This Could Become the Most Powerful El Niño Ever Recorded

In the middle of May 2026, a network of ocean buoys strung along the equator from Indonesia to Ecuador picked up a number that, just a few months earlier, almost nobody expected to see this soon. The weekly sea surface temperature in a stretch of the Central Pacific called the Nino 3.4 region had climbed to 0.9 degrees Celsius above the long-term average. That sounds small. It is not. It is nearly double the threshold scientists use to declare that an El Nino event is underway, and it was reached in a region of the ocean larger than the continental United States in a year when most forecasters had been bracing for something much milder. Beneath the surface, something even stranger was happening. A vast bulge of warm water, roughly 150 meters deep and thousands of kilometers long, was sliding eastward across the equatorial Pacific at the pace of an oceanic Kelvin wave. By the time it surfaces off the coast of South America, it will have done what it has done in every major El Nino event of the past half-century. It will rewrite the weather. The signal was clear enough that on the 12th of March, NOAA's Climate Prediction Center took the rare step of issuing an El Nino watch, placing the probability of an event emerging by July at 82%, and the probability of it continuing through the following winter at 96%. Some of the world's most reliable forecasting models now suggest the event could peak higher than anything observed since systematic record-keeping began. Others suggest it could stall halfway and become unremarkable. Which of those happens depends on a single question, whether the ocean and the atmosphere above it decide to start agreeing with each other. El Nino is not a storm, and it is not a heat wave. It is the largest natural disruption to the Earth system that scientists have ever measured, and it works on a scale and a timescale that is almost impossible to picture from a living room. To understand what is happening in the Pacific right now, it helps to start with what the ocean is supposed to look like when nothing unusual is going on. Most of the time, the trade winds along the equator blow steadily from east to west, from the coast of South America toward Indonesia and northern Australia. They are not dramatic winds, but they are persistent, and they do something that the rest of the climate system depends on. They drag the warm surface layer of the Pacific Ocean westward with them, hour after hour, year after year, piling it up against the islands of the Western Pacific until the sea surface itself sits roughly half a meter higher in the west than in the east. Imagine a swimming pool that someone has been blowing across one end of for centuries. The water tilts. The water that gets pushed away has to be replaced from somewhere, and in the Eastern Pacific, off the coasts of Peru and Ecuador, it gets replaced from below. Cold, nutrient-rich water rises from deep in the ocean to fill the gap. That upwelling is what feeds the anchovies and the seabirds and the entire Peruvian fishing economy. It is also what keeps the Eastern Pacific cooler than the Western Pacific by something like eight degrees Celsius. Beneath the surface, the boundary between the warm sunlit layer and the cold dark water below is called the thermocline, and it follows the tilt of the surface. A thermocline is the distinct underwater layer where water temperature drops rapidly with depth, acting as an invisible boundary separating the warm sunlit surface water from the cold, deep ocean water below. Because this boundary responds to the distribution of surface waters across the Pacific, its depth is not uniform but changes from one region to another. In physical terms, the thermocline is a density interface. The sharp gradient between warm, buoyant surface water and cold, denser deep water acts as a waveguide, channeling the equatorial Kelvin waves that carry the El Nino signal eastward. Its depth at any point is a direct fingerprint of how much heat the overlying ocean has absorbed. Scientists can track it remotely by combining sea surface height data from satellite altimetry with subsurface temperature profiles from the TAO slash Triton array and Argo floats. This makes the thermocline depth arguably the single most diagnostically powerful variable in ENSO monitoring. In the Western Pacific, where warm water is piled deep, the thermocline sits well below 150 meters. In the Eastern Pacific, where cold water is being pulled up from below, the thermocline rises to within 50 meters of the surface. The whole basin is held in a slanted equilibrium by the wind. The atmosphere above it is in the same slanted state. Warm water in the west evaporates, rises, releases its moisture as thunderstorms over Indonesia, then drifts back east at high altitude before sinking over the cooler Eastern Pacific and flowing back west along the surface as the trade winds themselves. The whole loop, ocean, and atmosphere together is called the Walker Circulation, and it is one of the most important