The Ocean Broke Its Heat Record in a Cooling Month

The extra-polar ocean hit 21.1°C (69.98°F) on August 22, the highest daily average in the Copernicus ERA5 series that Climate Pulse tracks from 1979. The previous high was 21.09°C in March 2024, when this band of water is usually at its warmest.

This reading landed in August, a month when that surface is normally cooling, with a strong El Niño still building toward winter rather than winding down.

A Hundredth of a Degree in the Cooling Season

The Copernicus Climate Change Service (C3S) posted a daily global average of 21.1°C for August 22, beating 21.09°C from March 2024 by 0.01°C. The average covers the ice-free ocean from 60 degrees south to 60 degrees north. ERA5 estimates the water at about 10 meters, a foundation temperature, not a thin skin at the very top.

A hundredth of a degree on a global mean is easy to shrug off. It is not a bathtub reading. It is the mean of a vast slab of water, so a tiny shift still means a large extra store of heat. The calendar is the sharper fact. Global ocean temperatures in this band usually peak in March and April, after summer in the Southern Hemisphere, where most of the planet’s water sits. August is when that surface is usually losing heat.

The same day also beat the previous August high of 20.98°C, set in 2023, by 0.12°C. C3S had already watched the water climb through the northern summer. From June 19 and through July, each daily extra-polar reading was the hottest on record for that date. July’s monthly average reached 20.96°C, above the prior July mark of 20.89°C from 2023. On July 31 the daily value was 21.05°C, only 0.04°C shy of the March 2024 all-time high. Three weeks later the series crossed it, in the wrong month.

The daily global average sea surface temperature has reached its highest level on record. On 22 August 2026, data from the #CopernicusClimate showed 21.1°C, exceeding the previous record set in March 2024.

🔗Read the press release: https://t.co/7UT31GeEQm

⬇️ pic.twitter.com/eMXWWa3tP8

— Copernicus ECMWF (@CopernicusECMWF) August 24, 2026

FOUR SST MARKS THAT FRAME THE RECORD

C3S still treats a single daily high as a snapshot that has to sit in a longer record. The service said the August 22 mark fits a long rise in ocean heat and the start of an extreme El Niño in the tropical Pacific. As of September 2 it has not announced a higher daily value.

Last Year’s La Niña Never Gave the Heat Back

The 2023-24 El Niño left the surface very warm. 2025 then spent much of the year in weak La Niña or near-neutral Pacific conditions, the pattern that usually trims global sea-surface heat. The trim was small. C3S’s Global Climate Highlights 2025 put the annual extra-polar sea-surface average at 20.73°C, 0.38°C above the 1991 to 2020 mean, the third-highest year in the series. That sat 0.13°C below 2024 and 0.07°C below 2023. C3S called 2025 the warmest La Niña year on record for both air and sea-surface temperature.

The water column did not follow the surface down. C3S’s ocean-heat indicator, which starts in 1993, found global ocean heat content at a record in 2025, including the upper 2,000 meters. The ocean takes up to 90% of excess heat trapped by greenhouse gases, then moves some of it into deeper layers. A team led by the Institute of Atmospheric Physics at the Chinese Academy of Sciences estimated that the upper 2,000 meters gained about 23 zettajoules, plus or minus 8, from 2024 to 2025, even as annual sea-surface temperature ran 0.12°C cooler than 2024 in that group’s own series.

THE 2025 HOLDING PATTERN

• Surface rank: Extra-polar sea-surface temperature finished third, behind 2024 and 2023, in a La Niña year.
• Gap to the peak year: The 2025 annual average sat 0.13°C below 2024 on the C3S scale.
• Heat in the water column: Global ocean heat content still set a record in a series that begins in 1993.
• Added energy: IAP/CAS put the 2025 gain in the upper 2,000 meters at about 23 zettajoules.

That split is the part a daily surface record does not show. La Niña can shave a little heat off the top while the ocean as a whole keeps taking energy. The next El Niño then starts from a raised floor, not from a reset.

