Compound extremes are of concern for ocean health, such as when ocean acidification extremes (OAX) and marine heat waves (MHW) co-occur. These compound events (OAX∩MHW) may amplify stress beyond the impact of each driver alone, yet their historical distribution remains poorly quantified. We used an observation-based product (OceanSODA-ETHZ) to investigate surface ocean OAX∩MHW from 1982-2020. OAX and MHW are defined when detrended surface hydrogen ion concentration and sea-surface temperature exceed their 95th percentiles. Events show distinct spatial and temporal patterns: they occur ten times more likely than by chance in the low-to-mid latitudes, but are rare in the eastern tropical Pacific and the high-latitudes. They occur primarily in summer and show strong variability associated with the El Niño-Southern Oscillation. Most events are small (< 106 km2) and brief (1 month), but several are exceptional, including: the Blob (2015) in the Northeast Pacific (8.2 × 106 km2), a year-long event spanning the equatorial Indian Ocean (2015-2016), and the most intense event in the Tasman Sea (2017). Two-thirds of such large and in-tense events occurred after 2008. Mechanistically, OAX∩MHW occur when warming-induced increases in [H+] are not offset by a reduction in dissolved inorganic carbon that typically accompanies MHW. This is typical of the permanently stratified low-to-mid latitude oceans, where the seasonal cycle of [H+] is controlled by temperature. Through characterizing past compound extremes and improving our understanding of these individual events, we highlight conditions that may lead to future ecosystems being at risk.
Gregor L. & Gruber N., 2025. Recent history of surface ocean acidification extremes that compound marine heat waves. ESS Open Archive. Article.
