Coral reefs and climate change: from cradle to an early grave
What are Coral Reefs?
Corals might look like plants or even rocks, but they are actually tiny animals called a ‘polyps’. Each polyp has a soft body, with a little mouth surrounded by tentacles that grab food – similar to a sea anemone. To protect themselves, corals use minerals from seawater to build hard skeletons made of limestone. When thousands of these polyps live together, their skeletons connect to form a coral reef.
Over time, coral reefs grow into large, colourful underwater cities that give shelter and food to all kinds of sea life - from tiny fish to giant turtles. That’s why they’re often called the “rainforests of the sea.”
The amazing thing about reefs
On their own, coral polyps are see-through, and their skeletons are plain white. So where do all those incredible colours come from? The answer is in the billions of tiny algae, called ‘zooxanthellae’, that live inside the corals. These algae use sunlight to make food through photosynthesis, just like plants on land, and they share that food with the polyps. In return, the polyps give the algae a safe place to live.
This relationship between the zooxanthellae and the coral polyps is an excellent example of a mutually beneficial relationship, also known as symbiosis.
Why are coral reefs so important?
Although they occupy less than 0.1% of the ocean floor, coral reefs are home to 25% of all marine life. That’s millions of fish and other species that find shelter, food, and safe places to raise their young among the coral.
Reefs are vital for people too – they protect coastlines from storms and waves that can cause flooding and erosion. They also provide food and livelihoods to many millions of people, worth around £300 billion each year.
What are the threats to coral reefs?
Over the last three decades, the world has lost half of its coral reefs. Today, over 80% of the world’s reefs are affected by bleaching, driven by rising ocean temperatures.[1] Destructive fishing, pollution from land, coastal development, and shipping have also damaged reef ecosystems.
In addition, our oceans absorb as much as 30% of human-made carbon dioxide emissions from the air (much like our forests do).
This extra carbon makes seawater more acidic, which weakens and corrodes corals’ limestone skeletons, leaving them more fragile and less able to recover.
What is Coral Bleaching?
Corals are very vulnerable to climate change, as they are sensitive towards changes in ocean temperature. When the water gets too warm, the corals become stressed and expel the algae (zooxanthellae) living inside them. These algae not only give a reef its colour, but also provide most of the food that corals need to survive.
Without algae, only the corals’ white skeletons are left behind, which is why reefs look “bleached.” While recovery is sometimes possible if conditions improve, it can take decades, and continued warming from climate change makes that recovery less and less likely.
The series of bleaching events between 2015 and 2017 killed vast areas of reef around the world, including nearly half of the Great Barrier Reef. And unfortunately, this trend has continued, as we are now in the midst of the fourth global mass bleaching event, with the vast majority of the world’s reefs affected.
Scientists warn that if the world warms by more than 1.5 °C above pre-industrial levels, we could lose over 90% of coral reefs by 2050. That makes it urgent for us to cut greenhouse gas emissions fast - before it's too late for our reefs.
What can I do?
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Sources
[1]NOAA Reef watch, Current Global Bleaching: Status Update & Data Submission, https://www.coralreefwatch.noaa.gov/satellite/research/coral_bleaching_report.php