Global sea surface temperatures have reached a record high, raising serious concerns for global Earth systems, according to new data released by the European Union’s Copernicus Climate Change Service (C3S).
On Aug. 22, the daily global average sea surface temperature, outside polar regions, reached 21.1° Celsius (69.98° Fahrenheit), exceeding the previous record set in March 2024 of 21.09° C (69.96° F). Even small temperature increases represent a huge gain in heat energy in the ocean because of its vast volume, noted the 2025 European state of the climate report. In 2025, too, the global ocean heat content — that is, the total amount of heat stored in the ocean — reached a new record high, passing the previous record set in 2024.
The increase in ocean warming is having far-reaching consequences.
“Our [latest] report highlights how sea surface temperatures contribute to sea-level rise, increase the likelihood and intensity of marine heatwaves, put critical ecosystems such as coral reefs and seagrass under greater stress, and threaten fisheries and food security,” Samantha Burgess told Mongabay in an email.
Burgess is the strategic lead for climate with the C3S at the European Centre for Medium-Range Weather Forecasts, which produced the report. She added that increased sea surface temperatures “can also increase the potential for some extreme weather events, putting coastal communities at risk, all the while reducing the ocean’s ability to act as a carbon sink for the planet.”
The world’s oceans have absorbed more than 90% of the excess heat produced by human activities since the 1980s, and the rate of warming has nearly doubled in the past two decades. The last four years have been the hottest on record for global ocean sea surface temperatures.
“The number of marine heatwave days globally has more than tripled since the early 1990s,” Burgess said.
Adding fuel to the fire of human-induced warming is a super El-Niño currently strengthening in the tropical Pacific Ocean.
“El Niño is adding heat to the system, but it is doing so on top of decades of human-driven warming,” Burgess said.
Globally, scientists are concerned about massive marine deaths from extreme ocean heat. Coral reefs are particularly sensitive to heat and a coral die-off could have cascading effects for the roughly 25% of all marine life that depends on them as a habitat.
Coastal communities are also at greater risk as warmer oceans damage fish stocks, create conditions for stronger tropical storms and increase sea level rise as a result of melting glaciers and thermal expansion.
Oceans are an enormous buffer for the climate system, Burgess told Mongabay. However, warmer oceans can mean significant changes to ocean circulation patterns and marine life and make the ocean less effective at absorbing excess carbon from human activity.
“In other words, the buffering service continues, but at increasing cost to ecosystems and society,” Burgess said.
Banner image: The ocean is a crucial player in the global carbon cycle. Image courtesy of NOAA.
