The case against renewed ocean iron fertilization field trials

Buesseler et al. call for larger and longer ocean iron fertilization (OIF) trials to assess climate-relevant carbon dioxide removal potential. We argue that such trials may document local ecosystem responses but cannot resolve the two central uncertainties for climate policy: removal potential and downstream consequences. Net atmospheric CO 2 removal depends on carbon export, air–sea re-equilibration, circulation, downstream nutrient redistribution, and ecosystem responses occurring beyond trial boundaries. Global models representing these processes consistently find removal potential to be low, and field trials cannot overturn that evidence. The same models identify delayed and far-field impacts on oxygen and ecosystems under basin-scale OIF, which trials can neither evaluate nor prevent, deferring judgment to the models they are intended to improve. Progressively larger OIF trials are therefore unwarranted. Research should instead prioritize observations of naturally fertilized systems and better models, which could test the controls on OIF carbon yield without deliberate perturbation.