On a Saturday in August, the average surface temperature of the world's oceans — outside the polar regions — ticked up to 21.1°C. That single decimal point made history. It edged past the previous record of 21.

09°C, which had been set on three separate days back in March 2024, according to data from Europe's Copernicus Climate Change Service. Small margin, enormous meaning. Here is the strange part. Ocean temperatures are not supposed to peak in August.

Their natural yearly high typically arrives in March or April, when the southern hemisphere — which holds more ocean surface than the northern hemisphere and therefore pulls global averages harder — is finishing its summer. The fact that seas are this hot right now, months ahead of schedule, is what is making scientists raise their eyebrows. "This record is another clear signal of an ocean under growing stress," said Dr Samantha Burgess, deputy director of Copernicus. El Niño, the periodic warming of the eastern tropical Pacific, is partly responsible.

But as Dr Burgess was quick to point out, it is doing so on top of decades of human-driven warming that was already baked in. What unnerves researchers even more is that El Niño has not yet reached its expected peak, which forecasters believe will arrive around Christmas time.