Ocean Breathing

INTRODUCTION

The ocean absorbs enormous amounts of excess heat from the climate system. As global temperatures rise, ocean waters are becoming warmer and marine heatwaves are placing increasing pressure on ecosystems around the world.

Coral reefs are among the ecosystems most vulnerable to this change. Reef-building corals live in a close partnership with microscopic algae inside their tissues. These algae provide corals with much of the energy they need and help give reefs their characteristic colours.

When ocean temperatures become unusually high, this relationship can break down. Stressed corals expel the algae, exposing their pale skeleton beneath the tissue. This process is known as coral bleaching.

A bleached coral is not necessarily dead. If temperatures return to normal quickly enough, some corals can regain their algae and recover. But prolonged or repeated heat stress can cause starvation, disease, reduced reproduction and eventually widespread coral mortality.

As marine heatwaves become more frequent and intense, coral reefs have less time to recover between bleaching events. What was once an occasional disturbance is becoming one of the defining challenges facing tropical marine ecosystems.

NOAA confirms that bleaching occurs when stressed corals expel their symbiotic algae and that recovery is possible when the stress is not prolonged. National Ocean Service

WHY IT MATTERS

Coral reefs are centres of marine biodiversity. Although they occupy only a tiny fraction of the ocean, coral reefs support at least one quarter of marine species.

Reefs support human communities. Hundreds of millions of people benefit from coral reefs through fisheries, tourism, livelihoods, cultural values and other ecosystem services.

Healthy reefs protect coastlines. Their complex structures help reduce wave energy and provide natural protection against storms, flooding and coastal erosion.

Bleaching can transform entire ecosystems. When large numbers of corals die, reefs can lose the complex three-dimensional structure that provides habitat, shelter and feeding areas for fish and other organisms.

Recovery requires time. Even corals that survive bleaching can have less energy available for growth and reproduction. Repeated heatwaves can therefore weaken reefs even when they do not immediately cause widespread mortality.

Coral reefs are an early warning system for climate change. Their extreme sensitivity to warming demonstrates how relatively small changes in global temperature can produce major ecological consequences.

UNEP describes coral reefs as among the planet’s most biodiverse ecosystems and highlights their importance for food security, livelihoods and coastal protection. UNEP - UN Environment Programme

KEY FACTS

Coral reefs support at least 25% of marine species despite covering less than 1% of the seafloor.

Coral bleaching does not automatically mean coral death. Corals can recover if heat stress ends soon enough, but prolonged bleaching increases the risk of mortality.

Ocean warming is increasing the frequency and severity of bleaching events. Marine heatwaves can expose entire reef systems to dangerous levels of heat stress.

The fourth global coral bleaching event was the largest recorded. NOAA reports that bleaching-level heat stress affected approximately 84% of the world’s coral reef area from early 2023 to mid-2025.

14% of the world’s coral was lost between 2009 and 2018, with climate-driven bleaching events playing a major role.

The difference between 1.5°C and 2°C of global warming matters enormously for coral reefs. IPCC assessments identify coral reefs as being at very high risk as warming increases, with the possibility of irreversible ecosystem changes.

These figures are supported by NOAA, UNEP and IPCC assessments. Nationale Miljø Satellitservice

Main Threats

Rising ocean temperatures. Long-term warming raises the baseline temperature of the ocean, making dangerous heat conditions more likely.

Marine heatwaves. Periods of unusually high ocean temperature can develop rapidly and expose coral reefs to intense thermal stress over weeks or months.

Repeated bleaching. Coral reefs need time to recover. When bleaching events occur too frequently, surviving corals may not regain enough energy, growth or reproductive capacity before the next disturbance arrives.

Ocean acidification. As the ocean absorbs additional carbon dioxide from the atmosphere, seawater chemistry changes, making it more difficult for many reef-building organisms to maintain calcium-carbonate structures.

Pollution and declining water quality. Nutrient pollution, sediment runoff, plastics, chemicals and wastewater can place additional stress on reefs already affected by warming.

Overfishing and destructive fishing. Removing ecologically important species can change reef food webs and reduce ecosystem resilience.

Coastal development. Dredging, sedimentation and habitat destruction can directly damage reefs and increase local environmental stress.

