Nearly half of reef-building corals face elevated relative risk of extinction.

Coral reefs are diverse marine ecosystems that provide ecological functions and economic and social benefits across over 100 tropical countries. Warming driven by climate change is a primary threat to coral reefs and is increasing the frequency of global coral bleaching events. Here we reassessed the IUCN Red List status of 892 reef-building coral species, first assessed in 2008 1 . Because species-level population trends are unavailable for most reef-building corals, population reductions were inferred using proxies based on global coral cover trends and projected annual severe bleaching under SSP2-4.5, both over three decades, adjusted using trait-based estimates of species vulnerability. Of the 774 species with sufficient data to be assessed, 340 (44%) were classified as threatened under IUCN Red List criteria. Thirty-three species were assessed as Critically Endangered, 251 as Endangered, and 56 as Vulnerable, most due to risk of population reduction associated with projected thermal stress. These findings predate the three hottest years on record and the fourth global coral bleaching event (2023-2025), and the first exceedance for an annual mean of the 1.5 o C global warming threshold. The findings reinforce the importance of limiting further warming to maintain coral biodiversity and reef ecosystem function. The Red List assesses the relative extinction risk of species, whereby species are placed in categories of increasing risk by comparing their population size, decline and geographic range against standardised thresholds. The Red List does not predict whether or when a species may go extinct. As the ranges of coral species remain under-surveyed, future assessments would be strengthened by species-level population data and thermal-stress and recovery metrics that better capture range-representative dynamics.