Global sea surface temperatures break record

Global sea surface temperatures have hit a new record high. The European Union's Copernicus Climate Change Service (C3S) reports the daily global average, excluding polar regions, reached 21.1° Celsius (69.98° Fahrenheit) on August 22.

This surpasses the previous record of 21.09° C set in March 2024. The 2025 European state of the climate report notes that even small temperature increases represent a huge gain in heat energy due to the ocean's vast volume. The global ocean heat content also reached a new record high in 2025, passing the 2024 record.

Consequences of Ocean Warming

Samantha Burgess, the strategic lead for climate with C3S, outlined the impacts in an email to Mongabay. She stated the latest report highlights how rising sea surface temperatures contribute to sea-level rise and increase the likelihood and intensity of marine heatwaves.

Critical ecosystems like coral reefs and seagrass are under greater stress. Fisheries and food security are threatened. Burgess added that increased temperatures can also raise the potential for some extreme weather events, putting coastal communities at risk.

Warmer oceans simultaneously reduce the ocean's ability to act as a planetary carbon sink. The world's oceans have absorbed over 90% of the excess heat from human activities since the 1980s. 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. Burgess said the number of marine heatwave days globally has more than tripled since the early 1990s.

Threats to Marine Life and Coastal Systems

Scientists are concerned about massive marine deaths from extreme ocean heat. Coral reefs are particularly sensitive. A coral die-off could have cascading effects for the roughly 25% of all marine life that depends on them as habitat.

Coastal communities face greater risks. Warmer oceans damage fish stocks and create conditions for stronger tropical storms. Sea level rise accelerates from melting glaciers and thermal expansion.

Burgess explained that oceans are an enormous buffer for the climate system. However, warmer oceans can significantly change ocean circulation patterns and marine life. They also 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 told Mongabay. A super El-Niño currently strengthening in the tropical Pacific Ocean is adding heat to the system on top of decades of human-driven warming.