features of the planet's climate. El Niño begins when the trade winds weaken. The reasons they weaken are not fully understood, but once they do, the tilted bathtub is no longer held in place. Warm water that was piled up in the west begins to surge back east in slow, deep pulses called downwelling Kelvin waves, the same kind of wave currently moving across the Pacific right now. As that warm water reaches the Eastern Pacific, it deepens the warm surface layer along the coast of South America. The thermocline gets pushed down, the cold upwelling that normally cools the Eastern Pacific gets suppressed, and the Eastern Pacific begins to warm. This is where the mechanism becomes self-reinforcing and this is the part that matters most. When the Eastern Pacific warms, the temperature contrast between the cool east and the warm west collapses. With less contrast, the trade winds have less reason to blow. They weaken further. With weaker winds, even less warm water gets pushed westward, so even more accumulates in the east. The east warms more, the winds weaken further, the east warms more again. This is the Bjerknes Feedback, named after the Norwegian meteorologist who first described it in 1969, and it is the engine of every El Niño that has ever been recorded. It is a positive feedback loop embedded within a coupled non-linear system, the kind of loop that complexity scientists recognize. What makes it unusual is its time scale in the physical world, months rather than milliseconds, while operating across a basin the size of the Pacific. The Bjerknes Feedback is, in a sense, the ocean atmosphere system discovering and amplifying its own instability. It is a loop that, once it starts, has a tendency to run away with itself. A small initial weakening in the winds can, given the right conditions, amplify into a full reversal of the Pacific's normal state within a few months. Nothing in nature runs away forever. El Niño eventually ends because the same Kelvin waves that brought warm water east have counterparts called Rossby, waves that move in the opposite direction, slowly bouncing off the western boundary of the Pacific and returning as upwelling waves that lift the thermocline back up in the east. Rossby waves travel much more slowly than Kelvin waves. They travel westward at roughly a third of the Kelvin wave speed, taking six months to a year to traverse the basin before collapsing. The interplay of these two wave types, one carrying the El Niño signal eastward, the other carrying the corrective signal westward, is sometimes described as the Delayed Oscillator Mechanism, first formalized by Schopf and Suárez in 1988. When that happens, the cold water returns, the temperature contrast rebuilds, the trade winds strengthen, and the system tips back the other way. Sometimes it tips all the way into La Niña. All of this is now textbook physics. None of it was understood a hundred years ago. So how did anyone figure it out in the first place? In the late 1800s, the fishermen working the long Pacific coastline of Peru and Ecuador noticed something that came back to bother them roughly every few years. Every December or January, the cold current that normally ran along their shore would stop or turn around entirely, and a slab of warm water would arrive in its place. The anchovies vanished, the seabirds went hungry, catches collapsed, sometimes for months. Because it tended to arrive close to Christmas, the fishermen gave it a name that sounded almost affectionate, given what it cost them. They called it El Niño, the Christ Child, the Boy. For a long time, nobody outside their part of the world paid much attention. The first scientific description came in 1892, when a Peruvian sea captain named Camilo Carrillo wrote up the warm current for the Lima Geographical Society. To Carrillo, it was an interesting local oceanographic curiosity, a thing that happened off the coast and was worth recording. Nobody, including him, suspected that the same warm water might have anything to do with weather on the other side of the planet. Half a world away, a British mathematician named Gilbert Walker was about to find the other half of the puzzle without realising it. Walker had been sent to India in the early 1900s to figure out why the monsoon kept failing. Failed monsoons meant famine. The British administration had been blindsided by several catastrophic ones already, and Walker was given the unenviable job of finding some way, any way, to predict them. He did what mathematicians do when they have no theory to work with. He collected enormous amounts of data and looked for patterns. What he found, sifting through decades of weather records from around the world, was that air pressure at the surface of the planet seemed to behave like a kind of seesaw. When pressure was high over the eastern Pacific, it tended to be low over Indonesia and the Indian Ocean. When one side went up, the other side went down, and the swings tracked closely with whether the Indian monsoon was strong or weak. By the 1920s, he had a name for the pattern. He