Copernicus said average sea surface temperatures were 20.97C in March, the second-highest value ever for the month, and the hottest since 2024 during the last El Nino, when global heat records toppled.

This reflected "a likely transition toward El Nino conditions", said Copernicus, the European Union's global warming monitor.

Several meteorological agencies had predicted the return this year of El Nino, a natural climate cycle that warms Pacific waters and can bring higher global temperatures and extreme weather.

The World Meteorological Organization (WMO) said last month an opposite, cooling La Nina cycle was tipped to give way to neutral conditions before swinging into El Nino later this year.

The most recent El Nino in 2023-2024 was one of the five strongest ever measured and contributed to making those years the second-hottest and hottest on record, respectively.

Scientists say La Nina and El Nino are natural cycles that cause short-term temperature swings, against a backdrop of human-driven climate change that is raising global temperatures and worsening extreme weather over the long term.

Oceans absorb most of the excess heat caused by humanity's carbon emissions and therefore play a key role in regulating the global climate.

Hotter seas can have damaging knock-on effects on the planet, fuelling stronger storms and rainfall, bleaching coral reefs, and contribute to sea-level rise through thermal expansion.

- 'Sobering story' -

More broadly, Copernicus said that last month was the fourth-hottest March on record, with global average temperatures 1.48C above the pre-industrial benchmark.

Almost the whole of Europe experienced warmer-than-average temperatures in March but the most pronounced extremes were in the United States where a prolonged heatwave gripped the west.

Much of the Arctic, and parts of Russia and Antarctica, also saw above-average temperatures.

Last month also saw the lowest Arctic sea ice cover on record for the month, another bellwether of climate change.

"Copernicus data for March 2026 tells a sobering story," Copernicus director Carlo Buontempo said in a statement.

"Each figure is striking on its own -- together, they paint a picture of a climate system under sustained and accelerating pressure."

Copernicus takes measurements using billions of satellite and weather readings, both on land and at sea, and their data extends back to 1940.