Coral reef

Coral Bleaching and CO2


Carbon dioxide (CO2)

I’m sure you’ve heard this combination of words before, perhaps even studied CO2 in your school science class. Maybe you've seen them on an advert on the train, or even on the TV, and we’re all now pretty used being told that ‘we’re using too much CO2’, but what exactly is carbon dioxide, and why is its effect on our environment so dire?

Carbon dioxide is a gas that occurs naturally in the atmosphere, it's produced by both humans and animals and released into the atmosphere every time we exhale, and it's also produced by combustion. All this carbon dioxide is then re-absorbed and removed from the atmosphere by trees and plants through photosynthesis.

So, if carbon dioxide is produced naturally, why is it so bad for our planet? Well, everything in moderation, right. The problem starts when more CO2 is being produced than able to be absorbed, resulting in an excess build up of carbon dioxide. Every single day we’re adding to our planet's already very high levels of carbon dioxide, by burning fossil fuels and depleting our natural resources such as coal, oil and gas.1

What effect does this have on our planet?

The amount of carbon dioxide in our atmosphere has been gradually increasing over the past 150 years, and with the increase of carbon dioxide has come the increase in the average temperature of our planet. As these levels have increased, more heat from the sun has been trapped, which has prevented the heat from escaping into space. Meaning, the more carbon dioxide in the atmosphere, the hotter our earth becomes.

Coming from a very hot country myself (Australia), which emits a lot of carbon dioxide and significantly lacks an Ozone layer, I’ve felt first hand the effects of climate change. It also feels like every year in the UK we have the ‘hottest April’ or the ‘hottest May’. In fact, this year we had the hottest April day since 1949. Even with records like this, I’ve only ever thought of the effects of climate change as being on land, and until recently, I didn’t even think that the effects of climate change also impacted our oceans. Silly, I know.

Why are the oceans important when it comes to climate change?

Not only does the ocean cover 71% of the earth's surface it supports a staggering 85% of the world's plant life. The ocean therefore plays a key roll in helping to limit the effects of climate change. Ocean waters currently absorb vast amounts of carbon dioxide, and they work like a buffer to help reduce the effects of climate change.

Nearly half of the carbon dioxide that we have produced in the last 200 years has been absorbed by our oceans.2 Microscopic algae also use carbon dioxide to grow, and when they die, they sink to the bottom of the ocean keeping that carbon dioxide out of our atmosphere.3 

Photo of a coral

A photo from Belize Barrier reef, photo courtesy of

How does this affect our oceans?

As the ocean absorbs carbon dioxide and becomes warmer, life in the ocean struggles. Unfortunately, each year our coral reefs around the world suffer bleaching events.

Like a clown fish and an anemone, coral and the photosynthesizing algae called zooxanthellae, live together in harmony. Basically, you can’t have one unless you have the other. Interesting, huh? 

Illustration of coral bleaching process

A chart on coral bleaching, photo courtesy of NOAA’s National Ocean Service.

A single coral animal is known as a polyp. Many species of coral grow in colonies and hundreds and sometimes thousands act as a single organism. Single-celled zooxanthellae live within coral’s tissues and they essentially help them eat. Coral reefs are very important as they support the majority of animals and fish who live in a reef ecosystem.4

Coral reefs are the foundations of reef ecosystems, but unfortunately with the increase of carbon dioxide in our atmosphere, and the warming of our oceans, coral reefs are facing up against mass bleaching events. The zooxanthellae algae that live in harmony with the coral, don’t like water that is too hot, so when the water becomes warmer the algae stops producing carbohydrates, which feeds the coral and instead it produces toxic waste. The coral polyps then reject the zooxanthellae and expels it. Once the coral has expelled the zooxanthellae, it turns white, known as bleaching.

There is a possibility that if the water temperature drops quickly, the coral has a chance of coming back to life! If not, the coral will eventually die, and leave its skeleton behind. The dead coral which is left could become covered in other types of algae which could change the species that live on the reef.5 Coral bleaching is happening fast, and since 2015, half of the coral in the great barrier reef off the east coast of Australia has died.6

A very alarming statistic.

Photo of a researcher on the great barrier reef

A researcher on the great barrier reef – photo credit: The Atlantic.

So, what can I do to help?

  • Walk, cycle or take public transport, or if you have to drive, car-pool.

  • Make climate-conscious political decisions. Government policy has a huge impact on how companies operate and energy is produced.

  • Eat less red meat. Livestock animals produce large amounts of methane, so fewer steaks will mean less CO2.

  • Make your home and household energy efficient. If you don't need as much energy to heat your home, less will need to be generated.

  • Buy energy efficient appliances. Less energy required to do your washing will cut your carbon footprint.

  • and, switch to renewable energy!

1BBC Bitesize 2, 3 WWF Open Ocean Importance 4Inverse 5Reef Teach 6The Atlantic