called it the Southern Oscillation. He did not know what caused it. He did not know it had anything to do with warm water off Peru. Walker died in 1958, still not knowing. He was working with the right data, in the right domain, with the right mathematical tools, and he found a real pattern. But the causal mechanism was hidden from him because it lay in a different domain entirely beneath the ocean surface, which he had no means of observing. This is a recurring condition in the history of science. The signal is legible. The source is invisible. It took a Norwegian meteorologist named Jacob Bjorknes, working at the University of California in the 1960s, to put the pieces together. Bjorknes had access to something the earlier generation had not. The first satellite observations of the tropical Pacific and a string of ocean buoys that could measure sea surface temperature in real time. When he laid the temperature data, alongside Walker's pressure records, the connection was almost embarrassingly obvious in hindsight. The warm water off South America that the Peruvian fishermen had named El Nino was the same phenomenon as the pressure seesaw that Walker had named the Southern Oscillation. They were not two things. They were one coupled system, ocean and atmosphere locked together, each one driving and being driven by the other. Bjorknes published the unifying paper in 1969 and the modern field of ENSO science was born within about 30 pages of writing. He gave the atmospheric loop its name too, the Walker Circulation, in honor of the man who had spent his career chasing it without ever knowing what he was looking at. What Bjorknes also gave the field was a way of thinking about feedbacks in the climate system. The idea that small changes can amplify themselves if the right conditions hold. That insight, the one that why El Nino exists at all, also explains why some El Ninos grow into something the world struggles to absorb, while others fade after a few months and barely register. The difference between an ordinary event and a historically destructive one comes down to how far that feedback loop is allowed to run, which raises the question that is now sitting in front of every forecaster watching the Pacific. What exactly makes one of these things a so-called super El Nino and is the one building right now actually going to become one? There is no such thing as a super El Nino, at least not in any official sense. The phrase does not appear in any NOAA category, any peer-reviewed classification system, or any forecast bulletin from the Climate Prediction Center. It is a media term, coined by headline writers somewhere along the way to describe the events that broke their own measuring stick. The scientists who study these things use more boring language and the boring language turns out to matter. What forecasters actually use is something called the Oceanic Nino Index, or ONI, which is a three-month rolling average of how warm the sea surface is in a specific patch of the central Pacific compared to the long-term average. The patch in question is called the Nino 3.4 region and it sits along the equator at roughly the longitude of the Galapagos Islands, stretching westward for several thousand kilometers. Its boundaries, five degrees north to five degrees south latitude, and 120 degrees west to 170 degrees west longitude, were chosen because research showed that this particular area captures the tropical Pacific sea surface temperature variability most reliably linked to atmospheric changes far beyond the equator. This makes it a kind of bottleneck, through which the ENSO signal passes most consistently. Sea surface temperatures in this region are monitored by multiple overlapping systems, the TAO slash triton buoy array and satellite-borne infrared radiometers. When the ONI for that region climbs to half a degree celsius above average and stays there for five consecutive overlapping seasons, an El Nino event is officially declared. When it falls to half a degree below average and stays there, it is La Nina instead. From there the categories are simple. An ONI between 0.5 and 0.9 is called weak, between 1.0 and 1.4 is moderate, between 1.5 and 1.9 is strong, and anything at 2.0 or higher is called very strong. That last category is the one the press tends to translate as super, and it is rare. In the entire instrumental record going back to 1950, only three events have crossed that threshold. The first was the El Nino of 1982 and 1983, which arrived almost without warning and caught the world's forecasters flat-footed. The second was the El Nino of 1997 and 1998, which peaked at an ONI of roughly plus 2.4 and became for a while the strongest event ever recorded by modern instruments. The third was the El Nino of 2015 and 2016, which edged past it at around plus 2.6 and took the title. Three events in 75 years. That is what the headlines mean when they say super. What complicates the picture, and what is going to complicate any comparison between the event building now and the events that came before it, is that the