Sydney, Australia. The world’s oceans recorded their hottest June on record, exceeding levels reached during the 2023–24 El Niño years. Scientists said average sea surface temperature is now just under 21°C across the world’s tropical and temperate oceans, up from about 19.6°C before widespread industrialisation in 1870.
Ocean warming accelerates
Scientists said the increase may appear small, but it reflects an enormous amount of accumulated energy. More than 90% of the extra heat trapped by greenhouse gases from burning coal, gas and oil has gone into the oceans.
They said ocean warming is occurring rapidly, with the heat added in 2025 estimated to be equivalent to about 12 Hiroshima-scale nuclear bombs exploding every second of every day.
Historical comparison
According to the scientists, a comparable climate state for the oceans would require looking back about 120,000 years to the period before the last ice age. At that time, gradual changes in Earth’s orbit caused warming over thousands of years, while current warming has occurred in a little over a century.
Effects on weather and climate
The scientists said ocean heat does not remain confined to the seas. Warmer oceans contribute to stronger cyclones, a more humid atmosphere, more intense rainfall and greater heat in air masses over the oceans, increasing the likelihood and intensity of heatwaves over land.
El Niño and regional impacts
The El Niño forming in the tropical Pacific is likely to be large, the scientists said. As it develops, warmer temperatures and extreme events such as marine heatwaves are expected in the western Indian, tropical Atlantic and eastern Pacific oceans.
Hotspots in Europe and nearby seas
Europe is experiencing a record-breaking heatwave, while the oceans surrounding the region and its enclosed seas are also exceptionally hot.