How Strong This El Niño Could Get

NOAA’s Climate Prediction Center, in its August 13 diagnostic, says El Niño is strengthening, with a greater than 90% chance of a very strong event in fall and winter 2026-27 and a 69% chance of historic event in October-December versus events back to 1950. Historic, in that note, means a three-month relative Niño index of 2.5°C or more. The next diagnostic is due September 10.

July already showed a coupled ocean-atmosphere system that had flipped into El Niño. Relative Niño values were 1.4°C in Niño-3.4, 1.7°C in Niño-3, and 2.9°C in Niño-1+2. Sea-surface anomalies exceeded 2.0°C in the eastern equatorial Pacific. The upper-ocean temperature index from 180 to 100 degrees west rose, and subsurface anomalies reached 10.0°C at depth. Low-level westerly winds and upper-level easterlies stretched from the western to east-central Pacific. Rainfall increased from the central to eastern Pacific and dried over Indonesia. The Southern Oscillation indices ran near 2 standard deviations negative.

The World Meteorological Organization said on July 3 that El Niño had developed and was forecast to strengthen rapidly into a strong event, with seasonal-average anomalies in the main monitoring regions expected to exceed 2°C. WMO Secretary-General Celeste Saulo said that would raise the odds of drought, heavy rain, and heatwaves on land and in the sea. El Niño typically peaks between November and February. C3S seasonal forecasts, in the August 24 note, said this event could reach levels not seen for several decades.

FROM A HOT EL NIÑO TO A RAISED FLOOR

1. 2023 to 2024: A strong El Niño helps drive successive sea-surface records, including the 21.09°C daily high in March 2024.
2. 2025: Weak La Niña and near-neutral Pacific conditions trim the surface only slightly; annual extra-polar SST still ranks third, and ocean heat content sets another record.
3. June 19, 2026: Daily extra-polar SST becomes the hottest on record for each date and stays there through July.
4. July 2026: The monthly extra-polar average hits 20.96°C, the highest July in the C3S series, while NOAA’s Niño indices climb.
5. August 13, 2026: NOAA keeps an El Niño Advisory and assigns greater than 90% odds of a very strong event through winter, with a 69% shot at a historic October-December.
6. August 22, 2026: Daily extra-polar SST reaches 21.1°C, an all-time high in a cooling month.

Art Miller, a research oceanographer at Scripps Institution of Oceanography, said forecast models agree on a moderate-to-large 2026-27 event, and some of them point to the largest in the historical record of the past 150 years. He also noted that a relative Niño index, which strips out the long-term warming trend, cuts recent peaks by about 0.5°C, so part of the “super” label is the warmer baseline itself.

Warmer Water Is Already Hitting Reefs and Coasts

C3S listed the harms that come with a hotter extra-polar surface, and they are already in motion rather than waiting on a winter peak. A warmer ocean expands and lifts sea level, which raises flood risk for coasts and low islands. It also feeds marine heatwaves that bleach coral and damage seagrass, then move through food webs into fisheries and fish farms. The same heat loads the air with more moisture, which can intensify rain and some storm systems. A warmer surface also takes up carbon dioxide less efficiently, which weakens a main natural brake on greenhouse gas in the air.

Samantha Burgess, strategic lead for climate at the European Centre for Medium-Range Weather Forecasts (ECMWF), tied the August 22 reading to that longer stress, not to a one-day spike.

This record is another clear signal of an ocean under growing stress. El Niño is adding heat to the system, but it is doing so on top of decades of human-driven warming. As ocean temperatures continue to rise, the risks to marine ecosystems and coastal communities also increase. The consequences are already visible: the number of marine heatwave days globally has more than tripled since the early 1990s, putting increasing pressure on marine ecosystems and the communities that depend on them.