ocean in which those earlier events occurred in is not the ocean we have today. The entire tropical pacific has warmed over the past several decades. The entire tropical atlantic has warmed. The entire tropical indian ocean has warmed. Half a degree celsius above average meant something specific in 1982, when the average itself was cooler. It means something subtly different now. This is not merely a calibration issue, it is a conceptual one. The ONI, in its original form, measured the anomaly of a region relative to a fixed historical climatology, much like measuring a person's height relative to a standard calculated decades ago. If the standard itself has grown, the measurement becomes misleading. It is the oceanic equivalent of using a ruler that has expanded, without accounting for the expansion. NOAA's Climate Prediction Center took the unusual step of acknowledging this directly. On the 1st of February 2026, they officially switched the index they used to monitor and classify ENSO events. The new version is called the Relative Oceanic Nino Index, or RONI, and instead of measuring how warm the Nino 3.4 region is compared to a fixed historical baseline, it measures how warm the Nino 3.4 region is compared to the rest of the tropical oceans right now. The reason for the change is worth understanding, because it tells you something about how climate change has scrambled the tools scientists use to rely on. Think of it this way. If the whole bathtub is warming, then measuring how hot one spot is, compared to its temperature 30 years ago, will tell you mostly about the bathtub, not about the spot. To know whether that spot is genuinely doing something unusual, you have to measure it against the rest of the water around it, today. That is what RONI does. The classification thresholds are the same, the names are the same, but the index is now sensitive to the actual ENSO signal, rather than the rising tide underneath it. The shift from ONI to RONI is, in a deeper sense, a change of reference frame, analogous to the shift in physics from absolute to relative measurement. The signal has not changed. What changed is our understanding of what it is being measured against. That may sound like a small difference, but it carries real-world consequences when it determines whether a disaster planning budget is mobilized or not. In practical terms, the 2026-27 event will be the first one ever monitored, classified, and forecast using the How it compares to 1997-98 or 2015-16 will depend on which index you use to compare them, and the answer in some cases will be different. What does not change is what an event of that size actually does to the world when it arrives. The numbers on a chart are easy to argue about, but the consequences are not. A number on a chart and several human beings are not the same kind of number. The ONI can sit at plus 2.4 or plus 2.6 for a few months and then fall back to neutral, and on the graph the line goes up and comes back down again, as if nothing much happened. What the line does not show is what is happening underneath it, on the farms and the coastlines and the reefs, while the warm water sits where it is not supposed to be. The history of the super events is mostly a history of those second numbers, and they are the reason anyone bothers to forecast the first ones at all. The earliest of the events, well documented enough to study in detail, arrived in the late 1870s, and it remains, by some measures, the worst climatic disaster in modern human history. Between 1876 and 1878, the monsoon failed across India for two consecutive years. Rains collapsed across northern China at the same time. Crops failed across northeastern Brazil. The historian Mike Davis, who pieced the story together in his book Late Victorian Holocausts, documented what followed in painful detail. Famines spread across three continents simultaneously. Cholera and malaria moved through the weakened populations. Estimates of the total death toll vary depending on which historian you ask, but the range that NASA's Earth Observatory cites is somewhere between 30 and 60 million people. For comparison, the entire global death toll from the First World War, including military and civilian, is generally estimated at around 20 million. The 1876 El Niño killed more people than that, and most of them never knew the warm water in the Pacific had anything to do with it. Modern history's first super event, the one that actually entered the instrumental record properly, was the El Niño of 1982 and 1983. It arrived almost without warning. The forecasting infrastructure of the time was not set up to catch it. The satellite coverage was thinner than what we have now, and by the time anyone realized what was happening, the ocean had already tipped. A recent study published in Nature Climate Change in January 2026 went back and quantified the long-term cost of that event in a way nobody had attempted before. Researchers found that average life expectancy at birth across the Pacific Rim countries fell by roughly half a year in the aftermath, and stayed depressed for years afterward. The monetary equivalent of those lost lives, totaled across all the affected countries, came to roughly 2.6 trillion US dollars. The event also caused what one report called the largest synchronous global crop failure in modern history. 