As climate change continues to push ocean temperatures to record highs, leading coral researchers around the globe are sounding the alarm: a potentially powerful El Nino weather pattern forecast for this year could deliver a fatal blow to reef ecosystems already reeling from repeated mass bleaching.
Meteorological forecasters have grown increasingly confident that the cyclical climate phenomenon, which emerges every two to seven years, will return in 2025 with unusual strength. El Nino disrupts established global weather patterns, triggering severe drought in some regions and catastrophic flooding in others. For coral reefs, the most dangerous impacts stem from El Nino’s tie to elevated ocean temperatures and reduced cloud cover across many tropical basins—two conditions that directly trigger mass bleaching.
“Every single global coral bleaching event in recorded history has coincided with an El Nino year,” noted Clint Oakley, a coral biologist at Victoria University of Wellington. He shared that he feels “dread, though not surprise” at the prospect of a strong event, which he says could prove “serious and devastating for reef systems across the world.”
To understand why warm water poses such an existential threat to corals requires looking at their symbiotic biological relationship: corals rely on tiny algae called zooxanthellae that live within their calcium carbonate structures. The algae use photosynthesis to produce nutrient-rich food for their coral hosts, and in exchange gain a stable habitat and access to the sunlight needed for photosynthesis. The algae are also responsible for the vivid, distinctive colors that make reefs so iconic. When ocean temperatures rise too far above historical averages, however, this delicate partnership breaks down. Researchers have not yet pinpointed the exact biological mechanism that triggers this collapse, but the outcome is consistent: the algae either leave the coral tissue voluntarily or are expelled by the coral itself. Without their algae symbionts, corals are left stark white, a state called bleaching, and slowly starve because they no longer receive the nutrients the algae provide.
If ocean temperatures drop back to safe levels quickly enough, corals can survive on stored energy reserves until the algae return. Even then, surviving bleaching leaves corals weakened, malnourished, far more susceptible to disease, and unable to allocate enough energy to reproduce. If heat stress persists or reaches extreme levels, the coral will starve to death before temperatures cool, explained Jen Matthews, a coral scientist at the University of Technology Sydney.
Occasional localised bleaching is a natural part of reef ecosystem dynamics, and can even help cull weaker corals to make space for hardier individuals. The modern crisis stems from repeated mass global bleaching events, which have become the new normal as climate change drives steady long-term ocean warming. When reefs are hit by bleaching before they have fully recovered and had time to produce new juvenile corals to replace lost individuals, the ecosystem enters an irreversible downward spiral, Oakley said.
The most recent global mass bleaching event was officially declared in 2024, and its impacts have already been devastating. In the Caribbean, multiple key coral species are now classified as functionally extinct, meaning they can no longer reproduce enough to sustain stable populations. Australia’s Great Barrier Reef, the world’s largest reef system and the only living structure visible from space, has already lost between 15 and 40 percent of its total coral cover across different regions since 2024.
A powerful “super El Nino” this year would add new heat stress to ocean temperatures that are already far above the safe threshold for most corals. Oakley pointed out that average global ocean temperatures over the past five years are already equal to the peak temperatures recorded during the 1998 global bleaching event, the first major mass bleaching event in modern history.
While a small subset of coral species and individual colonies have shown natural resilience to warm water, these hardier corals are not abundant enough to replace the massive losses caused by repeated bleaching. Scientists have pursued a range of experimental interventions to protect vulnerable reefs, from nutrient gels that feed starved corals to solar shading that cools reefs during heatwaves, and even genetic engineering to breed more heat-tolerant coral strains. These innovative strategies are important, Matthews said, but ultimately they only “buy time” for reefs rather than solving the core crisis.
Researchers emphasize that key details about this year’s El Nino remain uncertain: while an event is very likely, its exact strength and duration are still unpredictable, said Kimberley Reid, an atmospheric science research fellow at the University of Melbourne. El Nino is just one factor shaping regional ocean conditions, she added, with local ocean temperature anomalies and regional wind patterns also playing major roles in how much heat stress reefs will face.
Even if an unusually strong El Nino does not materialise this year, the long-term outlook for global coral reefs remains grim. Roughly half of the world’s total coral cover has already been lost over the past few decades. These ecosystems are not just tourist attractions: they provide critical spawning and nursery habitat for commercial fish species that feed billions of people around the world, and act as natural sea walls that absorb storm energy and protect coastal communities from flooding and erosion.
Matthews called the current trajectory a sobering reality. “If we don’t get our act together on climate change, then all we’re doing is buying time until our reefs, as we know them, disappear.”
