to strengthen the theoretical basis of iron-aluminum hypothesis and ocean aluminum fertilization. We also propose to verify the CDR efficacy of ocean aluminum fertilization and its potential environmental impacts at different temporal and spatial scales. The above two works are expected to provide basic scientific knowledge for the development and application of ocean aluminum fertilization as a CDR strategy.

Key words: carbon neutrality, carbon dioxide removal, negative emission, marine carbon sink, iron-aluminum hypothesis, ocean aluminum fertilization