Next generation ocean iron fertilization: Are there hotspots in the Southern Ocean that could sustain long-term removal of atmospheric CO2?

Meeting the goal of keeping global warming below 2°C requires not only rapid decarbonization but must be supported by actively pulling hundreds of gigatons of CO₂ out of the atmosphere. Ocean iron fertilization (OIF) initially sparked hopes of being capable to do this “job” but research over the past 2 decades has revealed a suite of uncertainties and potential side-effects that have dampened early enthusiasm. By working through a checklist of the most important physical, chemical, and biological criteria we map optimal regions in the Southern Ocean where OIF could lead to sustainable long-term CO₂ removal. These regions are relatively close to Antarctica where Fe-fertilized waters and forming blooms need to: 1) enable net biomass build-up under the given irradiance and grazing conditions; (2) rely on pre-formed nutrients that would otherwise be down-welled near Antarctica without being utilized by biology; (3) provide sufficient time for atmospheric CO₂ in-gassing in the aftermath of the bloom; (4) be entrained by the ocean circulation into Antarctic Bottom Water so that atmospheric CO₂ is locked in the deep oceans for century-to-millennial timescales; (5) pose little risk for negative side effects such as subsurface ocean deoxygenation or nutrient robbing. Our systematic analysis conforms with the current view that OIF will not be effective in large parts of the Southern Ocean and should not be seen as a “silver bullet” to counter climate change. However, it points towards high regional potential for atmospheric CO₂ removal which calls for a “next generation” of OIF in situ pilot studies near Antarctica. Indeed, our preliminary results indicate that OIF could become an efficient and sustainable smaller-scale component within a global diverse portfolio of different negative emission technologies.