1529. Year-Round Marine Heatwaves as an Emerging Risk: Impacts on Marine Ecosystems at ~1.5°C Warming

In recent years, anomalously high ocean temperatures—commonly referred to as marine heatwaves—have shown clear trends of expanding spatial extent, increasing intensity, and longer duration. These changes are already having significant impacts on marine ecosystems and the societies that depend on them. A recent study published in One Earth examines the record-breaking warm period from 2023 to 2024 and demonstrates that these impacts are occurring throughout the year rather than being confined to specific seasons.

According to the study, this period was characterized by unprecedented sea surface temperatures, alongside a sustained phase in which global mean temperatures hovered around 1.5°C above pre-industrial levels. These conditions represent an exceptional climate state. Traditionally, the impacts of marine heatwaves have been assumed to peak during summer months; however, the study challenges this assumption. It shows that marine heatwaves occur year-round, with approximately 30% of documented impacts outside the tropics taking place during seasons previously considered relatively cool.

The study further analyzes 201 impact events reported globally and finds that the vast majority were strongly associated with anomalously high temperatures. Notably, more than half of the documented cases involved mass mortality of marine organisms. These impacts include large-scale coral bleaching and ecosystem collapse, declines in seagrass and kelp habitats, mass mortality of fish and shellfish, and even breeding failures among polar species such as penguins, illustrating the wide-ranging consequences across diverse ecosystems.

In addition, the study highlights that these impacts are often not driven by heat stress alone. Instead, they frequently occur as compound events, where extreme temperatures coincide with other stressors such as low-oxygen conditions, extreme weather events, disease outbreaks, or harmful algal blooms. These overlapping hazards can amplify risks and exacerbate ecological damage.

Importantly, the study points out that traditional assessment approaches focusing primarily on summer conditions may fail to capture the full extent of marine heatwave impacts. Given that ecological responses and biological processes vary by season, anomalous heat outside the warmest months may trigger impacts that have been previously underrecognized.

Taken together, the study provides important insights into how marine ecosystems may respond under a climate regime approaching 1.5°C of warming. As the ocean plays a central role in the Earth system and supports food security, coastal economies, and biodiversity, addressing these changes is an urgent priority. Strengthening year-round monitoring, improving early warning systems, and expanding adaptation strategies will be essential to manage the growing risks associated with marine heat extremes.

Reference:
Shannon G. Klein et al., Year-round marine heat extremes severely impact marine life during the first year at 1.5°C warming, One Earth (2026).
https://www.cell.com/one-earth/fulltext/S2590-3322(26)00147-8

Contributor: IIYAMA Miyuki, Strategic Coordination Office