Climate change therefore acts together with local pressures. A reef exposed to warming, pollution and overfishing simultaneously can be less capable of recovering than one protected from additional stress.

NOAA and UNEP identify warming as the dominant global threat while also emphasizing pollution, overfishing and other local pressures. National Ocean Service

SOLUTIONS

Rapidly reduce greenhouse-gas emissions. The long-term survival of coral reefs ultimately depends on limiting global warming. Local conservation cannot permanently protect reefs from continually rising ocean temperatures.

Protect reefs from additional stress. Reducing pollution, sediment runoff, destructive fishing and overfishing can improve the conditions under which corals attempt to survive and recover from heat stress.

Expand effective marine protection. Well-managed protected areas can safeguard biodiversity, maintain healthier food webs and reduce direct human pressure on important reef ecosystems.

Restore damaged reefs strategically. Coral gardening, transplantation, larval propagation and other restoration techniques can support recovery in selected locations.

Develop heat-resilient restoration approaches. Scientists are investigating coral genetics, selective propagation and assisted adaptation to understand whether some coral populations can better tolerate future warming.

Improve early-warning systems. Satellites, ocean sensors and forecasting systems can identify developing marine heat stress and help managers prepare for bleaching events.

Connect reefs with surrounding ecosystems. Mangroves, seagrass meadows and healthy coastal waters form interconnected systems. Protecting these habitats can strengthen the wider ecological landscape surrounding reefs.

Work with local communities. Effective conservation and restoration depend on long-term participation by the people whose livelihoods, cultures and food security are connected to coral reefs.

Restoration can contribute to reef resilience, but UNEP explicitly warns that restoration is a complement to — not a substitute for — addressing climate change and other underlying stressors. UNEP - UN Environment Programme

WHAT YOU CAN DO

Support climate action. Reducing greenhouse-gas emissions is the most important long-term action for limiting ocean warming and protecting coral reefs.

Choose responsible seafood. Supporting well-managed fisheries can help reduce unnecessary pressure on reef ecosystems and marine food webs.

Reduce pollution. Plastics, chemicals, fertilizers and other pollutants can eventually reach coastal waters through rivers, wastewater and drainage systems.

Be a responsible reef visitor. When diving or snorkeling, avoid touching, standing on or collecting coral. Choose tourism operators that follow responsible reef practices.

Use reef-safe behaviour on land as well as at sea. Wastewater, sediment runoff and coastal development can affect reefs far beyond the immediate shoreline.

Support coral science and restoration. Research institutions and conservation organizations are developing new ways to monitor, protect and restore vulnerable reefs.

Support marine protected areas. Strong protection combined with effective management and enforcement can reduce local pressures and help maintain healthier ecosystems.

Share credible information. Coral bleaching makes climate change visible beneath the ocean surface. Helping others understand why reefs are bleaching can build wider support for climate and ocean action.

Think collectively. Individual choices matter, but protecting coral reefs at global scale requires action from governments, businesses, scientists, communities and citizens together.

SOURCES

NOAA Ocean Service — What is Coral Bleaching?
Clear explanation of how heat stress causes coral bleaching and under what conditions corals can recover. National Ocean Service

NOAA — What is Coral Bleaching?

NOAA — Corals and Climate Change
Explains how ocean warming increases the frequency and severity of coral bleaching and how repeated stress affects reef health. National Ocean Service

NOAA — Climate Change and Coral Reefs

NOAA Coral Reef Watch — Global Coral Bleaching Status
NOAA's monitoring and assessment of global bleaching-level heat stress, including the record fourth global bleaching event. coralreefwatch.noaa.gov

NOAA Coral Reef Watch

UN Environment Programme — Coral Reefs
Overview of coral biodiversity, climate risks, ecosystem services, reef decline and international conservation efforts. UNEP - UN Environment Programme

UNEP — Coral Reefs

IPCC — Oceans and Coastal Ecosystems
Scientific assessment of climate-change impacts and risks to marine and coastal ecosystems, including coral reefs. ipcc.ch

IPCC — Oceans and Coastal Ecosystems

UNEP — Coral Reef Restoration Guide
Guidance on when and how coral restoration can support ecosystem resilience while emphasizing that restoration cannot replace climate action.

Ocean Warming & Coral Bleaching