The Fourth Global Coral Bleaching: What El Niño Did to the World's Reefs

Published: May 22, 2026 · 10 min read

In April 2024, NOAA and the International Coral Reef Initiative made an announcement that marine biologists had been dreading: the fourth global coral bleaching event had begun. At least 62 countries and territories across all major ocean basins were affected.

It was the fourth such event on record. The first was in 1998 — during the strongest El Niño of the 20th century. The second in 2010. The third lasted from 2014 to 2017. The gap between events is shrinking: from 12 years, to 4, to 3.

A study published in Nature Climate Change in March 2025 revealed the mechanism: the summers of 2023 and 2024 saw 3.5 times more marine heatwave days than any previous year on record. Nearly 10% of the global ocean hit record-high temperatures. The driver was climate change, but the trigger — the extra heat that pushed ocean temperatures past lethal thresholds for coral — was the 2023-24 El Niño.

What Bleaching Actually Means

Coral bleaching is often misunderstood. Corals aren't dying when they bleach — they're starving.

Healthy corals host microscopic algae called zooxanthellae inside their tissues. The algae photosynthesize, producing up to 90% of the coral's energy. It's a partnership: the coral provides shelter and CO₂; the algae provide food.

When water temperatures exceed a threshold — typically 1-2°C above the normal summer maximum for that location — the algae's photosynthetic machinery produces toxic reactive oxygen. The coral evicts the algae to protect itself. Without algae, the coral turns white (hence "bleaching"). It can survive in this state for weeks to months. If temperatures cool, the algae can return. If they don't, the coral starves and dies.

The 2023-24 marine heatwaves pushed temperatures so far beyond normal that entire reef systems hit this threshold simultaneously. The critical metric is Degree Heating Weeks (DHW) — a measure of how much heat stress has accumulated over a 12-week period. Above 4 DHW, bleaching is likely. Above 8 DHW, mortality begins.

In Costa Rica's Gulf of Papagayo, the 2023-24 event reached 10.2 DHW — double the levels recorded during the 1997-98 El Niño. Bleaching thresholds were exceeded for seven consecutive months. Some sites lost over 90% of live coral cover.

Regional Damage Assessment

Florida Keys: Newfound Harbor Key accumulated heat stress nearly three times the previous record, occurring six weeks earlier than historical peaks. Coral nurseries — where conservationists had been growing resilient strains for reef restoration — were wiped out by the same heat that killed wild corals.

Great Barrier Reef: Australian government surveys confirmed that nearly 80% of coral outcrops bleached during the 2023-24 summer. This was the fifth mass bleaching event on the Great Barrier Reef since 2016. The interval between events has shortened so much that the reef has no time to recover between heatwaves.

Eastern Tropical Pacific: Costa Rica, Panama, Colombia, and Ecuador all experienced severe bleaching. The Gulf of Papagayo's 10.2 DHW reading was more extreme than anything recorded in the region during 1997-98 — suggesting that even though the 2023-24 El Niño was weaker than 1997-98 (5th strongest vs 1st), the higher baseline ocean temperatures amplified the bleaching impact.

Caribbean: Widespread bleaching from the Bahamas to the Lesser Antilles. The Caribbean has lost roughly 80% of its coral cover since the 1970s from a combination of disease, bleaching, and coastal development. The 2023-24 event hit reefs that were already severely degraded.

The Winners: Where Corals Survived

Not every reef bleached. Some held on — and understanding why matters enormously for conservation strategy:

• Seasonal upwelling zones: Reefs in areas where deep, cold water regularly rises to the surface — such as the Gulf of Papagayo's upwelling pockets — experienced buffered temperatures. The same upwelling that usually keeps these reefs cooler reduced the duration of heat exposure.
• Thermally tolerant symbionts: Some coral colonies hosted Durusdinium algae — a genus more heat-tolerant than the common Cladocopium. These corals bleached later and recovered faster.
• Depth refuge: Deeper reefs (below 30 meters) experienced less thermal stress than shallow-water reefs. In some systems, deep reefs are now being prioritized as "seed banks" for future restoration.

These patterns are driving a shift in coral conservation: from trying to protect all reef area to identifying and protecting "climate refugia" — reefs that are least exposed to warming, either because of local oceanography or coral genetics. The 2023-24 event provided a massive natural experiment in identifying where those refugia are.

Economic Costs Are Just Starting

Coral reef degradation isn't just an ecological tragedy. It's an economic one. Reefs provide:

• Fisheries: Roughly 500 million people depend on reef fish for protein and income. Peru's anchovy fishery — the world's largest single-species fishery — lost an estimated $1.4 billion during the 2023-24 marine heatwave due to stock collapse.
• Coastal protection: Reefs dissipate wave energy, reducing storm surge and erosion. A dead reef doesn't grow, doesn't keep pace with sea level rise, and gradually loses its protective function.
• Tourism: The Great Barrier Reef alone supports roughly 64,000 jobs and generates $6.4 billion AUD annually. Bleaching directly reduces tourism appeal.

What Changes With the 2026 El Niño

If the developing 2026 El Niño reaches strong or very strong intensity, the marine heatwave impacts will be severe for a simple reason: the baseline is even higher now than it was in 2023. Global ocean heat content set a new record in 2024 and has continued to climb.

A study in Nature Climate Change projected that, under current emissions trajectories, marine heatwaves could become 20-50 times more frequent and 10 times more intense by 2100. A 2022 IPCC report indicated that just 1.2°C of warming — where we already are — may be enough to severely degrade the majority of the world's coral reefs.

The interval between the 2014-17 bleaching event and the 2023-24 event was roughly 7 years. If the 2026 El Niño triggers another global bleaching, the interval shrinks to 3 years. At that frequency, recovery becomes impossible for all but the fastest-growing coral species. The reefs don't disappear overnight — but they transition from coral-dominated ecosystems to algae-dominated ones. The fish leave. The fisheries collapse. The coastal protection degrades. It's not a cliff. It's a staircase.

Data sources: NOAA Coral Reef Watch; International Coral Reef Initiative fourth global bleaching event declaration (April 2024); Nature Climate Change marine heatwave study (March 2025); "Impact of the 2023-2024 ENSO Event on North Pacific Coral Reefs of Costa Rica" — Diversity (2025); Australian Institute of Marine Science Great Barrier Reef monitoring (2024); IPCC Sixth Assessment Report, Working Group II (2022).