Estimates suggest that by 2050, more than 4 gigatons of carbon dioxide (Gt-CO) will be removed from the atmosphere. ₂ year ^-1 ) per year to meet international climate targets. One strategy for carbon dioxide removal is seaweed cultivation; however, its global potential remains highly uncertain. Here, we apply a dynamic model of seaweed growth that includes growth-limiting mechanisms such as nitrate supply to estimate the global potential yield of four seaweed species. We estimate that harvesting 1 Gt per year ^-1 of carbon from seaweed would require the management of more than 1 million km ² the most productive exclusive economic zones, located in the equatorial Pacific Ocean; the cultivation area would have to triple to achieve an additional 1 Gt per year ^-1of harvested carbon, suggesting a dramatic reduction in carbon sequestration efficiency beyond the most productive waters. Improving the accuracy of annual harvest estimates requires a better understanding of biophysical constraints such as the rate of seaweed loss (e.g., infestation, disease, grazing, wave erosion). (Isabella B. Arzeno-Soltero, Benjamin T. Saenz, Kristen A. Davis)