Explaining El Niño

Written by Haoyang Shi (Diverseas Intern)

On 11 June 2026, the US National Oceanic and Atmospheric Administration (NOAA) formally confirmed the emergence of El Niño in the equatorial Pacific Ocean. This has not come as a surprise to environmental scientists, as it has been widely forecasted that this year’s El Niño event will likely be one of the strongest on record (often referred to in media discussions as a ‘Super El Niño’). Of course, extreme weather is a widely-known consequence of anthropogenic climate change that increases strain on marine ecosystems (especially coral reefs, past tipping points). However, ‘weather’ is evidently no single factor amplified by carbon emissions, but a collection of complex, mutually-dependent systems (of which El Niño is merely one). Hence, as part of Diverseas’ mission to safeguard our marine ecosystems for future generations, we believe it is important to establish a correct understanding of why exactly 90% of coral reefs are projected to die by 2050, and the functional death of all coral reefs is expected since the environmental tipping point has been surpassed.

What El Niño is, and is not

El Niño (“the boy”) is defined as a concentration of heat +0.5ºC above average for 5 consecutive months in the equatorial Pacific Ocean. Inversely, a La Niña (“the girl”) signifies that temperatures in the equatorial Pacific are below-average. These fluctuations in temperature are expected seasonal events caused essentially by changing wind patterns — this weather system is collectively termed the El Niño–Southern Oscillation (ENSO). Peruvian fishermen have recorded these changes in temperature since the 1600s; hence, ENSO observations are not inherently abnormal, and is a dynamic environmental system historically in equilibrium before human-induced climate change.

Therefore, because ENSO is a Pacific Ocean system, this year’s ‘Super El Niño’ did not cause the May and June 2026 heatwave in Europe — that is caused by hot air from North Africa travelling northwards, certainly enabled by climate change from fossil fuel emissions. However, the contemporaneous heatwave in South Asia was enabled by El Niño conditions, as westward-travelling hot air masses disrupt the formation of the annual monsoon, creating hot and dry weather conditions causing thousands of deaths.

A ‘large’ El Niño is defined as average temperatures +1.5ºC above average; such events have occurred much more frequently in the last decades due to anthropogenic climate change. Even so, this is nothing compared to the +3.5ºC ocean temperatures expected in the upcoming months, stronger than anything on the historical record. Anthropogenic climate change is almost certainly responsible for this, since the probability of these deviations happening randomly is essentially zero.

What El Niño means for the ocean

El Niño events are closely linked to the death of coral reefs, as ocean temperatures increase far above what coral reefs can sustain. In high-temperature conditions, heat-stressed corals expel zooxanthellae (the symbiotic algae delivering energy for coral polyps), causing corals to ‘bleach’ by turning white. If these high temperatures are sustained for a long time, this causes corals themselves to die, also wiping out the surrounding ecosystem that depends on them for nutrition.

For example, the 4th mass coral bleaching event in 2023–24 is partially attributed to a strong El Niño in that period. This caused further damage to the Great Barrier Reef in eastern Australia, already heavily-impacted by bleaching events in the past. Notably, above-average water temperatures caused a recorded mass bleaching event in Western Australia for the first time.

June marks only the beginning of El Niño’s onset (with elevated ocean temperatures expected until early 2027). This means the Pacific is on course for another catastrophic coral bleaching event, with confirmation highly likely beginning next year.

It is these periods of extreme weather amplified by anthropogenic climate change that makes scientists’ predictions that 90% of coral reefs will die highly credible. This year’s supercharged ocean temperatures will subject the world’s densest area of ocean biodiversity to unsurvivable conditions, increasing the susceptibility of the world’s entire coral ecosystem to disease — next thing you know, they’ll be gone in a flash.