The planet has been steadily warming for decades as emissions rise but the stunning leap in ocean temperatures was so far above the long-term trend that scientists debated if climate forecasts were still accurate.

Warmer seas cause marine heatwaves that influence monsoons, intensify tropical cyclones, and cause coral bleaching and mass extinctions of ocean life.

Between March 2023 and April 2024, global sea surface temperatures were 0.25 degrees Celsius above the previous record -- a substantially large margin in climate terms.

Such an event was "very rare", said Jens Terhaar from the University of Bern in Switzerland, the lead author of a research paper on the anomaly published in the peer-reviewed academic journal Nature.

"We estimate that it occurs around every 500 years with the current warming trend," he told AFP.

While not entirely unexpected, without the influence of human-driven climate change "such an event would have been practically impossible", the authors concluded.

"If the warming trend increases, such events will happen more often and it if decreases, it will happen less often," said Terhaar.

Scientists are unanimous that burning fossil fuels has largely driven climate change and pushed global temperatures to levels likely not seen in thousands of years.

Annual global average temperatures were the hottest on record in 2023 and 2024, and a prolonged and unprecedented heatwave in the oceans caused particular alarm and a search for answers.

Scientists questioned if the rate of global warming could be accelerating, and whether this event defied the projections of the best-available computer models used by scientists to anticipate climate behaviour.

- Climate regulator -

Some research looked at whether other factors -- including reductions in cloud cover and a drop in heat-bouncing particles in the atmosphere -- could be helping drive up temperatures.

But the record-shattering spike in ocean temperatures was "not necessarily a sign of an unexpected additional acceleration of global warming", Terhaar said.

"Scientists never believed that warming will be linear but that it will accelerate as long as emissions increase," he added.

"Here, the question was raised if an additional unexpected acceleration has happened. We show that the jump is not impossible and that models can simulate such jumps."

Ocean temperatures have dropped but remain near record highs, following a pattern consistent with the projections of existing climate models, the authors wrote.

Terhaar said temperatures are expected to return to longer-term levels before September but if not "this could mean that we have underestimated the climate sensitivity and that future warming will be stronger".

Oceans cover about 70 percent of Earth's surface and are a vital climate regulator and carbon sink.

They store 90 percent of the excess heat trapped by humanity's release of greenhouse gases like carbon dioxide and methane.

Cooler waters can absorb greater amounts of heat from the atmosphere, helping to lower air temperatures.