Why A Record-Hot Ocean Is Supercharging The El Niño Effect

The ocean covers about 71% of the planet, from the Pacific side, Earth is almost nothing but water.

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A strengthening El Niño is forming over the hottest ocean ever recorded. After absorbing more than 90% of the excess heat from global warming, the ocean is beginning to release some of that energy back into the atmosphere. The consequences could shape weather around the world. Here’s why the ocean, not the air, is the climate story everyone should be watching.

This month a heatwave emptied Fourth of July parades across the eastern United States. In June, deadly heat gripped Europe, where Western Europe went on to record its hottest June ever measured. The two were reported as separate emergencies. They were not wrong to be. But both are part of a bigger story that we are still barely telling.

A strong El Niño is now building in the Pacific, over an ocean hotter than any we have recorded. The sea has already absorbed more than 90% of the extra heat trapped by greenhouse gases, according to NASA. Now some of that heat is moving back into the atmosphere and coming back to us as weather.

Picture the size of it. The ocean covers about 71% of the planet. From the Pacific side, Earth is almost nothing but water. That water is the biggest heat store we have. We watch the air because it is what we feel. But the heat is in the sea.

What the ocean has stored, and when it gives that heat back, tells you more about the weather this autumn and winter than any forecast map. The sea is not simply warm right now. It is a loaded spring.

The air temperature is what we feel. The ocean temperature is what sets the terms.

The ocean keeps breaking heat records, and this year it went public

The tank is filling faster, not slower. Ocean heat content rose again in 2025, the ninth record year running, Inside Climate News reported. Every second, the sea takes in about as much heat as 12 Hiroshima bombs. Kevin Trenberth of the National Center for Atmospheric Research calls it the clearest single sign the planet is warming: unlike the air, it barely wobbles. It just keeps rising.

This year the heat forced its way into the news. On 21 June, the world’s sea surface hit a record for the time of year: 21.0 degrees Celsius, beating 2023 and 2024, in the Copernicus Marine data. Two separate European systems, one built on observations, one on ocean modelling, reached the same result by different routes. June became the warmest June ever recorded for the oceans. And across the first half of the year, marine heatwaves spread until they had touched around 82% of the global ocean at some point. The Mediterranean was worst hit: almost the whole sea ran warmer than normal, and strong-to-extreme marine heat reached about 80% of it. Warm water does not just sit there. A sea this hot is a store of energy pressed against the underside of the atmosphere, and physics will not let it stay put.

The first payback already has a name: El Niño

The fastest way ocean heat reaches the rest of the world is El Niño, and it is switching on now. In June, NOAA declared an El Niño and put the odds of a very strong one, above 2.0 degrees Celsius in the tropical Pacific, at 63% for the peak in November to January. That would make it one of the biggest since records began in 1950. This month the World Meteorological Organization said it is strengthening fast, and warned it will raise the chances of drought, heavy rain and heatwaves worldwide.

El Niño works by pushing heat out of the Pacific and into the air, then spreading it. It nudges the jet stream and reshuffles where the rain falls and where drought sets in. The WMO’s July outlook points to a wetter southern Europe and southwestern United States, and a drier Australia, India, Horn of Africa, Central America and Caribbean. For the coming winter, NOAA expects the usual El Niño pattern: storms tracking across the southern United States, a drier Ohio and Tennessee valley, and a milder north.

There is a deeper link most headlines miss, running from the ocean to El Niño, not the other way round. The hotter the sea, the more fuel each El Niño has to work with. And the evidence increasingly points one way: the heat we have stored is making the strongest events more likely. A 2024 study in Nature Communications found that deep-ocean warming alone could make extreme El Niños 40 to 80% more common, and that the effect lasts even after we stop emitting. We have changed the machine that hands the heat back.

You can already sketch the likely map of trouble. On the wet side, a strong El Niño tends to throw powerful storms and “atmospheric rivers” at the US West Coast. The climate scientist Daniel Swain has written that the winter of 2026 to 2027 is shaping up like the years that leave California soaked and flooded, with a real chance of a tropical storm curving up into the drought-hit Southwest as early as this autumn. On the dry side, the signal over Australia, the Amazon and the Horn of Africa means the same thing on the ground: reservoirs running low, less hydropower, failing harvests and forests ready to burn. Hurricanes tend to split two ways. El Niño usually shreds Atlantic storms with high winds, which is why NOAA expects a quieter Atlantic season, while the same setup can build stronger hurricanes in the central and eastern Pacific.

