Ocean extremes as a stress test for marine ecosystems and society

In 2023–2024, widespread marine heatwaves associated with record ocean temperatures impacted ocean processes, marine species, ecosystems and coastal communities, with economic consequences. Despite warnings, interventions were limited. Proactive strategies are needed for inevitable future events.

The global oceans are warming at an unprecedented rate, and temperatures in 2023 and 2024 were exceptionally warm1. By March 2023, globally averaged sea surface temperatures (SSTs) had reached record highs and temperature extremes were recorded repeatedly over the following year1. This rapid warming, fuelled by human-induced climate change2, but amplified by El Niño conditions and record low global cloud cover, resulted in an unprecedented number of extreme marine heatwaves (MHWs), with global average MHW days in the summers of 2023–2024 being 240% higher than in any other year in the instrumental record (Fig. 1). These events spanned the globe, with 8.8% of the ocean experiencing the highest SSTs ever recorded, almost four times more than the historical annual average.