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 Niño/Southern Oscillation to regional variability. A forecast generated in late 2023 during the 2023–2024 El Niño event finds high likelihood for widespread marine heatwaves and ocean acidification extreme events in 2024.

海洋热浪和海洋酸化极端事件是指温度和酸化相对于正常变化达到统计极端水平（第 90 个百分位）的时期，可能危及生态系统。随着气候变化，海洋热浪和海洋酸化极端事件的威胁越来越大，因此需要提前数月到数年对事件进行巧妙的预测。以前的工作表明，气候模型可以提前 12 个月预测关键地区的海洋热浪，但由于导致极端事件的过程复杂且观测稀疏，预测海洋酸化极端事件一直很困难。在这里，我们使用社区地球系统模型季节性到多年期大型集合来预测海洋热浪和两种形式的海洋酸化极端事件，由氢离子浓度和文石饱和状态的异常定义。我们表明，该集合巧妙地提前 1 年预测了由文石饱和状态定义的海洋热浪和海洋酸化极端事件。由氢离子浓度定义的海洋酸化极端事件的预测能力较低，可能反映了模型和观测状态之间的不匹配。东太平洋的技能最高，反映了厄尔尼诺/南方涛动对区域变率的可预测贡献。在 2023-2024 年厄尔尼诺事件期间生成的 2023 年底预测发现，2024 年发生大范围海洋热浪和海洋酸化极端事件的可能性很高。