Ocean Health in 2026: Reef Bleaching & Dead Zones

The Great Barrier Reef experienced mass coral bleaching in 2024 and again in 2025, making this the second consecutive bleaching sequence on record after 2016–2017. AIMS states that the 2024 event was the reef’s fifth mass bleaching since 2016 and part of the fourth global coral bleaching event, which NOAA/ICRI declared in April 2024 after beginning in 2023. AIMS’ 2024/25 condition summary says the 2024 event had the largest spatial footprint ever recorded on the GBR, with high to extreme bleaching across all three reef regions. In 2025, the reef experienced its sixth mass bleaching since 2016; AIMS reported 281 reefs were surveyed across the Torres Strait, the northern GBR, and parts of the central region, and that regional coral-cover declines in 2025 ranged from 14% to 30% compared with 2024, with some reefs losing up to 70.8% of coral cover. NOAA’s global update says bleaching-level heat stress from 1 January 2023 to 30 September 2025 affected about 84.4% of the world’s coral reef area and that mass bleaching has been documented in at least 83 countries and territories.

Ocean dead zones are low-oxygen coastal and marine areas that can no longer support most marine life. The latest sources confirm that the main human-driven causes remain nutrient pollution from agricultural runoff, sewage, and industrial waste, which fuels algal blooms; when the algae die, decomposition consumes oxygen. NOAA and Woods Hole Oceanographic Institution both state that warming waters worsen the problem because warmer water holds less dissolved oxygen and stronger stratification reduces vertical mixing and oxygen replenishment. The problem is global and persistent. NOAA describes nutrient pollution as the primary human cause of dead zones, and a widely cited synthesis reported more than 400 dead zones worldwide and a spread that has accelerated since the 1960s. The Baltic Sea remains one of the largest and most persistent hotspots because its semi-enclosed geography limits water exchange, while the Gulf of Mexico continues to experience large seasonal hypoxia linked to Mississippi River nutrient runoff; NOAA’s 2026 forecast was 5,574 square miles, above the long-term average of 5,244 square miles. The issue affects fisheries, biodiversity, and coastal economies, and the scale of human dependence on ocean resources remains very large, but a precise global population directly exposed to dead zones is not available in the sources reviewed.