Year-round marine heat extremes severely impact marine life during the first year at 1.5°C warming

Periods of anomalous ocean heat are becoming more intense and persistent, with consequences for biodiversity, food systems, and coastal resilience. Yet ocean impact assessments often emphasize summer, creating a seasonal bias that can underrepresent the year-round consequences of marine heat extremes. In 2023–2024, ocean surface warming exceeded 1.0°C above pre-industrial levels, coinciding with year-round marine heat extremes across much of the ocean. Here, we provide a rapid global assessment of marine impacts during this exceptional year. We show that summer impacts were frequent and severe, but 29% of non-tropical impacts occurred in cooler seasons. Events involving compound hazards occurred across seasons, though their timing varied by hazard type. Summer assessments alone are therefore insufficient to capture when, where, and how ocean heat threatens marine ecosystems. Rapid, year-round assessments are needed to understand how anomalous heat affects marine ecosystems and dependent societies, while informing preparedness plans for inevitable future extremes.