15 years later came the El Niño that most people alive today still remember, the one of 1997 and 1998. Peru flooded, Indonesia and Borneo burned, with the Peatland fires releasing so much smoke that they covered six countries in haze for months. California saw storms that washed out highways and triggered mudslides through entire neighborhoods. The official global death toll from weather events attributed to that El Niño came to roughly 23,000 people, which was bad enough, but the longer economic shadow turned out to be far worse. The same Nature Climate Change analysis that examined the 1982 to 83 event found that the 1997 to 98 event reduced life expectancy across the Pacific Rim by about four-tenths of a year, with a monetary equivalent loss of roughly 4.7 trillion US dollars over the following years. A separate analysis in Science estimated total global income losses from the event at 5.7 trillion dollars, most of which fell on countries in the tropics that could least afford it. The most recent super event, the one of 2015 and 2016, peaked at an ONI of around plus 2.6, narrowly surpassing the 1997 to 98 record. It also did something the previous events had only hinted at. It killed reefs. The marine heatwave that rode in on the back of that El Niño produced the worst coral bleaching event ever recorded on the Great Barrier Reef, with surveys finding mortality across roughly a third of the reef's northern and central sections. Bleaching events of comparable scale hit reefs across the Indian Ocean, the Caribbean, and the Central Pacific in the same window. The Peruvian Anchoveta fishery, which is the largest single-species fishery on Earth, came so close to collapse that the government cancelled the second fishing season of 2016 outright. And in the atmosphere above it all, the heat that El Niño was pumping out of the Pacific helped push 2016 to become the hottest year ever recorded at that point in history, breaking a record that had been set only 12 months earlier in 2015. What is striking about all of these events taken together is that the warm water itself never moved more than a few thousand kilometers along the equator. It never left the tropical Pacific. And yet the consequences reached India and Indonesia, and Zimbabwe and California, and the Great Barrier Reef. To understand why that happens, you have to understand the strange way the atmosphere connects places that have no business being connected. How does a patch of warm water sitting on the equator in the middle of the Pacific end up causing drought in Zimbabwe, weakening the Indian monsoon, and suppressing hurricanes in the Atlantic Basin at the same time? The answer is the single most important concept in modern climate science, and it has a name that does not do it justice. Scientists call them teleconnections, which sounds clinical, but the physical reality is something closer to a stone dropped in the middle of a pond. The ripples do not stay where the stone went in. The mechanism starts with rainfall, of all things. When the central and eastern Pacific warms during an El Niño, the towering thunderstorms that normally cluster over Indonesia migrate eastward with the warm water, sometimes by thousands of kilometers. That shift sounds like a local change, but it is not. Those thunderstorms are doing the heavy lifting of moving heat and moisture from the ocean up into the upper atmosphere, where the jet streams live. When the storms move, the heat injection moves with them. And when you change where heat is being pumped into the upper atmosphere, you change the path of the jet streams. The jet streams are the rivers of fast-moving air that steer weather systems across continents. Move them, even slightly, and a storm track that normally crosses Oregon ends up over Los Angeles instead. A high-pressure ridge that normally sits over the central Pacific reshapes itself over the western United States. The rainfall that normally arrives in Indonesia in November does not arrive at all. The ripples spread outward through the atmosphere as standing waves, locking certain regions into patterns that can last for months. Southern Africa is one of the regions that has learned the hard way what those patterns do. During a strong El Niño, the rains that should fall across Zimbabwe, Botswana, Namibia, South Africa, and the southern half of Mozambique between December and February tend not to fall. The El Niño of 1991 and 1992 produced what the region still remembers as the worst drought in a century. An event the University of Arizona's Water Resources Research Center estimates affected around 86 million people, close to three quarters of the population at the time. More recently, the 2023 to 2024 event drove water levels at the Kariba Dam so low that hydroelectric output for both