Graveyards of the Sea: What is Coral Bleaching?

Aizah Malik
5 min readJan 29, 2022


Whether it’s through finding Nemo or a colouring book, most of us have been seeing coral reefs since we were kids. They’re bright, beautiful, and full of life — literally.

But these underwater rainforests are in big trouble. Anthropogenic climate change is quite literally sucking the life out of them. Here’s how.

Largest living structure in the world: Great Barrier Reef

What even are coral reefs?

Cities are hotspots for human civilization. Countless people from different walks of life wake up each morning and contribute something different to their community. A coral reef is similar to a bustling city because of just how many critters call it home.

A whopping 25% of marine life live in or around coral reefs, showing just how vital they are to the ocean. Everything from massive whales to microscopic plankton have a place in the rich tapestry of a coral reef ecosystem.

The species that inhabit a coral reef are like the citizens of the city, while the corals themselves are the foundation. One might even say they’re the ‘brick and mortar’ of the sea ;)

Corals are individual animals (yes, animals) that latch onto rocks at their larval stage of life. After they die, they leave behind a calcium carbonate skeleton that future generations of corals will grow upon.

After countless years of this process, coral reefs develop. These are when many different species of reef building corals have worked together to create a home for marine life that architects can only wish to recreate.

Every nook and cranny in a reef is teeming with life. Whether it’s tiny pygmy sea horses or viscous-looking moray eels, everyone who inhabits the coral reef can confirm they’re a great real estate investment. I’d go as far as to argue that trying to find a home in the coral reef is a marine organisms equivalent of finding a good deal for a NYC apartment.

The Threat

When most people hear ‘bleaching’, they think of a teenager going through a bad breakup and deciding to impulsively dye their hair.

Unfortunately, the word ‘bleaching’ in the context of corals isn’t quite so lighthearted.

From an ocean of colour to a bone-chilling sight, here’s how coral bleaching works:

  1. Coral reefs are made of individual coral polyps
  2. In the polyps of reef-building coral species, an algae called zooxanthellae lives
  3. Zooxanthellae produce the majority of the coral’s food by conducting photosynthesis, something corals can’t do on their own
The process of coral bleaching

4. Rising temperatures courtesy of climate change cause a coral to become irritated with their zooxanthellae and kick them out as though they were squatters. The mechanisms behind this process are widely unknown.

5. Without their main source of energy and colours (yep, the reason corals have all those pretty colours is thanks to zooxanthellae), the corals almost always end up dying.

6. Once the coral has died, their skeleton is often overrun by turning algae, as seen below

Dead corals that have been overrun with algae

Over the past few decades, coral bleaching has increased exponentially. Mass bleaching events are occurring at an unprecedented rate as temperatures rise due to global warming.

Nearly half of the Great Barrier Reef, the largest living structure in the world, is already dead or in the process of dying due to coral bleaching.

This is a good time to remember that this is the same Great Barrier Reef that is so big that it can be seen from space.

It’s not just the Great Barrier. Scientists estimate that 90% of coral reefs will be dead by 2030.

Satellite image showing the outlines of the Great Barrier Reef

Why Care

When you think of coral reefs, chances are that your mind jumps to a tropical vacation. While it is true that corals offer much to the world in terms of their beauty, there is more to them than meets the eye…

In fact, coral reefs:

  1. Protect coastal regions from severe storms

A physical barrier such as coral reefs acts as a buffer for storms and floods. This lowers the numbers of lives lost and the property damage in these vulnerable communities.

2. Are home to life-saving drugs

If you think about it, this news should come as no surprise. After all, coral reefs are hotspots for all kinds of marine organisms that have evolved chemical compounds over the years to protect themselves from predators and get an edge over competitors fighting for the same resources. Many times, the organisms will develop these compounds to become more resistant to diseases.

The idea of harnessing these compounds for scientific research and medicine is no pipe dream either. If we go back to the 1950’s, scientists were able to isolate two chemicals called spongothymidine and spongouridine from a species of sea sponge found in the shallow waters of the Caribbean.

These two tongue twister substances possess properties to fight HIV, AIDs, and herpes. In fact, they were the first marine drug approved for cancer treatment.

Perhaps the hidden treasure of the sea is not gold coins but the countless lives that could be saved by the naturally occurring compounds in marine life. It would take researchers decades (if it’s even possible) to find answers to problems the coral reef has evolved to solve already.

3. Provide countless jobs

Over half of the world’s federally run fisheries depend on coral reefs. They also provide jobs in tourism and recreation. Think about it. So many of the top tourist destinations in the world show beautiful oceans full of tropical fish.

Nearly half a billion people benefit directly from coral reefs because their local economy is dependent on them.

If that doesn’t convince you, a WWF report estimated that coral reefs contribute a net 30 billion dollars worth of ecosystem services worldwide.


Coral reefs are invaluable ecosystems that are the cornerstone of the ocean. They provide homes to countless species, jobs for millions of people, and are the medicine cabinets of the 21st century.

Unfortunately, they are getting closer and closer to extinction due to rising temperatures causing corals to expel their zooxanthellae — an algae that has a symbiotic relationship with the corals and help them make their food.

Due to this, the corals get ‘bleached’ and eventually die, becoming collateral damage of human foolishness.

To learn more about coral reefs, all they offer the world, and work done to slow down and reverse coral bleaching, click here.