Two things keep this honest, and the second is the worrying one. El Niño shifts the odds; it does not promise any single outcome. And every forecast is based on how past El Niños behaved, yet there has never been an El Niño on an ocean this hot. That part we simply cannot know from experience. Carlo Buontempo, head of the Copernicus Climate Change Service, calls the current conditions “uncharted territory.” The safest bet is not that this El Niño behaves like the last strong one, but that it behaves in ways we are less prepared for. And because an El Niño hits hardest late in its run, the global heat record is more likely to fall in 2027 than this year.

Even without El Niño, a hot sea rewrites the weather

El Niño is the loudest signal, but not the only one. A warmer sea evaporates more water, and warmer air holds more of it. That tilts the odds toward heavy rain, hands extra energy to storms that form over hot water, and takes away the cool nights that coastlines rely on. That last part was the quiet killer in this month’s heat: forecasters kept warning that it was the nights that never cooled down, not the afternoon peak, that were driving the death toll up. The Mediterranean shows it most clearly. A sea running 5 to 6 degrees above normal in early summer is a huge store of moisture, and it often comes down later in the year as sudden, violent autumn rain. Europe saw this in 2023, when a record-hot North Atlantic was followed by brutal heat, deadly floods in Spain and fierce fires around the Mediterranean.

One caveat matters. A hot ocean does not automatically mean more storms everywhere. It provides the fuel; winds and other conditions decide whether a storm actually forms. But where they line up, the extra heat shows up as extra force, and the World Meteorological Organization reported this week that record heat in the upper Pacific is already helping drive stronger cyclones in the South-West Pacific. The message is not that ocean heat guarantees any single disaster. It quietly loads the dice, region by region, toward the extremes.

The long-term bill is bigger, and part of it is locked in

The seasonal effects are only the visible edge. The deeper changes are bigger, slower and, on any human timescale, permanent. The ocean carries heat downward, so warmth added at the surface keeps sinking, and even if we stopped all emissions tomorrow the deep sea would go on warming for centuries. Warm water also expands, which is a big part of why the seas are rising. It holds less oxygen. And as it soaks up carbon dioxide, it slowly turns more acidic. Coral is the clearest victim. The world has just come through its fourth global mass bleaching, which NOAA’s Coral Reef Watch says hit about 84% of the world’s reefs across all three oceans between 2023 and mid-2025. A study in the journal Coral Reefs found the heat stress ran unbroken from 2018 to 2025, and warned that bleaching is becoming a near-yearly event.

This is not only about nature. Heat pushes fish toward cooler water and throws the fisheries that depend on them into chaos. During the last strong El Niño, warm seas helped collapse the world’s biggest single-species fishery, Peru’s anchoveta, and an entire season was canceled. A hotter ocean is a poorer and less predictable one, and the bill lands on food supplies, coastal economies and the roughly three billion people who get their protein from the sea.

A child born today will live into the 2100s, long enough to see how far this goes

Ask the hardest question plainly, because the answer belongs to the people who will inherit it. A child born this year could easily live into the 2100s. The reefs go first. The world’s warm-water corals, home to as much as a quarter of all sea life, are on track to be all but gone within that child’s lifetime, as heat compounds the pollution and overfishing already straining them. The IPCC found that 70 to 90% of them die off at 1.5 degrees of warming, and more than 99% at 2 degrees. Warming already reached 1.37 degrees in 2025. That line is no longer far away.

And 2 degrees is closer to the best case than to where we are headed. On today’s policies the world is on track for about 2.8 degrees this century, the UN’s 2025 Emissions Gap Report finds, and if we keep burning fossil fuels it points to 3 to 4 degrees or more by 2100. At that kind of heat the ocean does not settle down. Oxygen drains out of the water, acidity climbs, and the IPCC warns that past 2 degrees the risk of species dying out, and whole ecosystems collapsing, rises fast. A dying ocean is not science fiction. It is the far end of the path we are on, and it falls inside a life that has already started.

And in the sea it is crueler than on land. When the heat comes for us, we move: into the shade, into a cool room, into a night that finally drops below body temperature. A reef cannot move at all. And almost everything else in the ocean eventually runs out of places to go. There is no water colder than the poles, no oxygen in the depths the sea has already lost, no outrunning heat that comes faster than a species can adapt. How much of this a child born today actually lives to see is not fixed yet. It depends on what we do next.

