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Place 01 · The Great Barrier Reef · Chapter 7 of 7
When It Goes Wrong
White coral is not dead coral. It is coral that just lost its roommates.
About 20 minutes · 1 video · 1 live dashboard · 1 quick activity
Start here
Two problems, and the news mixes them up
Two different things are happening to this reef. One is a chemistry problem. One is a heat problem. Almost every news story blends them together. Telling them apart is the single most useful thing you will take out of these two weeks.
Watch this
See it move
National Geographic — First Ever Footage: Watch Coral Bleaching Happen Before Your Eyes.
While you watch, look for
- What the coral pushes out of its body
- How long the whole thing takes
- Something you did not expect. Hold onto it.
The short version
Three things going on here
Problem one
Chemistry: fewer bricks
Acidification is slow, global and invisible. It does not poison corals. It makes building more expensive, and a reef that builds slower loses ground to erosion.
Problem two
Heat: the tenants leave
When water stays too warm for too long, corals push out their algae. What you see is the white skeleton through clear tissue. The coral is starving, not dead, for now.
The pattern
Eight events, six since 2016
Mass bleaching hit the Great Barrier Reef in 1998 and 2002, then in 2016, 2017, 2020, 2022, 2024 and 2025. The clustering is the worry. The reef gets hit again before it has recovered.
Photos: Sandra Filipe, Karsten Sporenberg, Francesco Ungaro / Pexels
Play with it
Check the reef’s temperature right now
This is the same live dashboard scientists use to watch for bleaching. It shows heat stress on the central Great Barrier Reef, updated every day.
Live data: NOAA Coral Reef Watch (public domain).
Two questions to answer while you are in there
- What alert level is the central Great Barrier Reef at today?
- It is spring in Australia right now. When in the year would you expect the highest heat stress, and why?
Try it yourself
Check yourself
Try it
Heat or Chemistry?
Eight clues. For each one, decide whether it describes bleaching, which is heat, or acidification, which is chemistry.
Try it · interactive
Two Threats Sorter
Sort twelve statements into heat or chemistry. Every answer explains itself.
Open the Two Threats Sorter in its own window (opens in a new tab).
Words you will hear
Four terms, that is all
Bleaching
Corals pushing out their algae under heat stress
Thermal stress
Water that is too warm for too long
Degree heating weeks
How scientists add up heat stress
Resilience
How fast a reef can bounce back
Check your understanding
Practice the whole chapter
Everything in this chapter, in eight quick questions. Retry as often as you like. Read the feedback on every answer, right or wrong, because that is where the learning is.
Practice
Practice: When It Goes Wrong
Eight questions covering the video, the three cards, the four terms and the interactive above.
If you want more
Go deeper
- Tropical Marine Biology 3.3, 3.5 and 3.6 — Acidification, Bleaching, Warming — LibreTexts · 10 minutes · the assigned reading
- AIMS — Great Barrier Reef bleaching events — The official Australian record · optional
Photo credits (Pexels): wewe yang, mingche lee, Sandra Filipe, Karsten Sporenberg, Francesco Ungaro.
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All Places
- The Great Barrier Reef
- Monterey Bay
- The Galápagos
- The Ridge and the Trench — in development
- The Southern Ocean — in development
- The North Pacific Garbage Patch — in development