The global average sea surface temperature reached 21.1C, or 69.98F, on Friday and again on Saturday, exceeding the previous record of 21.09C set in March 2024, according to the European Union's Copernicus Climate Change Service, reported by Al Jazeera. The measurement was announced on Monday.
A hundredth of a degree, across an ocean
The margin over the old record is 0.01C, which sounds trivial and is not.
This is an average across the surface of every ocean on the planet. Moving it by a hundredth of a degree requires an enormous quantity of heat, and the number is best read as a position on a trend rather than as an event. The trend is what the figure attached to human activity describes: long-term fossil fuel emissions have already raised ocean temperatures by 0.8C.
Samantha Burgess, strategic lead for climate at the ECMWF, made the distinction between the two contributions explicit: "This record is another clear signal of an ocean under growing stress. El Nino is adding heat to the system, but it is doing so on top of decades of human-driven warming."
That is the sentence to hold onto. El Nino is a cyclical redistribution of heat that comes and goes. The 0.8C does not.
The cycle underneath
El Nino is intensifying rapidly, and forecasters expect it could reach a strength not seen for several decades, peaking between November and January.
Which means this record is unlikely to stand. A warming phase reaching its peak four or five months from now, on a baseline already at a record, is the arithmetic of further records rather than a reason to expect a return to normal.
What a warmer ocean does
Warm water expands, so sea level rises without a single additional gram of ice melting. Marine heatwaves damage coral reefs and seagrass beds. Food chains are disrupted, with consequences for fishing and aquaculture. A warmer ocean also absorbs less carbon dioxide, which returns the problem to the atmosphere, and it supplies more energy to storms.
Jane Lubchenco, an oceanographer and former head of NOAA, put the consequences in one line: "An ocean this warm destabilises weather patterns and threatens food security, economic prosperity and marine life."
We should be precise about what that quote is. It is an expert characterization of risk, not a measurement, and reasonable specialists differ on how quickly these effects arrive.
What we could not establish
We could not establish the baseline period Copernicus uses, whether polar waters are included in the average, the current El Nino index value, or whether any specific reef systems are already bleaching. We verified this account from a single publication.
