Ocean iron fertilization or artificial ocean alkalinization? Study reveals divergent effects on climate

Global warming poses a significant threat to human society. Rapid and substantial reductions in greenhouse gas emissions are necessary measures to mitigate global warming; however, substantially reduced emissions alone may not be sufficient to achieve the temperature control targets of the Paris Agreement. Carbon dioxide removal (CDR) is a crucial approach to potentially curb global warming, the core idea of which is to artificially increase terrestrial or marine carbon sinks and reduce atmospheric CO₂ concentrations, thereby achieving global cooling. However, the similarities and differences in the impacts of different CDR methods on the carbon cycle and climate system represent an important open question that holds significant research value.

Currently, studying the climate system’s response to artificial CDR methods relies primarily on Earth system models. With this in mind, researchers from Laoshan Laboratory and Zhejiang University conducted a simulation study using an Earth system model to comparatively analyze the similarities and differences in the impacts of two marine-based CDR methods—ocean iron fertilization (OIF) and artificial ocean alkalinization (AOA)—on the climate system and carbon cycle. The findings were recently published in Atmospheric and Oceanic Science Letters.

The study reveals that OIF promotes marine carbon sinks by adding iron to “high-nutrient, low-chlorophyll” marine regions (e.g., the Southern Ocean), which enhances the photosynthesis of surface phytoplankton and thereby increases marine carbon sinks. In contrast, AOA improves seawater alkalinity by adding alkaline materials (such as limestone) to the ocean; through carbonate chemical processes, this method drives the ocean to absorb more CO₂ from the atmosphere.

In this work, under the RCP8.5 scenario, which represents high CO₂ emissions in the future, the researchers designed two sets of simulation experiments to ensure that OIF and AOA promoted the global ocean to absorb an equivalent amount of atmospheric CO₂. The simulation results showed that under the same carbon reduction target, due to the fundamental differences in the mechanisms by which the two methods enhance marine carbon sinks, their impacts on the marine carbon cycle system also differed.

The study indicated that, compared with the RCP8.5 high-emissions scenario, OIF could effectively increase marine net primary productivity while enhancing the absorption of atmospheric CO₂ by marine plankton, thereby reducing atmospheric CO₂ concentrations. However, this method also exacerbates deep-ocean acidification, leading to a decrease in global marine oxygen content. In contrast, AOA increases marine carbon sinks through inorganic carbon chemical processes. While reducing atmospheric CO₂ concentrations and achieving global cooling, the increase in seawater alkalinity raises the seawater pH value, which can effectively alleviate acidification in both surface- and deep-ocean layers. Additionally, its impact on the global marine oxygen content is relatively minor.

This study demonstrates that, even when the same target of increasing marine carbon sinks is achieved, different artificial CDR methods exert distinct impacts on the physical and biogeochemical processes of the Earth system due to differences in their mechanisms. A strong grasp of the similarities and differences in the impacts of different CDR methods on the climate system and carbon cycle is of great significance for deepening our understanding of the advantages and disadvantages of different climate change mitigation measures.