Coral Reef Bleaching

Lesson notes

Overview

Heat stress makes corals expel their symbiotic algae and turn white. This topic is rated advanced and is number 93 of 98 in Earth & Space. Deeper models, formal results and harder reasoning.

What you are seeing

The panel above shows the steps of a process, laid out in order. Click a step to see what happens next. The animation is drawn live from the model, so everything you see responds to the sliders. Press Space to pause, use Restart to replay from the start, and change the speed to slow down quick events.

In depth

Corals live in symbiosis with tiny algae that give them food and colour. When the water is too warm, even 1 to 2 °C above the usual summer maximum for a few weeks, the algae produce toxic compounds and the corals expel them, turning white. Bleached corals may recover if the heat subsides, but prolonged stress kills them. Mass bleaching events, such as those on the Great Barrier Reef in 2016, 2017 and 2020, have become more frequent with global warming.

Reading the relationship

The expression "thermal stress" is a compact summary of the idea. It uses the symbols t, h, e, r, m, a; name what each one stands for before you start. Then use the sliders to see how changing one quantity changes the others.

A guided exploration

1. Run the animation once with the default settings and describe in words what happens. 2. Change one slider at a time and note which quantity responds most. 3. Push each slider to its extreme and decide whether the result still makes sense.

Try this

Try to explain why the pattern holds, not just what it is. Look for limiting cases and what happens at the edges.

Common pitfalls

Earth and space systems work on very different scales of time and distance. Check which scale a number refers to before comparing it with another.

How it connects

It builds on “The Ocean Carbon Pump”, the topic before it in Earth & Space. Once you are comfortable, continue with “Deforestation Feedbacks”.

Why it matters

Earth and space science helps us understand our planet, its climate and the wider universe.