AP® Environmental Science review sheet from Aim for Five (aimforfive.com/enviro/units/9/9-6)
Unit 9 · Topic 9.6
9.6 Ocean Warming
The ocean has absorbed about 90 percent of the extra heat trapped by greenhouse gases. Warmer water causes coral bleaching, pushes marine species toward the poles or deeper water, holds less oxygen and expands, adding to sea level rise.
Key terms
- ocean warming
- coral bleaching
- zooxanthellae
- habitat shift
- thermal expansion
The ocean as a heat sink
Water can absorb a huge amount of heat with only a small rise in temperature; it has a high specific heat. Because of that, and because the ocean is so vast, it has taken up about 90 percent of the extra energy trapped by greenhouse gases since the 1970s. That slows warming on land, but it changes the ocean itself. Most of the heat is in the upper few hundred meters, though warming is reaching deeper water too. Marine heat waves, periods of unusually warm water lasting days to months, have become more frequent and intense.
Coral bleaching
Reef-building corals live in partnership with tiny photosynthetic algae called zooxanthellae, which live inside the coral's tissues. The algae give the coral up to about 90 percent of its food from photosynthesis, along with much of its color; the coral gives the algae shelter and nutrients. This is mutualism, a relationship that benefits both partners.
When water stays even about 1 °C above the usual summer maximum for several weeks, the stressed coral expels its zooxanthellae. Without them, the coral's tissue turns transparent, revealing its white skeleton: it is bleached. A bleached coral is not dead yet. If temperatures drop soon enough, the algae can return. If the heat lasts, the coral starves, becomes more vulnerable to disease, and dies.
Mass bleaching events have struck reefs around the world repeatedly, including several on Australia's Great Barrier Reef since 2016, and a global bleaching event that began in 2023. When coral dies, the reef's structure slowly breaks down and the many species that depend on it lose their habitat.
Other effects of ocean warming
- Habitat shifts: many fish and other marine species are moving toward the poles or into deeper, cooler water to stay within their range of tolerance. That disrupts food webs and fisheries.
- Less oxygen: warm water holds less dissolved oxygen, and warming increases layering of the ocean so less oxygen mixes down from the surface, expanding low-oxygen zones (compare topic 8.6).
- Changes in metabolism and reproduction: most marine animals are cold-blooded, so warmer water speeds up their metabolism and raises their need for food and oxygen. Heat can also shift spawning times and lower the survival of eggs and larvae.
- Sea level rise from thermal expansion of the water itself (topic 9.4).
- Stronger storms: warmer surface water supplies more energy to hurricanes and typhoons, increasing their rainfall and intensity.
- Melting ice: warmer ocean water melts ice shelves at the edges of Antarctica and Greenland from below.
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Reasoning chain: from warming to fish decline
Explain how a marine heat wave lasting two months could lead to a drop in reef fish populations a year later.
Show the solutionHide the solution
- Step 1: The heat wave raises water temperatures above the corals' tolerance for weeks.
- Step 2: Stressed corals expel their zooxanthellae and bleach.
- Step 3: Because the heat lasts, many corals starve and die before the algae can return.
- Step 4: Dead coral loses its structure and becomes covered by algae, so fish lose the food, shelter and nursery areas the living reef provided.
- Step 5: With less habitat and food, fewer young fish survive, and reef fish populations fall.
Answer: Heat stress → bleaching → coral death → loss of reef habitat and food → fewer fish survive and populations decline.
- Example 2
Trap: bleaching versus acidification
A student writes that 'coral bleaching happens because the ocean is becoming more acidic and dissolves the coral.' Correct this statement.
Show the solutionHide the solution
- Step 1: Bleaching is caused mainly by heat stress, which makes corals expel their zooxanthellae, turning them white.
- Step 2: Ocean acidification is a separate problem caused by the ocean absorbing CO₂, which lowers pH and makes it harder for corals to build calcium carbonate skeletons (topic 9.7).
- Step 3: Both are driven by rising CO₂ and both harm reefs, so they often act together, but they are different processes.
Answer: Bleaching comes from warming, which makes corals expel their algae; acidification is a different process that weakens skeleton building.
Common mistakes
- Saying a bleached coral is dead. It is severely stressed and can recover if temperatures drop in time.
- Confusing coral bleaching (caused by heat) with ocean acidification (caused by CO₂ lowering pH).
- Calling zooxanthellae a parasite. The relationship is mutualistic; both partners benefit.
On the exam
- Expect to explain the coral–zooxanthellae relationship and what happens to it under heat stress.
- If asked for an effect of ocean warming other than bleaching, species moving poleward, reduced dissolved oxygen or thermal expansion are all specific answers.
Connected topics
Videos
Check yourself
4 questions on 9.6 Ocean Warming. Pick an answer to see if you got it, and why.
| Month | Average water temperature at reef (°C) | Corals showing bleaching (%) |
|---|---|---|
| March | 27.5 | 2 |
| April | 28.5 | 5 |
| May | 30.0 | 38 |
| June | 30.5 | 61 |
Hypothetical monitoring data from a tropical reef
Which statement best explains the trend in coral bleaching?
Over several decades, the ranges of many fish species in the North Atlantic have shifted toward the poles. Which explanation is best supported?
If unusually warm water lasts for many weeks, bleached corals often die. Which statement best explains why?
The ocean has absorbed about 90% of the extra heat trapped by greenhouse gases, yet ocean surface temperatures have risen less than air temperatures over land. Which statement best explains this?
0 of 4 answered