Copenhagen Study Reveals Warming Oceans Disrupt Coral Oxygen Supply Before Visible Bleaching Appears

A peer-reviewed study published in Science Advances on May 21, 2026, by University of Copenhagen researchers identified a previously unknown mechanism by which warming ocean temperatures kill corals. The research found that rising seawater temperatures damage the microscopic cilia — hair-like surface structures on corals — that generate micro-scale water flows essential for absorbing dissolved oxygen from the surrounding seawater. At a critical thermal threshold of approximately 37°C, ciliary motion collapses and corals can experience severe damage or death without any outwardly visible bleaching symptoms. The discovery has significant implications for coral monitoring: conventional bleaching surveys may be missing a large fraction of thermally stressed corals that show no discolouration. The study found that changes in ciliary motion could serve as an earlier warning indicator of thermal stress than the visual bleaching typically used to trigger management responses. Conducted in collaboration with institutions across Europe, Australia, and Saudi Arabia, the research underscores the urgency of reducing global greenhouse gas emissions, as the newly identified mechanism implies corals are more thermally vulnerable than bleaching-only assessment data currently suggests.