Unit 9 · Topic 9.6 Beta

Ocean Warming

The ocean absorbs about 90% of the extra heat.

Practice 5: Data AnalysisPractice 1: Concept Explanation

Part 1 · Hook

Why this matters

In 2016 and 2017, water off northeastern Australia stayed unusually warm for weeks. Large parts of the Great Barrier Reef turned white and many corals died. The ocean had been quietly soaking up heat for decades, and the reef showed what that heat can do.

Part 2 · Before you start

What this builds on

Part 3 · Prerequisite check

Quick check before you start

1. Which change raises sea level?

  1. Melting land ice
  2. Melting sea ice
  3. More sea ice
  4. More snow on land
Show the answer

Land ice adds water to the ocean; floating sea ice does not.

  • Correct: Melting land ice:
  • Melting sea ice:
  • More sea ice:
  • More snow on land:

2. How does temperature affect dissolved oxygen in water?

  1. Warmer water holds less oxygen
  2. Warmer water holds more oxygen
  3. Temperature has no effect
  4. Only salt water is affected
Show the answer

Gases dissolve less in warm water, which is why thermal pollution lowers oxygen.

  • Correct: Warmer water holds less oxygen:
  • Warmer water holds more oxygen:
  • Temperature has no effect:
  • Only salt water is affected:

Part 4 · See it

See it first

A central box: the ocean absorbs heat; it has taken up about 90% of the extra heat trapped since 1970. Arrows lead to five effects: water expands, so sea level rises; warm water holds less dissolved oxygen, so fish and shellfish struggle; heat-stressed corals expel their algae and can starve (coral bleaching); fish shift toward the poles or into deeper, cooler water; and a warm surface layer floats on top, so fewer nutrients rise to it.
The ocean absorbs most of the extra heat, with five main effects. LevlPrep original diagram.

Part 5 · Step by step

How it works, step by step

  1. Greenhouse gases trap extra heatthe ocean absorbs about 90% of it, because water stores heat well
  2. Warmer water takes up more space (thermal expansion)sea level rises, alongside water from melting land ice
  3. Warmer water holds less oxygen and mixes lessoxygen falls and fewer nutrients reach the surface
  4. Heat stress and shifting temperaturescorals bleach, fish move toward the poles, and currents can change

Part 6 · Key ideas

Key ideas

  • The ocean has absorbed about 90% of the extra heat trapped since 1970.
  • Thermal expansion: warmer water takes up more space. With melting land ice, it causes most sea level rise. It is caused by heat, not by dissolved CO₂.
  • Warming lowers dissolved oxygen, bleaches corals, shifts species ranges and makes the surface layer mix less with nutrient-rich deep water.
  • The thermohaline circulation (ocean conveyor belt) is driven by temperature and salinity; fresh meltwater could weaken it.

Part 7 · Misconception

A common mistake

The wrong idea: Seawater expands because it absorbs carbon dioxide.

What actually happens: Thermal expansion is caused by heat: warmer water molecules move faster and spread apart. CO₂ dissolving in seawater changes its chemistry, which is a different problem covered next.

Part 8 · Check yourself

Check yourself

Exam-style questions. Anything you miss goes into your review queue.

1. What is thermal expansion of seawater?

  1. Water takes up more space as it warms
  2. Water gains mass as it absorbs CO₂
  3. Ice on land slides into the sea
  4. Warm water rises and cold water sinks
Show the answer

Thermal expansion: the same mass of water fills a larger volume when it is warmer. It is one of the two main causes of sea level rise.

  • Correct: Water takes up more space as it warms: Right: warmer water molecules move faster and spread apart, so the same mass fills more volume.
  • Water gains mass as it absorbs CO₂: Dissolved CO₂ adds little volume; thermal expansion is caused by heat.
  • Ice on land slides into the sea: That is land ice adding water, a separate cause of sea level rise.
  • Warm water rises and cold water sinks: That describes circulation, not expansion.

2. Which two processes cause most of today’s sea level rise?

  1. Melting sea ice and more rainfall
  2. Melting land ice and thermal expansion
  3. Dissolved CO₂ and melting sea ice
  4. More rivers and sinking icebergs
Show the answer

Land ice adds water; thermal expansion makes the existing water take up more room.

  • Melting sea ice and more rainfall: Sea ice already floats; rain is part of the water cycle and does not add new water.
  • Correct: Melting land ice and thermal expansion: Right: water from glaciers and ice sheets, plus warmer water expanding.
  • Dissolved CO₂ and melting sea ice: Neither raises sea level much.
  • More rivers and sinking icebergs: Rivers return water that came from the ocean; they do not add new water.

