Key messages
- Ocean warming is accelerating. Global mean sea surface temperature records were continuously broken from April 2023 to June 2024. Marine heatwaves (MHWs) have become more frequent, intense, and persistent globally over the past four decades.
- Exceptional sea surface temperatures tend to strengthen extreme weather events (e.g., heatwaves, cyclones) and increase the likelihood of intensifying hurricanes (Atlantic, Caribbean, and Pacific).
- MHWs are causing severe, widespread, and in some cases likely irreversible ecological impacts, affecting biodiversity and coastal livelihoods, such as fisheries, tourism, and coastal protection.
- The ocean is a critical carbon sink, but its capacity to uptake carbon dioxide from the atmosphere is reduced as its surface warms.
The global average temperature of the ocean surface serves as a key indicator of climate change, and in April 2023, it reached record-breaking levels. For the following 13 consecutive months, new records were set for global sea surface temperature: May 2023 exceeded all previous Mays, June 2023 all previous Junes, and so on. As the largest sink for Earth’s accumulating heat, the ocean sets the pace for global warming, and that pace is accelerating. Impacts on ocean life are widespread, often severe, and in some cases probably irreversible.
Mean global sea surface temperature for 2024 was 0.6°C warmer than the average between 1981–2019, slightly warmer than 2023, and about 0.9°C warmer than pre-industrial levels. Between April 2023 and March 2024, temperatures exceeded previous records set in 2015–2016 by an average of 0.25°C. While it is not unexpected that El Niño years break records, the 2023–2024 El Niño was not particularly intense and the magnitude of exceedance is large: one study showed that the acceleration of the underlying warming trend, driven by the Earth’s energy accumulation over the past decade (see Insight 1), is physically plausible and now statistically detectable. Furthermore, the acceleration of global mean sea surface temperature is consistent with the well-established accelerations in ocean heat content and sea level rise.
The rise in global ocean temperature is accompanied by an increasing incidence of marine heatwaves (MHWs), which last days to months and exert catastrophic ecological and socio-economic impacts. The persistence of MHWs detected, based on a fixed baseline (Box 1), has increased. They last a week longer on average than they did four decades ago. The intensity and persistence of MHWs, detected based on a fixed baseline (Box 1), has increased. Annual MHW days have risen by 54% in the past four decades, from 20–30 days long in the 1980s to 40–50 days during 2000–2016. These changes are in part driven by the weakening interaction between the upper and the deeper ocean, as the upper waters warm faster and become relatively more buoyant (less dense and therefore strengthening stratification). Climate models consistently project further increases in both the frequency and intensity of MHWs under continued global warming.
MHWs also occur in the subsurface ocean, where the majority of fish live and diurnally migrate. Heatwaves in this layer can be more intense than their surface counterparts and most of them do not co-occur with surface ones. They are often caused by ocean eddies and are intensifying more rapidly (0.1–1°C per decade) than the rise in mean temperature (around 0.1°C per decade). Despite a growing recognition of the ecological importance of subsurface MHWs, there is a scarcity of observations, making it difficult to fully understand their dynamics and impacts.
Oceanic warming has consequences beyond the oceans themselves: the weather and seasons experienced by human populations are strongly determined by the warmth of the ocean. Exceptional sea surface temperatures tend to strengthen European heatwaves and increase the likelihood for intensification of Atlantic, Caribbean, and Pacific hurricanes. Several studies also link MHWs to extreme weather events like hurricanes, cyclones, flooding and atmospheric heatwaves, which have vast economic costs. The recovery from Cyclone Gabrielle, which was fuelled by an MHW, for example, cost USD 7.5–8.5 billion; the closure of the Peruvian anchovy fishery following a shift in species range (Figure 2) led to a USD 1.4 billion loss; and closures or reduced quotas in North American fisheries often follow MHWs. A warmer ocean surface also reduces the uptake of CO₂ from the atmosphere: recent estimates point to a global net reduction of 8% during MHWs over 1990–2019, reducing the mitigation of human carbon emissions.
Declines in foundation species like macroalgae, seagrass and corals in many coastal ecosystems around the world are associated with MHWs, as highlighted in Figure 2. The fourth global coral bleaching event (a heat-stress response in which the algae that give corals their colour are lost) was declared in 2024. Even in the tropical Atlantic, where corals are considered more resilient to bleaching, massive bleaching events occurred in response to increases in frequency and intensity of MHWs over the last two decades. In the Mediterranean, MHWs worsened outbreaks of disease and increased mortality in fish and shellfish, and satellite observations identified shifts in the size and biomass of phytoplankton linked to MHWs in the Western Baltic Sea, South Atlantic, and eastern boundary upwelling systems.
The responses of marine species can vary and often depend on where within a species’ geographic range the MHW occurs, complicating efforts to predict and interpret biological impacts. Trophic models, which show the flow of energy and nutrients through a food web or ecosystem, indicate that at the community scale, MHWs significantly reduce biomass across all consumer levels, with higher species most affected, altering ecosystem structure and function. Some “wins” are reported, with corals bred for heat tolerance demonstrating resistance to bleaching, and conservation efforts show some potential for preserving endangered species.
But the widespread impacts driven by MHWs are occurring more often and more intensely. Ultimately, to mitigate future ecological, economic, and societal losses, rapid measures to reduce greenhouse gas emissions and limit ocean warming are essential.
Definition of Marine Heatwaves
Marine heatwaves (MHWs) are periods of abnormally high sea surface temperatures that persist for days to months or even longer and can extend across thousands of square kilometres. MHWs are commonly defined as seawater temperatures exceeding the 90th percentile relative to a baseline climate conditions for at least five consecutive days. These events can occur at the surface or subsurface and have wide-ranging ecological, biogeochemical, and socio-economic impacts.
Policy implications
- Quantifying the economic toll of marine heatwaves (MHWs) on fisheries, aquaculture, and tourism highlights the need for climate-smart marine policies and early warning systems to support the resilience of “blue economies”.
- The intensification of MHWs strengthens the case for incorporating marine-specific targets, especially for ocean-based livelihoods and biodiversity, as part of the Global Goal on Adaptation indicators.
- To enable proactive adaptation and ecosystem-based management, coastal and ocean governance frameworks such as Marine Spatial Planning and Marine Protected Areas should incorporate MHW risk projections.
- Existing initiatives, such as WMO’s Early Warnings for All (EW4All), should expand coverage to MHWs and ocean hazards, particularly in the Global South, where ocean-dependent communities face disproportionate risks.
- At the multilateral level, coordination is necessary to enhance the exchange of best practices in coral reef restoration, adaptive fisheries policies, and nature-based ocean solutions. These efforts should be anchored to Article 7 of the Paris Agreement (on the Global Goal on Adaptation) and regional frameworks, such as the SAMOA Pathway or the Mediterranean Strategy for Sustainable Development.
- While the points above focus on adaptation, the unfolding impacts of MHWs add urgency to the call for accelerating global mitigation efforts. As listed above, meeting the Paris Agreement temperature goals requires phasing out fossil fuels across energy systems, tripling renewable energy capacity, doubling energy efficiency, and halting/reversing deforestation by 2030. The new Nationally Determined Contributions (NDCs), especially those of the G20 countries, will have to reflect greater ambition, including targets for sustainably scaling up carbon dioxide removal (CDR), as well as much more effective implementation.
Figure 2. The impacts of the exceptional marine heatwaves in 2023–2024, which was also the period of occurrence of the warmest sea surface temperature (relative to the seasonal normal) in the satellite record since 1985. Dataset: ESA Climate Change Initiative Sea Surface Temperature v3 (Embury et al. 2024).
