Study Finds Climate Change, Not El Niño, Drove Nearly All Coral Bleaching Since 1998

Nearly every global coral bleaching event over the past 40 years would not have occurred without human-caused climate change, according to new research published in Oceanography and led by Climate Central. The study found that during the 2018 to 2025 bleaching event, only one of 71 regions where bleaching was observed would have faced even a moderate bleaching risk in a world without climate change, with similar results across the 1998, 2010 and 2014 to 2017 events. Researchers also project that under current warming rates, climate change's contribution to bleaching risk will outweigh El Niño's in every coral-containing region worldwide by 2028.

 

How Scientists Separated Climate Change From El Niño

 

Coral bleaching has long been popularly associated with El Niño, the periodic warming pattern in the Pacific Ocean that raises sea surface temperatures and has coincided with several major bleaching events. That association has made it easy to treat bleaching as a natural climate cycle rather than a consequence of human emissions, since El Niño itself is a naturally occurring phenomenon that predates industrial greenhouse gas emissions by millennia.

To test whether that popular framing holds up, researchers used a multi-model extreme event attribution method, a technique that compares observed climate outcomes against simulations of a world without human-caused warming, to assess how much of the ocean temperature rise behind each historical bleaching event could be attributed to climate change versus El Niño specifically. They then applied those attribution results to the coral bleaching risk model developed by NOAA's Coral Reef Watch, a widely used tool for forecasting bleaching risk based on sea surface temperature thresholds. That combination allowed the team to isolate how much of the temperature rise pushing reefs past the bleaching threshold came from the long-term warming trend rather than the temporary boost El Niño provides in a given year.

 

Read more: Nature Finds Its Reporting Line: Two Years On, the TNFD Moves From Framework to Practice

 

Why the 2018-2025 Event Is the Clearest Evidence

 

The most striking finding concerns the 2018 to 2025 global bleaching event, the most recent and geographically widespread episode in the dataset. Of the 71 regions where bleaching was actually observed during that period, the analysis found only one would have reached even a moderate bleaching risk in a counterfactual world without human-caused climate change. That result indicates climate change was not simply an amplifying factor layered on top of a naturally occurring bleaching event, but the fundamental precondition without which the event would not have reached global scale at all.

Lead author Andrew Pershing, chief program officer at Climate Central, said the study shows that without climate change, coral bleaching would be a rare and isolated occurrence rather than the mass, multi-region events documented over the past four decades. That distinction matters for how the risk to reefs should be understood going forward: if bleaching events were primarily El Niño-driven, they might be expected to ease during La Niña or neutral climate phases, but if climate change is the dominant driver, the underlying risk continues rising independent of natural climate cycles.

 

What the 2026-2027 Projection Reveals

 

Applying the same methodology to the El Niño event currently under way, researchers project that natural El Niño-driven warming alone would produce only low bleaching risk in a small number of regions, while human-caused climate change is likely to drive bleaching across the globe, with particular risk concentrated in the southern Caribbean, Central and South American coasts, and the coasts of eastern Asia. That geographic breadth mirrors the pattern found in the historical analysis, reinforcing that climate change rather than the current El Niño cycle is the primary force behind the elevated risk this cycle carries.

The finding that climate change's contribution to bleaching risk is projected to exceed El Niño's in every coral-containing region globally by 2028 represents a forward-looking threshold rather than merely a historical finding, suggesting that the distinction between climate-driven and El Niño-driven bleaching risk will become progressively less meaningful as warming continues, since climate change will dominate the risk calculation everywhere regardless of where a given year falls in the natural El Niño cycle.

 

Explore OneStop ESG Marketplace: Monitoring and testing

 

Why the Findings Matter Beyond Reef Science

 

The research adds to a growing body of evidence that coral reefs face increasing risk despite documented capacity to adapt to warming conditions over time, since the pace of ongoing temperature rise appears to be outstripping what natural adaptation can accommodate. Pershing noted that millions of people and entire countries depend on healthy coral reefs for livelihoods and food security, framing the finding not solely as an ecological concern but as one with direct human and economic consequences tied to fisheries, coastal protection and tourism economies that depend on reef health.

The study's authors frame their findings as underscoring that without meaningful reductions in the carbon emissions driving ocean warming, coral reef ecosystems as they currently exist face a genuine risk of disappearing, a conclusion that reframes coral bleaching from a matter of periodic natural stress into a direct and measurable consequence of continued fossil fuel emissions, one that will keep intensifying under current warming trajectories regardless of how future El Niño cycles unfold.