Global warming driven by the emission of greenhouse gases is threatening all sectors of human society. Carbon dioxide (CO₂) removal (CDR) besides emission reduction is needed to limit global warming to below 2^oC compared to that before the pre-industrial revolution. The ocean is the largest active carbon pool. It has a huge potential for removing CO₂ from the air. Here I report the effects of aluminum (Al) on the marine carbon sink and the possibility of ocean Al fertilization as an ocean-based CDR. Our studies show that Al could favor the growth of phytoplankton by facilitating the utilization of dissolved organic phosphorus, iron (Fe), and dinitrogen, and in this manner enhance carbon fixation in the upper ocean. Also, Al may preserve biogenic matter from decay and decomposition, which would facilitate the export of the fixed carbon to ocean depths and its sequestration there. In addition, the Antarctic ice core records show that atmospheric CO₂ was low when the input of dust Al and Fe was abundant over the past 800,000 years. Therefore, we have proposed the Iron-Aluminum Hypothesis based on the Iron Hypothesis, to highlight the importance of Al in marine carbon sink and climate change.

Our new data suggest that the addition of Al at environmentally relevant levels (40 to 200 nM) increased the net carbon fixation by marine diatoms (up to 30%) and decreased their subsequent decomposition (by up to >50%). Based on these results, a simple calculation suggests that the addition of environmentally relevant levels of Al will lead to 1 to 3 orders of magnitude increases in the particulate organic carbon exported to 1000 m. Increases of Al in the range from 20 to 200 nM in the upper ocean might have occurred in glacial times, as a consequence of the significant increase in the dust deposition over the world ocean during that period. These results implicate that Al may have significantly increased carbon export to the ocean interior over the world ocean during the glacial period and contributed to the glacial climate. Therefore, we suggest that ocean Al fertilization may help ocean Fe fertilization by increasing Fe utilization efficiency and facilitating carbon export and sequestration in deep ocean depths. Ocean Al fertilization could be a potential CDR strategy used alone or together with other CDRs such as ocean Fe fertilization, artificial upwelling, and alkalinity enhancement by adding minerals, to alleviate global warming.