What this lesson teaches
I can trace how climate change affects oceans, the atmosphere and living things.
Syllabus K226, Topic 3.2, Key Question 3. How might climate change affect natural systems?
Make a guess
Scientists find a reef where seawater pH is falling. Is this coral bleaching?
- Yes, because the sea has turned into an acid.
- No. Falling pH is acidification, a separate process.
- Yes. Bleaching and acidification are the same thing.
- No, because warm water raises pH instead of lowering it.
Show the answer
No. Falling pH is acidification, a separate process.
Bleaching is caused by unusually warm water. Acidification happens when more carbon dioxide dissolves in seawater, leaving less carbonate for shells.
Warmer seas bleach coral and raise sea level, while extra carbon dioxide makes seawater more acidic and harms shell-builders.
The oceans absorb most of the extra heat trapped by greenhouse gases. Warming, fresh water from melting ice and changing winds affect how dense seawater is, how it mixes and how currents flow. A warmer surface layer mixes less, so nutrient-rich deep water reaches the surface less often. Changing currents also move heat and sea life differently. Effects differ by region; currents have not all stopped.
When water gets unusually warm, corals lose the algae that give them much of their energy, and turn white. This is coral bleaching. Bleached corals can recover, but long or repeated heat makes death more likely. When reef habitat is lost, other animals lose food and shelter, and the food web changes.
Ocean acidification is a separate process. More carbon dioxide dissolves in seawater, which lowers its pH. This leaves less carbonate for creatures that build shells or skeletons from calcium carbonate. Acidification means becoming more acidic. Seawater stays above pH 7; it is just less alkaline.
Warm water expands, raising sea level. Melting land ice adds more water. Melting sea ice, which is already floating, does not add much directly. Rising seas and damaged ecosystems then affect coastal habitats, fishing and towns. Follow the chain from physical change to ecosystems to people.
- Coral bleaching
- Heat makes corals lose their algae.
- Acidification
- Dissolved carbon dioxide lowers pH and carbonate.
- Sea level rise
- Warm water expands and land ice melts.
Step by step
Heat stress
Warm water makes corals bleach.
Chemical change
Carbon dioxide lowers pH and carbonate for shells.
Wider losses
Damaged reefs affect food webs and people.
Real examples to use
Great Barrier Reef, Australia
Unusually warm seas caused mass coral bleaching in 2016, 2017, 2020, 2022 and 2024.
Rising seas
Global average sea level has risen by about 20 cm since 1900, as water warms and expands and land ice melts.
Worked example: From reef to fishing village
Long heat stress kills corals. The reef breaks down, and fish lose their habitat. Catches fall, so a fishing community earns less. The impact also depends on overfishing and other jobs available.
Watch out for this
Acidification means the ocean has become an acid below pH 7.
It means pH is falling. Seawater can become less alkaline and still stay above pH 7.
Check your understanding
A reef study finds falling pH and less carbonate. What directly explains this?
- A daily sea breeze that mixes the surface water.
- Corals losing their algae as the water warms.
- More carbon dioxide dissolving into the seawater.
Show the answer
More carbon dioxide dissolving into the seawater.
Yes. This is ocean acidification.
Check your understanding
What is the difference between coral bleaching and ocean acidification?
- They are one process, given different names by the biologists and chemists who study it.
- Bleaching comes from heat stress; acidification from dissolved carbon dioxide.
- Acidification is caused by warmer water, and bleaching by plastic pollution.
Show the answer
Bleaching comes from heat stress; acidification from dissolved carbon dioxide.
They are different processes, though both harm reefs.