Prof Jason Hall-Spencer uses natural gradients in seawater carbon dioxide to assess the likely ecological effects of ocean acidification. With colleagues at Tsukuba University, he recently compared intertidal and subtidal marine communities at increasing levels of pCO2 at volcanic seeps off Japan (34° N). This region is of interest as it has naturally low levels of surface seawater pCO2(280–320 μatm). Marine communities exposed to mean levels of pCO2 predicted by 2050 experienced periods of low aragonite saturation and high dissolved inorganic carbon. These two factors combined to cause a major decline in biodiversity, including the loss of key habitat-forming species, with even more extreme community changes expected by 2100. These results provide empirical evidence that ocean acidification has already had impacts worldwide, and that near-future levels of pCO2 shift the coastal ecosystems that typify south China to fleshy algal dominated systems, accompanied a major simplification of the ecosystem. Knock-on effects of these changes on fish communities and seawater oxygen levels will be discussed.