The world’s oceans reached their warmest daily average temperature on record on 22 August, according to the European Union’s Copernicus Climate Change Service. The figure was 21.104C, measured across the extra-polar ocean between 60 degrees south and 60 degrees north.

The previous record, 21.09C, was set in March 2024. The same value as Sunday’s was also recorded the day before, on 21 August.

A hundredth of a degree is not, on its own, a dramatic number. What has drawn attention from climate scientists is the calendar.

Why August is the wrong month for this

Global ocean temperature has a seasonal rhythm. Because the southern hemisphere holds far more ocean than the northern one, the global average tracks the southern summer, and normally peaks in March and April. By late August the global figure should be well down from its annual high and still falling.

Instead it set a record. Both 21 and 22 August ran about 0.67C above the 1991-2020 average for that point in the year — the widest such gap recorded in 2026 so far. The reading also comfortably beat the previous highest August value of 20.98C, set in 2023.

Copernicus described the result as a sign of an ocean under growing stress, and noted that it followed a run of record and near-record monthly sea surface temperatures through 2026.

Two causes, stacked

Scientists quoted on the finding pointed to a combination rather than a single driver. Samantha Burgess, strategic lead for climate at the European Centre for Medium-Range Weather Forecasts, said El Niño is adding heat to the system, but is doing so on top of decades of human-driven warming.

El Niño is a recurring pattern in which warm water spreads eastwards across the tropical Pacific, releasing heat into the atmosphere and shifting rainfall and storm tracks worldwide. It is a natural cycle. It does not create long-term warming; it moves stored heat around and briefly raises global averages. The World Meteorological Organization expects the current event to strengthen in the coming months, and the Copernicus seasonal forecasting system suggests it could reach a strength not seen in several decades.

The second cause is the baseline the cycle is now operating on. Zachary Labe, a climate scientist at the research non-profit Climate Central, said the record was not surprising to him and was consistent with decades of research on how the climate responds to warming. The ocean has absorbed the large majority of the extra heat trapped by greenhouse gases since industrialisation. Each El Niño therefore starts from a warmer floor than the last.

What “on record” means here

The Copernicus daily series begins in 1979, which is when satellite coverage became consistent enough to produce a genuinely global daily figure. So the strict claim is that this is the warmest day in 47 years of this particular dataset.

Some coverage has gone further, describing a temperature that may not have been reached for millennia. That is an inference drawn from longer-run reconstructions of ocean temperature from proxies such as coral cores, sediments and shells, which are less precise than instruments and are not directly comparable to a daily satellite average. It is a reasonable inference, and it is a different kind of statement from the measurement itself. Both are worth keeping separate.

Why a warm ocean matters on land

Four consequences follow from warmer surface water, and none of them stays at sea.

• Marine heatwaves. Prolonged periods of unusually warm water bleach corals, damage seagrass meadows and disrupt food webs. Copernicus reports that the number of marine heatwave days globally has more than tripled since the early 1990s. The US National Oceanic and Atmospheric Administration says more than 40 per cent of the world’s oceans are currently in one.
• Fisheries. Fish move when water temperature changes. Fleets, quotas and the communities that depend on them do not move as easily, and stocks that cross national boundaries create disputes over who is entitled to catch them.
• Sea level. Water expands as it warms. Thermal expansion is one of the two main contributors to sea level rise, alongside melting land ice.
• Storms and rainfall. A warmer ocean transfers more heat and moisture into the atmosphere, which can increase the intensity of some rainfall and storm systems. This is a statement about the energy available to storms, not a prediction that any particular storm will form.

What comes next

If the seasonal pattern reasserts itself, the ocean should cool through the northern autumn and reach its next annual peak in the spring. Climatologists quoted this week suggested that, if the El Niño strengthens as forecast, the record set on Sunday may not last past March or April 2027.

Records that fall out of season, and then fall again in season, are the kind of pattern that tells you the underlying baseline is moving rather than the weather being unusual. That, more than the specific figure, is what made 22 August notable.

Sources

• Down To Earth, on the Copernicus ERA5 figures and the seasonal timing, 24 August 2026
• Euronews, “World’s oceans hit hottest temperature ever recorded in August”, 24 August 2026
• Scientific American, with comment from Climate Central, 24 August 2026
• Reuters via The Star, with comment from ECMWF, 24 August 2026