Record breaking marine heat waves in 2023 covered 96% of oceans and lasted four times longer than average

The global marine heat waves (MHWs) of 2023 were unprecedented in their intensity, persistence, and scale, according to a study published in the journal Science on July 24, 2025.

The findings provide insights into the region-specific drivers of the marine heat waves, linking them to broader changes in the climate system. They may be signs of an emerging climate tipping point, the study warned, that could trigger irreversible changes to marine ecosystems worldwide.

MHWs are extreme rises in ocean temperature for an extended period of time. They can occur at different locations in the ocean. Their magnitude and frequency have increased over the last couple of decades, with harmful impacts on ecosystems, marine industries and human activities.

To better understand the MHWs of 2023, Tianyun Dong from China’s Eastern Institute of Technology and colleagues conducted a global analysis using combined satellite observations and ocean reanalysis data, including those from the ECCO2 (Estimating the Circulation and Climate of the Ocean-Phase II) high-resolution project.

The researchers reported that MHWs of 2023 set new records for intensity, duration, and geographic extent. MHW activity totalled “53.6 billion °C days square kilometer—more than three standard deviations above the historical norm since 1982”.

MHWs lasted four times the historical average and covered 96 per cent of the global ocean surface. Regionally, the most intense warming occurred in the North Atlantic, Tropical Eastern Pacific, North Pacific, and Southwest Pacific, collectively accounting for 90 per cent of the oceanic heating anomalies.

The North Atlantic experienced the most prolonged marine heat wave on record, persisting for 525 days after beginning in mid-2022. The Southwest Pacific event broke prior records with its vast spatial extent and prolonged duration. In the Tropical Eastern Pacific, temperature anomalies peaked at 1.63°C during the onset of El Nino.

The study discovered diverse regional drivers contributing to the formation and persistence of the marine heat waves — increased solar radiation due to reduced cloud cover, weakened winds, and ocean current anomalies.

While humans feel the impact of land-based heat waves more directly, MHWs are affecting human activities in alarming ways too.

Across warming seas, record breaking MHWs in 2023 underscored the growing vulnerability of ecosystems and human livelihoods and led to events like coral bleaching, mass migration of sea life and disruption of marine food webs.

This translates to reduced availability of seafood resources and job and economic losses in many marine-dependent industries.

“These events can stress ecosystems beyond recovery thresholds, potentially triggering coral reef collapse, reducing species richness, increasing mortality rates, and causing redistribution of fish species, which has already led to the decline of key fisheries, such as the Pacific cod fishery,” the study said.