Surface and subsurface compound marine heatwave and biogeochemical extremes under climate change

This is the accepted version. The published version is available on this DOI: https://doi.org/10.1029/2025GB008514 

Abstract:

Marine species are increasingly threatened by extreme and compound events, as warming,deoxygenation, and acidification unfold. Yet, the surface and especially the subsurface distribution andevolution of such compound events remain poorly understood. We present the current and projecteddistributions of compound marine heatwave (MHW), low oxygen (LOX), and high acidity (OAX) eventsthroughout the water column, using observation‐based data from 2004 to 2019 and large ensemble Earth systemmodel simulations from 1890 to 2100. Our findings reveal that compound MHW‐OAX and OAX‐LOX eventsare prevalent in the low to mid latitudes at the ocean surface. At 200 and 600 m, MHW‐OAX and MHW‐LOXevents are frequent in the high latitudes and parts of the tropics, while OAX‐LOX events occur globally. Subsurface compound events are often associated with vertical displacements of water masses, with theclimatological vertical gradients of ecosystem stressors typically explaining their occurrence patterns.Projections show a strong rise in compound event frequency over the historical period and under continuedglobal warming, primarily driven by shifts in mean oceanic conditions. The portion of the top 2,000 m affectedby extreme or compound events rises from 20% to 98% under 2°C of global warming in a high emissionsscenario using a preindustrial baseline, and to 30% using a shifting‐mean baseline. However, physical andbiogeochemical changes may also lead to regional decreases in subsurface events, highlighting complexities inhow warming, deoxygenation, and acidification unfold in the ocean interior. Increasing compound eventfrequency poses a major threat to marine ecosystems, potentially disrupting food webs and biodiversity.

Citation: Le Grix, N., Burger, F. A., & Frölicher, T.L. (2025). Surface and subsurface compound marine heatwave and biogeochemical extremes under Climate change. Global Biogeochemical Cycles, 39, e2025GB008514. https://doi.org/10.1029/2025GB008514