Recent History of Surface Ocean Acidification Extremes That Compound Marine Heatwaves

Abstract

Compound extremes are of concern for ocean health, such as when ocean acidification extremes (OAX) and marine heatwaves (MHW) co-occur. These compound events (OAX$\cap$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$\cap$MHW from 1982 to 2024. 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 roughly four times more often 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 ( ${\text{km}}^2$) and brief (1 month), but several are exceptional, including: the Blob (2015) in the northeastern Pacific ($12.6\times 10^6$ ${\text{km}}^2$), the North Atlantic marine heatwave lasting nearly a year (2023–2024), and the most intense event off the Western Australian coast (2011). Mechanistically, OAX$\cap$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. By characterizing past compound extremes and improving our understanding of individual events, we highlight conditions that may lead to future ecosystems being at risk.