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Place 01 · The Great Barrier Reef · Chapter 3 of 7
The Warm Belt
Reefs live in a stripe around the middle of the planet. Ask why, and you end up talking about a tilt.
About 15 minutes · 1 video · 1 live map · 1 quick activity
Start here
Draw every reef on a map and you get a belt
Not a scatter. A belt. Reef-building corals live between roughly 30° north and 30° south, and the reason has nothing to do with corals and everything to do with the angle at which sunlight hits a round, tilted planet.
Watch this
See it move
NOAA — 22 Years of Sea Surface Temperatures. Every color is a temperature. Red is warm, blue is cold.
While you watch, look for
- The warm band hugging the equator
- How the band shifts north and south with the seasons
- Something you did not expect. Hold onto it.
The short version
Three things going on here
The geometry
Sunlight spread thin
Near the equator, sunlight arrives almost straight down. Near the poles the same beam is smeared across far more ground. Same sun, less heat per square meter.
The window
Roughly 18 to 30 °C
Reef-building corals need warm water all year. Too cold and they cannot build. Too hot, as Chapter 7 will show, and the partnership inside them breaks.
The exception
Currents move the line
Warm currents carry reefs farther from the equator on some coasts, like Bermuda. Cold currents shut them out on others, like Peru. Place 03 is built on this idea.
Photos: Magda Ehlers, Tom Fisk, Ben Jackson / Pexels
Play with it
Watch the ocean’s temperature live
This is a live map of today’s ocean. The colors are sea surface temperature, and the moving lines are currents. It opens over the Great Barrier Reef. Drag to spin the globe, and click anywhere to read the temperature.
Live map: earth.nullschool.net by Cameron Beccario, using NOAA and other public data.
Two questions to answer while you are in there
- Click the water off Queensland. What is the temperature on the Great Barrier Reef today?
- Spin north to the Gulf of Maine. How much colder is it, and could a reef live there?
Try it yourself
Check yourself
Try it
Could a Reef Live Here?
Seven places. For each one, decide whether a coral reef could survive there. Check four things every time: warm, clear, shallow, salty.
Try it · interactive
Sun Angle Slider
Move the latitude and watch the same sunbeam spread thinner and thinner.
Open the Sun Angle Slider in its own window (opens in a new tab).
Words you will hear
Four terms, that is all
Insolation
Sunlight energy arriving on each square meter
Heat budget
Energy coming in versus energy going out
Tropics
The belt between 23.5° north and south
Sea surface temperature
SST, what satellites measure every day
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: The Warm Belt
Eight questions covering the video, the three cards, the four terms and the interactive above.
If you want more
Go deeper
- Webb 8.1 — Earth’s Heat Budget — LibreTexts · 8 minutes
- Webb 6.2 — Temperature — LibreTexts · 5 minutes
Photo credits (Pexels): Tuấn Vũ, Asad Photo Maldives, Magda Ehlers, Tom Fisk, Ben Jackson.
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← The Great Barrier Reef — place overview
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