Samantha Burgess, Strategic Lead for Climate, ECMWF

WHAT A HOTTER SURFACE ALREADY DOES

• Sea level: Warmer water expands, adding to the rise that already threatens coasts and low islands.
• Marine heatwaves: Global heatwave days have more than tripled since the early 1990s, with bleaching and seagrass loss among the first hits.
• Food webs: Damaged reefs and seagrass disrupt fisheries and aquaculture.
• Storms and rain: Extra heat and moisture in the air can intensify rainfall and some storm systems.
• Carbon uptake: A warmer ocean is less efficient at taking carbon dioxide out of the air.

Zachary Labe, a climate scientist at Climate Central, said hotter oceans can worsen sea-level rise, scramble weather, and endanger sea life such as coral, with knock-on effects for the economy, coastal works, and security. He also said the new high is unlikely to last long as a headline number.

Fuel Still Sitting in the Western Pacific

The surface record is the visible layer. The fuel for the next months is still below it. Miller said this event is expected to be large because of the heat sequestered below the surface in the western tropical Pacific, which then feeds warmer water in the central and eastern Pacific, the region that defines El Niño’s size. A Pacific Meridional Mode pattern, stretching from Baja California toward waters south of Hawaii, has been in place for months and has helped trip the tropical event.

Sarah Purkey, a physical oceanographer and Argo lead at Scripps, said the strong El Niño signal in the tropical Pacific now lines up with a maximum in global ocean temperature. The overlay of those two heat anomalies, she said, is what produces the marine heatwaves and heavy-rain events that hit people on land. Argo’s robotic floats sample the interior, not just the skin, which is why a surface series can look like a rounding error while the water column is still taking energy.

Off North America the overlay is already local. Miller said a strong, persistent marine heatwave has been in place off the U.S. West Coast and Baja since early 2026, and that El Niño would add to it. Wind-driven upwelling, which usually brings cold, nutrient-rich water up in spring and summer, has been weak so far in Southern California. El Niño can warm that coast two ways: by changing North Pacific winds so warmer offshore water moves in, and by sending subsurface waves along the shore that push down the surface layer and suppress cool upwelling.

Daniel Rudnick, a physical oceanographer who directs Scripps’s Instrument Development Group, said California Underwater Glider Network lines have been seeing the highest temperatures off that coast in the last 20 years in this heatwave. Those gliders dive to about 500 meters, which filters out thin surface noise. If upwelling stays weak into fall, the coastal warming has a path to persist after the August global average has come and gone as a news spike.

Will This Record Survive the Winter?

ECMWF scientists working for C3S are still checking whether the August 22 exceedance is a one-day spike or a new plateau, and Labe said it is very possible the mark falls again in the coming months as El Niño adds heat. The August 22 value is the number in the book. It is not a peak-season reading, and the Pacific event that can add more warmth is still on the way up, with NOAA’s next full diagnostic due September 10 and the usual El Niño peak window still months off.

WHAT WE KNOW

• The daily mark: Extra-polar SST averaged 21.1°C on August 22, 0.01°C above March 2024 and 0.12°C above the prior August high.
• The July step: The July 2026 monthly extra-polar average was 20.96°C, the hottest July in the C3S series.
• The 2025 floor: Last year stayed the third-warmest extra-polar SST year and a record year for ocean heat content, in a La Niña setting.
• The El Niño odds: NOAA assigns greater than 90% odds of a very strong event through winter 2026-27 and a 69% chance that October-December is historic back to 1950.

Those figures are already published. What is still moving is the daily series itself, and the peak strength of the Pacific event.

WHAT IS UNCONFIRMED

• A lasting plateau: ECMWF has not yet said whether the 21.1°C exceedance is persistent or a brief overshoot.
• A record El Niño: The 69% historic odds are a forecast, not a measured peak; NOAA’s next note is due September 10.
• A higher daily SST: C3S has not posted a later daily value above 21.1°C, but Labe said another break in the coming months is possible.

The August reading is already in the books, while ECMWF still checks whether that daily exceedance holds. NOAA’s next ENSO diagnostic is due September 10. Until then the 21.1°C mark stands, set in a month when this band of ocean is supposed to be cooling, with a strong El Niño still adding heat toward winter.