Data

Dissolved oxygen that seawater can hold at each temperature

Most oxygen full seawater can hold at each temperature, mg/L (invented data for practice, close to measured values)
Water temperature (°C)Dissolved oxygen (mg/L)
011.2
510.0
109.0
158.1
207.4
256.8
306.2

3. Describe the relationship in the data.

  1. As temperature rises, oxygen rises
  2. Oxygen stays about the same across temperatures
  3. Oxygen rises up to 15 °C, then falls
  4. As temperature rises, oxygen falls
Show the answer

An inverse relationship: warmer seawater holds less oxygen.

  • As temperature rises, oxygen rises: The oxygen values fall as temperature goes up.
  • Oxygen stays about the same across temperatures: Oxygen changes by 5 mg/L across the table.
  • Oxygen rises up to 15 °C, then falls: Oxygen falls the whole way.
  • Correct: As temperature rises, oxygen falls: Right: from 11.2 mg/L at 0 °C to 6.2 mg/L at 30 °C.

4. By what percent does the oxygen the seawater can hold change as it warms from 10 °C to 20 °C?

Type a number, then its unit (%). Scale words (2.5 million) and powers of ten (2.5 × 10^6) are read.

Show the answer

(7.4−9.0)9.0×100\frac{(7.4 - 9.0)}{9.0} \times 100≈−17.8%\approx -17.8\%, a decrease of about 18%.

  • Answer: -17.8 %

5. A cod population needs at least 7 mg/L of dissolved oxygen. Using the table, above about what temperature would full seawater fall below this level?

  1. About 12 °C
  2. About 22 °C
  3. About 30 °C
  4. About 2 °C
Show the answer

Find where the values cross 7 mg/L: between 20 °C (7.4) and 25 °C (6.8).

  • About 12 °C: At 15 °C the water still holds 8.1 mg/L.
  • Correct: About 22 °C: Right: 7.4 mg/L at 20 °C and 6.8 mg/L at 25 °C, so it falls below 7 between them.
  • About 30 °C: At 30 °C it is already 6.2 mg/L, well below 7.
  • About 2 °C: Cold water holds the most oxygen, over 10 mg/L.

Model or diagram

Effects of ocean warming

A central box: the ocean absorbs heat; it has taken up about 90% of the extra heat trapped since 1970. Arrows lead to five effects: water expands, so sea level rises; warm water holds less dissolved oxygen, so fish and shellfish struggle; heat-stressed corals expel their algae and can starve (coral bleaching); fish shift toward the poles or into deeper, cooler water; and a warm surface layer floats on top, so fewer nutrients rise to it.
LevlPrep original diagram.

6. Which effect in the figure raises sea level?

  1. Water expands
  2. Less dissolved oxygen
  3. Coral bleaching
  4. Species move
Show the answer

Thermal expansion, together with melting land ice, raises sea level.

  • Correct: Water expands: Right: thermal expansion makes the ocean take up more space.
  • Less dissolved oxygen: Oxygen loss harms life but does not change sea level.
  • Coral bleaching: Bleaching harms reefs but does not change sea level.
  • Species move: Shifting ranges do not change sea level.

7. The figure shows less mixing in a warmer ocean. Why could this lower productivity at the surface?

  1. The surface gets too dark for photosynthesis
  2. Fewer nutrients rise from deep water
  3. Warm water has too many nutrients
  4. Phytoplankton sink into the warm layer
Show the answer

A warm, light surface layer floats on cold, dense water, which blocks mixing that brings nutrients up.

  • The surface gets too dark for photosynthesis: Warming does not darken the surface layer.
  • Correct: Fewer nutrients rise from deep water: Right: phytoplankton at the surface depend on nutrients mixed up from below.
  • Warm water has too many nutrients: The problem is fewer nutrients reaching the surface.
  • Phytoplankton sink into the warm layer: Phytoplankton stay near the surface; nutrients are what is missing.

Part 9 · Summary

Summary

The ocean absorbs about 90% of the extra heat. Warmer water expands (thermal expansion), which with melting land ice raises sea level. Warm water holds less oxygen and mixes less, so oxygen and surface nutrients fall. Heat stress bleaches corals and pushes species toward the poles. The thermohaline circulation, driven by temperature and salinity, could slow as fresh meltwater enters the North Atlantic.

Part 10 · Up next

What comes next

Part 11 · Connections

Connections

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