Every two to seven years, a huge shift takes place in the tropical Pacific Ocean. Trade winds that normally push warm surface water westwards weaken or reverse.

Cooler, nutrient-rich water that usually rises from the deep ocean is suppressed, and unusually warm water spreads across the surface. This is El Niño, a naturally occurring phase of the El Niño-Southern Oscillation (ENSO), one of the most powerful demonstrations of the ocean’s role in the world's climate.

As the ocean releases heat into the atmosphere, it alters atmospheric pressure, rainfall and winds. Ocean and atmospheric currents shift in response, redistributing heat and moisture far beyond the ocean itself. A change in the temperature of one part of the Pacific, which covers one-third of the planet, can therefore influence weather thousands of kilometres away.

El Niño’s effects vary by region, but they typically include drought and extreme heat in parts of South-east Asia and Australia, heavier rainfall across parts of the Americas, and changes to storm activity across the Pacific and Atlantic. The strongest events have been associated with some of the world's most severe floods, droughts, wildfires and coral-bleaching episodes over the past half-century.

El Niño in a warmer world

Peruvian fishermen gave El Niño its name in the 1800s, but evidence from coral samples and other paleoclimate indicators suggests that the phenomenon dates back tens of thousands of years. However, El Niño now operates in a different world—and a different ocean.

The world's oceans have absorbed 90% of the excess heat generated by greenhouse-gas emissions. The daily global sea-surface temperature has already hit a record high in 2026, and marine heatwaves have more than tripled in the past century. An El Niño event can add an additional wave of heat to an ocean that is already unusually warm.

Some climate models suggest that El Niño episodes might become more frequent and more intense as a result of global warming. Stronger El Niños have undoubtedly become more common. The interaction is not straightforward. Scientists have not established whether anthropogenic global warming is linked to stronger or more frequent El Niños, and why El Niño happens in the first place remains one of the ocean’s biggest mysteries. But the consequences of a strong El Niño are now being felt against a warmer baseline, potentially compounding ocean warming and other climate extremes.

A Super El Niño

The 2026-27 El Niño could exceed all its predecessors. Climatologists principally monitor the phenomenon through sea-surface temperatures in the Niño 3.4 region of the central and eastern tropical Pacific. A sustained sea-surface-temperature anomaly of at least 0.5°C, together with characteristic atmospheric changes, is used to identify El Niño conditions, and anything above 2°C is often called a “super” El Niño. The latest data suggest a rise of 3.05°C, higher than the previous record of 3.02°C set in 2015. And this is early in the calendar year, months before an El Niño usually peaks; an analysis of 674 models projects a peak of around 4.1°C in November and December. Some models anticipate that this El Niño will be the largest since records began, bringing global temperatures to record levels. This event is also developing unusually far east in the Pacific, adding another layer of uncertainty to forecasts of what comes next.

What is already clear is that its effects are global. In September the World Meteorological Organization (WMO) said that El Niño was firmly established and expected to intensify into a very strong event, with an exceptionally high likelihood that it would persist until the end of February 2027. The organisation warned of heightened risks of extreme heat, drought and flooding around the world. These patterns characterising El Niño have been under way since June. Droughts in Central America and the Caribbean, increased wildfires in Indonesia, reduced monsoons in India, and heavy rain and snow in Chile and Peru are all hallmarks. The economic consequences could be substantial, potentially reaching US$10trn in economic losses by 2032. The World Food Programme expects it to push 50m people into acute hunger.

Impacts below the surface

The first major ecological effects occur in the equatorial Pacific, where warmer surface water suppresses the normal upwelling of cold, nutrient-rich water from the deep ocean. This sets off a chain of impacts through the marine food web. With fewer nutrients reaching surface waters, phytoplankton production falls, reducing the food available to fish and other marine life. Species such as tuna may move in search of more productive waters, while seabirds and marine mammals can struggle as their prey disappears or relocates. The Super El Niños of 1982-83, 1997-98 and 2015-16 were associated with widespread disruption of marine ecosystems, including coral bleaching and seabird mortality. Kelp forests can also suffer because they depend on cool, nutrient-rich water and can be further damaged by changes in waves and storms. 

Marine ecosystems are accustomed to adapting to the natural cycle between El Niño and its cooler counterpart, La Niña. What has changed is the increasingly warm baseline. Marine heatwaves can leave ecosystems with less time to recover between extreme El Niño events. Changes in temperature, nutrients, prey and habitat can persist for longer, forcing species to alter their ranges, behaviour and reproductive cycles. Tropical species can move into newly suitable waters, bringing them into competition with species already living there. The longer these conditions persist, the harder recovery can become. A species may survive an individual heat event but still face declining food availability, disrupted reproduction or a shrinking habitat, and disruption is more difficult to overcome amid a long-term warming trend.