What Is Coral Bleaching & Can Our Reefs Recover?

A scuba diver exploring a reef suffering from coral bleaching.

Our oceans are heating up, and with heat comes coral bleaching.

Consequences of climate change seem just around the corner, but we still have time to act.

The loss of our coral reefs would not only harm countless species – hundreds of millions of us also depend on them for survival.

So how does this bleaching happen, and can coral reefs recover?

Let’s take a closer look behind the scenes of coral bleaching, and discuss what needs to be done about it.

What is coral bleaching?

Despite what you may think, corals are actually animals.

These tiny marine invertebrates form massive colonies made up of individual polyps, and they often have a symbiotic relationship with single-cell organisms called zooxanthellae, sometimes simply called algae.

Bleached coral with brown stems and translucent white tips.
Image by OSU via Flickr

zooxanthellae: symbiotic single-celled marine organisms

Like plants, these creatures undergo photosynthesis using the sun, supplying themselves with energy. In exchange for a safe home, they pay rent to their coral friends with some of that energy.

Easy to follow so far, but why does coral turn white?

When ocean temperatures rise or waters become too polluted, coral polyps expel their algae; and since the algal chlorophyll is what gives corals their beautifully diverse colors, the loss of these organisms leaves coral reefs translucent and white – or bleached.

A comparison of before and after coral bleaching.

Bleached coral can still survive, but the loss of coral algae means less productive reefs – and corals which are more vulnerable to disease.

To put it simply, coral bleaching is the disruption of a healthy symbiotic relationship between coral and algae, in which vast biodiverse reefs are left damaged and lose their color and ability to survive.

What causes coral bleaching?

Many different issues come together to cause coral bleaching, with some of them impacting healthy reefs more than others:

Higher water temperatures. The main cause of coral reef bleaching is a general rise in ocean temperatures, as well as particularly high-heat events throughout the year. For this reason, coral bleaching has even been described as weather in order to optimize solutions to such an overwhelming issue.

Stress and damage from solar radiation. Alongside the heat itself, irradiance and ultraviolet radiation (UV) from the sun also damages coral and disrupts important cellular functions they need in order to thrive. This can be devastating for reefs in low-tide areas, which are heavily exposed to the sun when tides are low.

Pollution and runoff from human activities. While rising temperatures are thought to be the main cause of coral death, reefs are also constantly bombarded with industrial pollutants.

Crop-lands which will eventually drain into coral reef ecosystems.

Fertilizer and pesticide runoff from farms, sediment and heavy metals from mining operations, and thousands of other substances constantly flow from rivers into coastal reef habitats.

Overfishing and excessive tourism. Reefs like the Great Barrier Reef in Australia are some of the most popular ecotourism destinations in the world. But their popularity comes with a range of issues, including physical damage from careless divers and sewage or waste dumping.

An ecotourist diver exploring an artificial shipwreck reef.

While separate issues, overfishing and even whaling have clear, cascading effects on coral reefs – remember, these are ecosystems that rely on stable populations of thousands of different species, not just corals!

Ocean acidification and CO2 concentration changes. Oceanic CO2 levels are increasing as our reliance on fossil fuels continues, which can have negative effects on the calcium carbonate shells of marine organisms. When it comes to corals, ocean acidification isn’t thought to be a major driver of bleaching events, but likely still plays an indirect role.

What are the effects of coral bleaching?

Vast fields of coral interconnect in shallow waters, with some of them stretching for thousands of miles – making them some of the largest networks of living structures on earth.

An aerial shot of a section of the Great Barrier Reef.
Image by Sarah Lou via Flickr

But corals don’t suffer alone. Countless other marine species rely on these biodiversity hotspots, often regarded as oceanic versions of the rainforests along the equator.

So, how does coral bleaching affect marine life and our society?

Coral bleaching leads to ecosystem collapse and biodiversity loss. Corals are the foundation for oceanic ecosystems that may contain over a million different marine species, many of which have yet to be discovered.

eddy reef dive site
Image by Paul Toogood via Flickr

These important nurseries provide cracks and crevices for animals who need to hide and hunting grounds for predators, especially shallow reefs rich with life. So bleaching events not only lead to less coral – biodiversity loss is another major consequence of dying reefs.

Millions of people rely on coral reefs to survive. It’s hard to put an exact number on how many human lives are supported by reefs, but somewhere around 500 million people will be affected as these ecosystems become less productive and ecotourism profits dwindle.

Traditional reef fishing boats in Malaysia.

