El Niño is back, and ocean temperatures are already near record highs—that can spell disaster for fish and corals

The return of El Niño—even in its early stages—carries implications far beyond the familiar headline warnings of heatwaves and extreme weather. This recurring climate phenomenon, marked by the warming of Pacific surface waters, is already reshaping marine ecosystems at a time when ocean temperatures are already at historically high levels. The timing amplifies risks to coral reefs, already stressed by decades of rising ocean acidity and prolonged marine heatwaves. For fish populations, particularly those in tropical and subtropical regions, the disruption of currents and nutrient upwellings could trigger cascading effects on food security and coastal economies. What makes this El Niño noteworthy isn’t just its arrival but its potential intensity. Forecast models suggest a two-thirds probability of a strong to very strong event by late 2026, a scenario last seen in 2015-2016. That event coincided with the worst global coral bleaching on record, with nearly a third of the Great Barrier Reef dying in a single year. Today, coral reefs face even greater vulnerability: global average sea surface temperatures have risen by roughly 0.1°C since then, pushing many reefs closer to thermal thresholds. Fish populations that rely on these ecosystems—from parrotfish to tuna—could face habitat loss, altered migration patterns, and shifts in reproductive cycles. The broader significance lies in the compounding pressures on marine biodiversity during a period when ocean warming has already accelerated. Unlike past El Niño events, this one unfolds against a backdrop of record-low Arctic sea ice, intensifying marine heatwaves, and a rapidly changing ocean chemistry. The interaction between these factors remains poorly understood, leaving scientists cautious about the full extent of ecological disruption. For policymakers, the challenge is immediate: preparing for potential fisheries collapses, protecting vulnerable coastal communities, and addressing the long-term resilience of marine protected areas. Yet the uncertainty around this El Niño’s duration and intensity underscores the need for adaptive management strategies rather than reactive measures. As the planet braces for another period of climatic volatility, the ocean’s response may well serve as a barometer for the broader stability of Earth’s ecosystems in an era of accelerating change.