Is ocean iron fertilisation research trapped in a dilemma of scales?

Ocean iron fertilisation has re-emerged as a candidate marine carbon dioxide removal approach, leading to proposals for larger and longer field experiments. Here, I argue that iron fertilisation research may be trapped in a dilemma of scales. The three critical bottlenecks – (1) predicting plankton community responses and carbon export, (2) quantifying nutrient reallocation and carbon additionality, and (3) assessing deep-ocean oxygen loss – occur at scales larger than those accessible even to proposed meso-scale experiments. While such studies would undoubtedly improve our understanding of ocean biogeochemistry, their outcomes may remain contingent on ecological conditions and too limited to predict deployment-scale effectiveness and environmental risk. Rather than continuing to scale experiments in space and time, I propose a complementary strategy that scales replication by conducting many experiments across diverse ecological and oceanographic settings. This approach may more likely identify reproducible patterns, improve model parameterisations, and support credible estimates of fertilisation performance and environmental consequences.