What happened
Average sea surface temperature across the world’s ice-free oceans has risen to 21.1°C, according to the EU’s Copernicus Climate Change Service. The reading is marginally above the previous high of 21.09°C recorded in March 2024.
The dataset combines observations from buoys, vessels and satellites. It covers seawater temperatures to a depth of 10 metres beneath the surface.
The timing has drawn attention because worldwide ocean temperatures usually peak in March or April, at the end of the Southern Hemisphere summer. The Southern Hemisphere has a larger ocean area than the Northern Hemisphere, giving it greater weight in the global average.
Why it matters
Warmer seas can supply additional moisture and energy to storms, intensifying extreme weather. They can also reduce the cooling effect of sea breezes in coastal areas. As seawater warms, it expands, contributing to sea-level rise and higher coastal flooding risk.
The evidence also identifies unusually warm waters around the UK and Europe, including persistent marine heatwaves west of the English Channel. This is a regional climate signal relevant to European and Türkiye-based readers with exposure to coastal conditions and marine ecosystems.
More frequent marine heatwaves add pressure on ecosystems and on communities that depend on them. The source attributes the longer-term warming trend to human-driven climate change, while noting that oceans take up more than 90% of the excess heat retained by greenhouse gases from fossil-fuel use.
Background
El Niño is warming surface waters in the central and eastern tropical Pacific. Copernicus said this natural event is adding heat to a system already warmed over decades by human activity. Its effect was expected to strengthen before reaching its maximum.
What's next
A researcher at the National Oceanographic Centre in Southampton said the early record could indicate that El Niño is strengthening. Under current conditions, another global ocean-temperature record could be set in March or April 2027.