Zambia and Zimbabwe collapsed, and entire cities ran on rolling blackouts for months. East Africa, only a few thousand kilometers north, gets the opposite. Rainfall there tends to increase during El Niño years, sometimes catastrophically. The flooding brings displacement and crop damage, but the longer shadow comes afterward in the form of disease. Studies of El Niño-sensitive areas of East Africa have found that cholera incidence triples during and after these events, as standing water and damage sanitation infrastructure create the conditions the bacteria need to spread. This link between El Niño and infectious disease is not limited to cholera. Research across multiple epidemiological datasets has documented strong ENSO signals in the incidence of malaria, dengue fever, rift valley fever, and hantavirus, all driven by temperature and precipitation anomalies that change the habitat and reproductive cycles of disease vectors. El Niño, in this sense, is not merely a climate event. It is an epidemiological forcing, one whose health costs are distributed unevenly but reach every inhabited continent. Indonesia and the islands of the Western Pacific experience drought, which usually means fire. The Indonesian peatland fires of 1997, the ones that covered six countries in haze for months, were not a freak accident. They were what happens when the rainforest dries out enough for the underlying peat to ignite, and El Niño is the thing that dries out that rainforest. Northern Australia gets the same pattern, drought and fire risk, while the southern half of the country can swing in unpredictable ways depending on which El Niño it is. The Indian monsoon is the one teleconnection that scientists are most careful about, because the link is real but it is not deterministic. Strong El Niños tend to weaken the southwest monsoon, which is the rainfall that feeds roughly a billion farmers across South Asia. But there are well-documented cases where a strong El Niño arrived, and the monsoon was perfectly normal anyway. The Indian meteorological department has been pushing back against the simple correlation for years. The relationship is statistical, not mechanical, and there are other factors, particularly the state of the Indian Ocean Dipole, that can either amplify or partially cancel the El Niño signal in any given year. And then there is the Atlantic, which gets the strangest gift of all. When the Central Pacific warms during an El Niño, it changes the wind patterns high above the Atlantic Basin in a way that increases what meteorologists call vertical wind shear. Wind shear is the change in wind speed and direction with altitude, and tropical storms hate it. A storm trying to organize itself into a hurricane needs a stable column of rising warm air to build vertically. Strong wind shear at the top of that column tears it apart before it can intensify. The result is fewer hurricanes, weaker hurricanes, and a quieter Atlantic season overall. That mechanism is the reason for one of the most consequential forecasts of the year. On the 22nd of May 2026, NOAA released its official outlook for the upcoming Atlantic hurricane season. And for the first time in a decade, the agency called for a below normal season. The forecast ranges 8 to 14 named storms, 3 to 6 hurricanes, and 1 to 3 major hurricanes. An average Atlantic season produces 14 named storms, 7 hurricanes, and 3 major hurricanes. The last time NOAA issued a below normal forecast was 2015, which was also an El Niño year. A quieter hurricane season is the closest thing El Niño offers to a benefit, and even that comes with a warning. Forecasters are quick to point out that even quiet seasons can produce devastating individual storms. The teleconnection lowers the odds. It does not eliminate them. All of which brings the story back to where it started. To the warm water now moving east beneath the surface of the Pacific, and to the question of what kind of event it is actually going to become. The ocean side of the equation is already in place. The Kelvin wave is moving east. The subsurface heat content across the equatorial Pacific has been climbing steadily since December. The Niño 3.4 region is sitting just shy of the official threshold, with weekly readings already well past it. The Southern Oscillation Index, the air pressure seesaw that Gilbert Walker spent his life chasing, fell to negative 11.2 in April, which is exactly what the start of an El Niño is supposed to look like in the atmosphere. Almost everything that needs to happen for a strong event to develop has happened, or is happening now. What has not happened yet is the part that makes a strong event into something historic. Michelle Leroux, the physical scientist at NOAA's Climate Prediction Center, who has been speaking publicly about the forecast, has been careful to frame it as a conditional. El Niño depends on the ocean and the atmosphere, reinforcing each other, she has said. The ocean is doing its part. The atmosphere needs to respond in a way that keeps the trade winds weak, that keeps the warm water