Surface and Subsurface Compound MHW and Biogeochemical Extremes Under Climate Change
Abstract
Overview Climate coffee with Natacha Legrix De La Salle (Univ. Liverpool) held on 27 November 2025 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 and evolution of such compound events remain poorly understood. We present the current and projected distributions of compound marine heatwave (MHW), low oxygen (LOX), and high acidity (OAX) events throughout the water column, using observation‐based data from 2004 to 2019 and large ensemble Earth system model simulations from 1890 to 2100. Our findings reveal that compound MHW‐OAX and OAX‐LOX events are prevalent in the low to mid latitudes at the ocean surface. At 200 and 600 m, MHW‐OAX and MHW‐LOX events 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 the climatological 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 continued global warming, primarily driven by shifts in mean oceanic conditions. The portion of the top 2,000 m affected by extreme or compound events rises from 20% to 98% under 2°C of global warming in a high emissions scenario using a preindustrial baseline, and to 30% using a shifting‐mean baseline. However, physical and biogeochemical changes may lead to regional decreases in subsurface events, highlighting complexities in how warming, deoxygenation, and ocean acidification unfold in the ocean interior. Increasing compound event frequency poses a major threat to marine ecosystems, potentially disrupting food webs and biodiversity. Paper https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2025GB008514 Our speaker Natacha Legrix De La Salle (Univ. Bern) What is a Climate Coffee? #climatecoffees are short (circa 40 min: 20 min talk + 20 min Q&A), relaxed meetings for scientists to share ideas, discuss methods, and communicate new results. They are open to speakers of all seniority; we especially encourage early-career scientists to become speakers. The Coffees are an exciting opportunity for scientists to build a network and disseminate recent results peer-to-peer. We invite researchers from across the climate science community to join us for this series of regular online knowledge exchange events. Organisation The Climate Coffees are organized by the Horizon Europe projects TipESM, ObsSea4Clim, OCEAN ICE, the National Center for Climate Research - DMI of the Danish Meteorological Institute and the European Climate Research Alliance. Disclaimer Funded by the European Union. Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or is agencies CINEA and REA. Neither the European Union nor the granting authority can be held responsible for them.