Coral reefs are an undeniably critical part of the ocean. Although these ecosystems only occupy 0.01% of the ocean floor, they support 25% of all marine life, providing crucial habitat for a myriad of fish and invertebrate species. Coral reefs also have a significant impact on coastal communities, with one billion people benefiting from their existence. They provide food and livelihoods, reduce storm surge and flood risk to coastlines across the tropics, protect against erosion and attract tourists to over 100 countries and territories.

Despite their importance, coral reefs face local and global threats including nutrient runoff from land sources like agriculture or deforestation, overfishing and climate change. Without immediate action to protect and restore coral reefs, they could cease to provide the essential goods and services valued by communities worldwide. Addressing these issues requires an understanding of what coral reefs are, relevant trends, threats and reasons for optimism about the future of these incredible ecosystems.

Answering Common Coral Queries

The ins and outs of coral reefs can be confusing. Here’s a deep dive into what makes a coral reef, why they’re threatened and how they’re doing now:

What Are Coral Reefs?

Coral reefs are vast, three-dimensional structures comprised of coral animal colonies that secrete calcium carbonate, also known as limestone. Over time, these limestone secretions build up and create structures, some of which can be seen from space. Reefs are built by a variety of hard corals, of which there are 800 different species. The coral colonies that form these structures can take on a multitude of shapes and sizes. These colonies grow close together on the reef to create vibrant underwater cities for thousands of invertebrate species and over 4,000 fish species.

Orange, spiky coral glowing against a dark background.
Vibrant orange corals in the ocean at night. Photo by Krishna Desai

A coral animal, or "polyp," has a simple, transparent, tubular body with a ring of stinging tentacles. While they have a central mouth that filters food, 90% of their nutrition comes from microscopic algae within a polyp's tissues, known as "zooxanthellae." Zooxanthellae photosynthesize sugars, which the coral animal depends on for the energy it does not get from filtering food. This algae also provides coral reefs with their notoriously vivid colors.

What Are the Main Threats to Coral Reefs?

Coral reefs are threatened by both local and global threats, including overfishing; sediment, nutrient and marine pollution; and increasing ocean warming and acidification.

Overfishing is the most pervasive local threat to coral reefs. It can alter the ecological balance on the reef through removing herbivorous fish that control the macroalgae growing on coral. Sedimentation from land clearing also poses a major threat, as sediments within the water column can bury the corals and reduce the amount of sunlight that reaches the zooxanthellae, therefore limiting their access to nutrients from photosynthesis. Additionally, nutrient pollution from agriculture and sewage can increase nutrient levels that promote algal cover at the expense of corals. Ships can damage reefs with anchors or chains, discharge pollutants or introduce invasive species that can disrupt the ecosystem.

Globally, ocean warming due to climate change is a rapidly growing threat. The zooxanthellae within corals’ tissues are sensitive to ocean temperature, and ocean warming can cause the corals to expel their colorful algae — a process known as “coral bleaching.” This leaves behind the appearance of a bright white skeleton and deprives the polyps of an important source of nutrition. The corals eventually die if the symbiotic algae don’t return, if there is inadequate time between bleaching for corals to recover or if other threats impede their recovery.

A close up of a coral reef that underwent coral bleaching, leaving behind a white coral.
A coral that underwent coral bleaching, leaving behind a white skeleton. Photo by The Ocean Agency/Ocean Image Bank

On top of that, increasing carbon dioxide in sea water is slowly causing oceans to become more acidic. This decreases the availability of aragonite, a mineral which corals need to build their skeletons. A lack of aragonite slows coral growth and results in less dense, weaker structures that are more prone to erosion and damage. Aragonite saturation levels have consistently decreased in the last century, and this trend is projected to continue over the next century under current CO2 emissions.

How Are the World’s Coral Reefs Doing Now?

Unfortunately, there is no simple answer on the state of coral reefs. The extent of damage to the world’s coral reefs vary, and some have recovered. However, most present a grim outlook. Around half of the world’s reefs are likely degraded from climate change, pollution and overfishing. Hard coral cover has declined significantly in some regions, and there has been a clear change in coral community structure, with loss of susceptible coral species and loss of diversity.

A recent report by the Global Coral Reef Monitoring Network (GCRMN) paints a more nuanced chronology of coral decline. The report utilizes data from 35,000 coral reef surveys collected in 73 countries over the past 40 years to reflect fluctuation in live hard coral cover and algal cover, two key indicators of coral reef condition. Although the findings are based on limited data, they suggest that average live hard coral cover was reasonably stable prior to the first mass coral bleaching event in 1998. That event prompted an 8% loss of coral cover globally, but most coral reefs recovered during the subsequent decade.

