Environmental context. A ‘climate stabilising’ feedback system known as the CLAW hypothesis, which involves the phytoplankton driven influence on cloud reflectivity through the cycling of sulfur was proposed ~20 years ago, and because of its complexity, it remains unproven today. Since the CLAW proposal, experiments that have added iron to the ocean have proven that iron can significantly limit phytoplankton productivity and can also affect the marine sulfur cycle in a complex manner. Because of a range of possible feedbacks between iron, sulfur and climate, it is likely that future advances in understanding the CLAW hypothesis will require a comprehensive process-based description that can be tested in fully coupled earth-system models.