Multi-month forecasts of marine heatwaves and ocean acidification extremes

Marine heatwaves and ocean acidification extreme events are periods during which temperature and acidification reach statistically extreme levels (90th percentile), relative to normal variability, potentially endangering ecosystems. As the threats from marine heatwaves and ocean acidification extreme events grow with climate change, there is need for skilful predictions of events months to years in advance. Previous work has demonstrated that climate models can predict marine heatwaves up to 12 months in advance in key regions, but forecasting of ocean acidification extreme events has been difficult due to the complexity of the processes leading to extremes and sparse observations. Here we use the Community Earth System Model Seasonal-to-Multiyear Large Ensemble to make predictions of marine heatwaves and two forms of ocean acidification extreme events, as defined by anomalies in hydrogen ion concentration and aragonite saturation state. We show that the ensemble skilfully predicts marine heatwaves and ocean acidification extreme events as defined by aragonite saturation state up to 1 year in advance. Predictive skill for ocean acidification extremes as defined by hydrogen ion concentration is lower, probably reflecting mismatch between model and observed state. Skill is highest in the eastern Pacific, reflecting the predictable contribution of El Ni & ntilde;o/Southern Oscillation to regional variability. A forecast generated in late 2023 during the 2023-2024 El Ni & ntilde;o event finds high likelihood for widespread marine heatwaves and ocean acidification extreme events in 2024. One type of ocean acidification extreme event, as well as marine heatwaves, can be confidently predicted up to 1 year in advance, according to forecasts stemming from an Earth system model ensemble.