Global ocean surface temperatures reached an unprecedented average of 21.1°C in August, marking the hottest temperature on record and signaling escalating stress on marine ecosystems. This new high is particularly striking because global sea surface temperatures usually peak in March and April, following the southern hemisphere's summer, due to its larger ocean coverage.
The findings, released by the European Union’s Copernicus Climate Change Service (C3S), are based on satellite records dating back to 1979 and exclude polar regions, though the Arctic remains the fastest-warming area on Earth. Dr. Samantha Burgess, representing C3S, described the milestone as another clear signal of an ocean under growing stress. Burgess noted that while a rapidly developing El Niño event is contributing heat, it is doing so on top of decades of human-driven warming. She added that the real-world impacts are already evident, with the annual number of global marine heatwave days more than tripling since the early 1990s.
This year's El Niño, projected to be the hottest in at least a century, has already altered global weather patterns. It has contributed to weak monsoon rains in India, severe wildfires in Indonesia, and elevated bushfire risks in Australia. This comes amid a northern hemisphere summer already defined by intense drought, wildfires, and heatwaves across Europe and North America, alongside severe typhoons and torrential rain in Asia.
The physical consequences of warmer seas are profound. Oceans absorb more than 90% of the excess heat trapped by carbon dioxide emissions from fossil fuel combustion. However, as ocean temperatures rise, their capacity to absorb CO2 diminishes, potentially weakening one of the Earth's primary natural defenses against global heating. Warmer oceans contribute to sea level rise because of thermal expansion of the water, increasing flooding risks for coastal communities and low-lying island nations. Higher ocean temperatures also supercharge extreme weather events, because warmer seas transfer more energy and water vapour to the atmosphere, intensifying rainfall and storms.
To combat this trend, conservationists are urging immediate global intervention. Brian O’Donnell, director of Campaign for Nature, emphasized that two primary actions are essential. Fossil fuel emissions need to be rapidly reduced, while countries must also protect critical coastal and marine ecosystems like kelp beds, mangroves, marshes, and coral reefs, which absorb carbon and defend against storms. He noted that over 190 governments have pledged to conserve at least 30% of the planet by 2030, stating that now is the time to turn that commitment into effective, durable conservation on the ground.
Hotter oceans exacerbate the sea level rise and extreme storms that threaten billions of people, as well as driving marine heatwaves that decimate sea life and undermine coastal livelihoods, scientists warned.
Most of the planet-heating CO2 from fossil fuel burning and forest destruction is absorbed by the oceans. However, a hotter ocean is less efficient at absorbing CO2. This is potentially weakening one of the planet’s most important natural buffers against global heating, the scientists said.