Between 2009 and 2018, coral cover progressively declined by 14%, primarily due to recurring large-scale coral bleaching events and inadequate time between events for coral to recover. Local disturbances and threats also contribute to coral decline and hinder recovery after coral bleaching, creating an opportunity for algae to occupy the space. As a result, algal cover increased by 20% over that period. Transition from coral to algae dominance in a reef community reduces the physical and biological complexity of coral habitat, which is essential to support important ecosystem services.

What is the Outlook for Coral Reefs?

The decline in live hard coral cover over the past 40 years isn’t the end of the story. Projections of future ocean warming and the associated increased frequency of coral bleaching make coral reefs highly susceptible to further declines. By the 2030s, most coral reefs are projected to experience coral bleaching at least twice per decade, and possibly every year by the 2040s. This frequency would prevent coral recovery between episodes. Without drastic change, coral reefs could disappear by 2100.

While this may look bleak, there are signs of hope. Reef resilience, better understanding of these ecosystems and improved reef management can help prevent the worst-case-scenario.

The GCRMN report, among other studies, show that coral reefs can recover under certain conditions. In some cases, coral reefs with particularly high coral cover and diversity show evidence of natural resistance to higher ocean temperatures. Reducing local and global pressures on coral reefs is also critical to help reefs recover and maintain their resilience. This includes preventing destructive fishing practices and overfishing, minimizing pollution and sedimentation, managing dredging and preventing direct physical damage to reefs.

Networks of scientists, coastal managers and conservation professionals are also mobilizing to better understand the factors which aid coral persistence and recovery. Approaches are being tested, including:

  • Amplification of alerts of impending elevated sea surface temperatures. These alerts show when corals are expected to experience stress, which better enables coastal managers to reduce local threats.
  • The strategic development of marine protected areas (MPAs). When reefs in protected areas are able to survive and reproduce, their coral larvae can drift into degraded reefs and help them repopulate, as well. This can also disperse more heat-tolerant algae to bleached reefs.
  • The development of more successful coral restoration techniques.

Furthermore, coral reefs are becoming increasingly protected through their inclusion in new and expanded MPAs. More coral reefs are also in “fully and highly protected areas,” which typically include zones where fishing is prohibited. Good management practices — like the implementation of “no take” zones from which the removal of resources, living or dead, is restricted, as well as practices that reduce pollution and physical disturbance within these areas — can help to reduce local threats and promote coral resilience. Meanwhile, global actions to reduce global greenhouse gas emissions and control warming will also help reduce threats to reefs.

Tools that help increase knowledge on coral reefs and address local and global threats are also critical to reef protection. Data and visualization platforms, such as the Global Coral Reef Profile, are especially valuable in providing this knowledge and increasing understanding of the complexity of coral reefs, the threats they face, their enormous values and what is needed to help them persist.

Tools that provide insight into interactions on a regional scale have additional utility to stakeholders on the ground. The Coral Reef Regional Dashboards includes some of the most requested data to support decision-making relevant to coral reefs. The regional dashboards provide an overview of the value of coral reefs for fisheries, tourism and shoreline protection values; reef dependent populations; and the relative social and economic vulnerability of people within the region to coral degradation. They also provide information on the extent of coral reef and mangrove habitat, as well as how many are within MPAs and fully protected areas. Finally, the dashboards include mapping and indicators of current and future threats, locally and globally, and summarizes current knowledge about changes in the extent of live coral cover and algal cover within the region.

What Must Be Done to Ensure a Future for Coral Reefs?

There is no one solution to saving coral reefs — many coordinated steps must be taken toward a future where corals persist.

On a local level, threats to coral reefs can be addressed by managing fisheries sustainably, eliminating destructive fishing and addressing all sources of pollution. Moreover, management and financial support for MPAs and other area-based conservation measures must become more connected and efficient, with different departments recognizing potential areas for coordination.

On a global scale, efforts to keep warming within 1.5 degrees C are paramount to lessen the risk of coral bleaching and acidification. While talks about the ocean are becoming more prominent at international climate events like the United Nations Framework Convention on Climate Change, progress is slow. The importance of ocean environments like coral reefs must gain momentum and take a more central role in climate mitigation strategies.

Finally, tools that help regional policy makers and global leaders make informed decisions can offer new hope to protect coral reefs. These changes will not come easy, but they must come now to save coral reefs and those that depend on them.