from being pushed back westward, that lets the feedback run rather than stalling out. If that coupling holds through the summer, the event becomes something serious. If it does not, the event fades. The numbers the forecasters are putting on this are unusual in how confident and how uncertain they are at the same time. There is, according to the latest Climate Prediction Center discussion, a 2 in 3 chance that the event peaks as a strong or very strong El Niño during the November to January window, and a 1 in 3 chance that it ends up weaker than that. Some of the most reliable forecasting models in the world are pushing the upper end of those probabilities into territory that has no precedent in the instrumental record. The American climate forecast system has been projecting an only peak in the range of plus 3 degrees Celsius for October and November, which would surpass every El Niño ever measured. The European ECMWF model is showing similar peak values across parts of the ENSO region. If either model is correct, the event arriving by Christmas will be the strongest in modern history. The complication, and the reason no forecaster is willing to commit to that yet, is that the world this El Niño is arriving in is not the world the previous super events arrived in. The entire tropical ocean has warmed, the background heat content of the planet is different, and there is now evidence that the teleconnections themselves, the patterns that linked El Niño to drought in southern Africa and flooding in Peru and quiet hurricane seasons in the Atlantic, may not behave the way they used to. A study published in Nature in January 2026 found that the typical impacts of the 2023-24 event were partially offset by the unusual warmth of other ocean basins at the same time, which scrambled the standard playbook in ways that surprised the modelers. This is perhaps the most scientifically unsettling aspect of ENSO in the current era. The teleconnection patterns that formed the basis of regional climate forecasting over the past 50 years were calibrated against a world where the Pacific was anomalously warm and the other ocean basins were closer to neutral. When the Atlantic and the Indian Ocean are both running warm simultaneously, the atmospheric bridge that carries the ENSO signal to the extra tropics is operating against a changed background state, and the destination of the signal shifts in ways the historical record cannot fully anticipate. The past is still our best guide. It is just a less reliable one than it used to be. What the next seven months hold then is a kind of natural experiment that nobody designed and nobody can stop. If the coupling locks in, and the ocean and atmosphere reinforce each other the way they did in 1997 and in 2015, the world will be hit by an event larger than any of those that came before it. In a climate already running close to a degree and a half above pre-industrial temperatures. James Hansen, the climate scientist who first warned Congress about global warming in 1988, has said publicly that even a moderately strong El Nino in the next 12 to 18 months could push the average global temperature to roughly 1.7 degrees above pre-industrial levels. He also said he does not expect the planet to meaningfully cool back below 1.5 degrees once the El Nino fades. The other possibility is that the atmospheric piece does not hold, and the event peaks somewhere in the moderate range, and the world gets a year that looks more like 2009 than like 2016. Forecasters will know which one is happening within a few months. The ocean tends to commit by late summer, and once it commits it does not change its mind. What is harder to know, and what may not be answerable until the event is well behind us, is whether the categories we have used to describe these things for the past half century still mean what they used to mean. The instrumental record only goes back to 1950. The three super events we have to compare this one against happened in a cooler ocean, against a different baseline, with different teleconnection patterns. The methodology used to measure them was officially retired in February. Proxy records from coral cores, tree rings, and deep sea sediments extend the ENSO record back several centuries and even millennia. They reveal considerable natural variability, with past events that may have rivaled or exceeded the strongest in the instrumental era. But those proxies cannot capture the interactions between a large El Nino and a background climate state as warm as the present one, because that combination has not occurred within the range of the proxies. We are, in the most literal sense, in uncharted territory. Not because El Nino is new, but because the ocean it is occurring is. The next El Nino will arrive in a world it has never arrived in before, and it will be measured by an index that has never measured one of these before. Somewhere underneath the surface of the Pacific, the warm water is still moving east. By the time it gets to where it is going, the world will already be inside the answer.