KEY HEADLINES • Global-scale patterns and processes of ocean acidification are superimposed on other factors influencing seawater chemistry over local to regional space scales, and hourly to seasonal time scales. • Future ocean conditions will depend on future CO2 emissions; there is now international agreement that these should be reduced to net zero, thereby reducing the consequences of both climate change and ocean acidification. • Assessments of ocean acidification by the Intergovernmental Panel on Climate Change (IPCC) gave high or very high confidence to chemical aspects, but a much wider range of confidence levels to projected biological and biogeochemical impacts. Biotic impacts will depend on species-specific responses, interactions with other stressors and food-web effects. • Previous MCCIP statements are considered to still be valid, with increased confidence for some aspects. • Observed pH decreases in the North Sea (over 30 years) and at coastal UK sites (over 6 years) seem more rapid than in the North Atlantic as a whole. However, shelf sea and coastal data sets show high variability over a range of timescales, and factors affecting that variability need to be much better understood. • UK research on ocean acidification has been productive and influential. There is no shortage of important and interesting topic areas that would improve scientific knowledge and deliver societally-important outcomes