Fisheries near coral reefs produce 60% less food than they did in the 1950’s, and many of those who rely on fish as a source of protein live in developing nations, where there really isn’t another affordable option to fall back on.

Losing our coral reefs will impact future scientific discoveries. Coral reefs aren’t only a source of food and guided tours – these ecosystems are the birthplace of a variety of medically significant compounds.

You can find lots of examples of this, like chemotherapy drugs derived from reef sea sponges; so as bleaching events worsen, we lose access to incredible future potential in medicine and other industries.

Can corals recover from bleaching?

It’s clear that losing reefs will lead to catastrophic effects, but the good news is that, in many cases, bleaching is reversible.

In fact, bleaching in moderation could even be compared to forest fires – a standard seasonal occurrence in many regions that isn’t really that harmful in small amounts.

Healthy ecosystems have evolved to deal with or even benefit from seasonal waves of stress and damage, so random bleaching events are something that coral populations can recover from.

Soft and vibrant corals in a healthy, non-bleached reef.
Image by NOAA via Flickr

Reefs show a remarkable ability to heal under the right circumstances. If a coral reef has a resilient and biodiverse foundation, it’s likely that it can recover from bleaching events.

Coral polyps can repopulate bleached areas, especially with the help of marine biologists and conservationists, who are able to transfer healthy coral to areas in need.

bigeye coral habitat

Let’s go over a few of the key factors that determine if a reef can bounce back from a bleaching event:

  • The density of juvenile corals after a bleaching event
  • How well a bleached reef can provide shelter for new adult corals
  • The variety in species of corals involved
  • Competition between different coral species

In other words, the severity of a coral bleaching event is largely defined by how much coral survives, and how well they can adapt and recover. These ecosystems have a natural ability to repair themselves, the main issue is the increasing frequency of coral bleaching.

What solutions to coral bleaching exist?

Climate change and coral bleaching are directly related, and things are looking a bit dire as we close in on various climate tipping points. At this point, it’s likely that we’ll need to directly intervene in order to save our reefs – in addition to keeping global temperatures as low as possible.

So what’s being done to help corals, and what changes can you make in your life to lower your impact?

Coral reef restoration and stabilization. Many methods of restoring and protecting reefs have shown promise in aiding our corals; for example, shading reefs which are exposed to heavy UV radiation.

An artificial reef made from a past shipwreck, now home to corals.

Physically building up reef substrate is also common, where things like concrete and metal mesh are dropped near reefs to serve as a habitat for many species who rely on thriving reefs.

Gene editing, bio-control, coral treatments. Along a different line, selectively breeding coral and manipulating their genes is an option – a rapidly growing field aiming to build up the biological resilience of various coral species.

Other interesting solutions include mass relocating coral polyps, controlling the population of species that are fierce competitors, and even feeding reefs nutrient-dense probiotics to encourage growth.

Lowering global temperatures should remain a priority. This is an entire subject of its own, but living a low-impact life is the best way you can help keep global temperatures below or near 2 °C – after which, corals are unlikely to survive.

A graph showing the global monthly temperature growth from 1880-2022.

Educating yourself about all the best ways to reduce your carbon footprint doesn’t have to be stressful, it can be as simple as using less plastic or starting your own garden instead of relying on supermarkets.

Industrial runoff needs to be reduced. Chemicals from monoculture farming, raising animals, mining operations, and various other human activities are part of the problem and need to be drastically reduced.

Learning how to go vegan is one option, as agricultural runoff is one reason why meat and dairy are so bad for the environment. Even your hygiene routine could be contributing to coral bleaching. Switching to a zero waste sunscreen, which won’t contain things like oxybenzone – one of many skincare ingredients to avoid – is a good place to start.

Not all coral reefs can be saved. It would be fantastic if humanity rapidly came together and every reef was rescued, but the reality is not every reef will be saved. One interesting concept that explores this is conservation triage, where efforts are focused on the most important areas instead of being spread too thin.

In terms of coral, this would mean focusing on important areas like controlling Great Barrier Reef bleaching versus saving a handful of other, smaller reefs that aren’t host to such biodiversity.

Final thoughts

Mass bleaching events are on the rise as global temperatures soar, and losing these aquatic ecosystems will have disastrous consequences if we don’t take action now.

But bleached coral reefs can rebound, and making small but impactful changes to your lifestyle plays an important role in finding a better path forward.

How might you be impacted by the loss of our coral reefs?

We love hearing from you.

Feel like contributing or have a question? Comment below.