The world just took a real step to protect the ocean, and almost no one noticed

The science is clear and the stakes are plain. What matters now is what the people in charge do about it, and the record is mixed. Start with the good news, because there is some. On 17 January 2026, after almost twenty years of talks, the High Seas Treaty came into force. It is the first binding deal to protect ocean life in the two-thirds of the sea that lies beyond any country’s borders, and more than 80 nations have signed on. For the first time, the world can set up protected areas on the high seas. It is exactly the kind of shared action this moment needs, and proof it can still be done. And yet, if you missed it, you were not alone. A deal covering half the surface of the Earth took effect with a tiny fraction of the attention given to a single interest-rate decision. That is the quiet problem under this whole story: the ocean is the biggest and most important climate system we have, and the one we watch least.

The systems that watch the ocean are politically fragile

The contrast with Washington is sharp. In May, the National Science Foundation ordered the shutdown of most of the Ocean Observatories Initiative, a network of about 900 deep-sea sensors, built for some $386 million, that had sent back live data on ocean heat, chemistry and currents for more than ten years. After an outcry from scientists and a rare bipartisan revolt in Congress, the decision was reversed in June: the NSF said it would stop pulling out equipment, put back what it had already removed, and set up an expert panel. Reversing it was the right call. But that such a system could be put on the chopping block at this moment, just as the ocean enters territory we have never seen and a very strong El Niño builds, shows how easily our eyes on the sea can be switched off by one political decision. Helen Findlay of Plymouth Marine Laboratory summed up the stakes: take away steady monitoring, she said, and we are left to “navigate an increasingly volatile ocean with diminishing visibility.”

We still control the heat we add next

All of it comes down to one number we still control: how much more heat we add before we stop. The ocean keeps warming until greenhouse gas emissions reach net zero. That sets the order of what must happen.

First, stop adding heat. Cutting fossil fuel emissions is the only thing that changes the path, and the good news, which I have written about before, is that the money is already moving: clean energy now attracts almost twice as much investment each year as fossil fuels.

Second, cut methane and the other short-lived climate pollutants. They leave the atmosphere far faster than carbon dioxide, so cutting them is the fastest way to slow warming in the near term. Done at speed, these reductions could avoid up to about 0.5°C of warming by mid-century, buying time for people, ecosystems and the ocean while the world completes the harder work of ending fossil fuel emissions.

There is a catch buried inside that first step. Burning fossil fuels also releases sulfate particles that reflect sunlight and cool the planet, a dirty umbrella the IPCC reckons has hidden around 0.4 degrees of warming. Cleaning that pollution up, which we have to do because it kills millions of people a year, lets some of that hidden heat through. The ocean has already felt it. After a 2020 rule cut the sulphur in ship fuel by about 80%, NASA satellites saw the reflective “ship tracks” over the busiest shipping lanes drop sharply, and some studies suggest the sudden loss of those aerosols may have contributed to the acceleration of ocean warming behind the 2023 records. The answer is not to keep the air dirty. It is that cleaning it makes fast methane cuts more urgent, not less, so we are not tearing off the umbrella faster than we turn down the heat underneath.

The rest is about resilience while the heat already locked in works its way through. Protect and restore the “blue carbon” habitats, the mangroves, seagrasses and salt marshes that store carbon and shield coastlines. Ease the overfishing, pollution and habitat loss that leave sea life less able to cope with heat. And pay for the marine heatwave forecasts that now give fishing fleets and coastal towns weeks of warning.

We already know the cost, and we know the payoff

For investors, this is where the story turns financial. The ocean supports more than 3% of the world’s GDP, according to the OECD, and feeds some three billion people. Letting it break down is not free: the High Level Panel for a Sustainable Ocean Economy, a group set up by 14 heads of state, estimates that staying on the current path could cost more than $8 trillion by 2050. It also puts a figure on the other choice, and it is strikingly clear: every $1 put into healthy-ocean projects returns at least $5, the World Resources Institute found in the Panel’s work, a return most ordinary investments cannot match. Yet the ocean is still one of the least funded of all the big global goals. The money stays away not because the returns are bad, but because the risk has been mispriced, treated as an environmental footnote instead of a line on the balance sheet. A sea breaking record after record is the signal that this will not hold much longer.

The sea has been paying our debts quietly. Now it is calling them in

For decades the ocean soaked up the heat we could not see and asked for nothing back. That was never a gift. It was a delay.

The record warmth in the sea this summer is that delay running out. It is coming back as a strengthening El Niño, as heavier autumn rains and deeper droughts, as bleached reefs, shifting fish, and a heat record more likely to fall next year than this one. We cannot cool the ocean by looking away. And we cannot ask it to carry any more than it already does.

The ocean has bought us time. What we do now decides whether that time was wasted.