Modelling climate change at Higher Level: Different examples from SL: ocean heat, the last ice age, Greenland's ice sheet, Alaska's lakes and the test of old climate models. At HL, link these loops to tipping points (6.2.13) and to how climate models are built (6.2.11).
Practise this as you read
- Draw a closed loop and give its sign.
- Explain which loops speed up warming and which slow it.
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Energy in, energy out: Earth's temperature is set by its energy balance. When the two are equal, global temperature stays steady.
The points to remember
- Sunlight (short-wave) arrives; some is reflected straight back by clouds, ice and snow.
- How much a surface reflects is its albedo: fresh snow is high, dark ocean is low.
- The rest is absorbed; the warm surface gives off infrared (long-wave) heat.
- Greenhouse gases absorb infrared and send some back down, keeping Earth warm.
- More greenhouse gas, or a lower albedo, keeps more energy in: an imbalance, so Earth warms.
Real example (HL): the oceans. About 90% of the extra energy kept by Earth goes into the oceans. Measurements show the heat stored in the upper ocean set a new record in 2023 and again in 2024.
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The input to the system, sunlight, is not quite constant. Small natural changes in it can start a change in climate, which feedback loops then make bigger.
The points to remember
- The Sun's output rises and falls by about 0.1% over an 11-year sunspot cycle.
- Slow changes in Earth's orbit and tilt, over tens of thousands of years, change where sunlight falls. They set off ice ages.
- Feedback loops amplify the small start: more ice reflects more sunlight; colder oceans take up more carbon dioxide.
- The Sun's output has not risen since the 1950s, but global temperature has, so recent warming is not caused by the Sun.
Real example (HL): the last ice age. About 20,000 years ago, ice sheets covered much of North America and northern Europe. Orbit changes started the thaw; rising carbon dioxide, from about 190 to 280 ppm, and shrinking ice amplified it until about 11,700 years ago.
A common slip: Do not blame today's warming on the Sun. Its output has been flat or slightly lower since the 1950s, while temperatures rose.
A feedback loop is drawn in four steps. Here is the ice-albedo loop for Greenland's ice sheet.
Draw an arrow to what it causes, and write the link on the arrow.
Keep adding effects until the next one changes temperature again.
Close the loop back to the start. Each step makes the change bigger, so it is positive.
The points to remember
- Start with the change (temperature rises).
- Each box is one effect; each arrow is labelled.
- The last arrow returns to the start: an open chain is not a loop.
- Amplifies the change: positive. Reverses it: negative.
Real example (HL): in July 2012, for a few days, almost the whole surface of Greenland's ice sheet melted. Dark algae and soot on the bare ice lower its albedo further.
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Permafrost holds the frozen remains of plants that lived thousands of years ago. Thawing it starts a methane loop.
The points to remember
- Northern permafrost holds about 1,500 billion tonnes of carbon, about twice what is in the air now.
- When it thaws, microbes decompose the remains, releasing methane and carbon dioxide.
- Methane traps about 80 times more heat than carbon dioxide over 20 years.
- Methane hydrates can break down as the water warms.
- More greenhouse gas means more warming and more thawing: a positive loop.
Remember it as: Thaw, rot, release, warm, thaw again.
Real example (HL): Alaska. Methane bubbles up from lakes formed in thawing permafrost, and in winter it can be trapped in the lake ice. NOAA's 2024 Arctic Report Card found the Arctic tundra, long a carbon store, is now a source of carbon, as wildfires spread.
Not every loop speeds warming up. A negative loop works against the change, so it slows it down.
The points to remember
- Clouds: warming increases evaporation; more low cloud raises albedo, reflecting sunlight (negative).
- Plants: more carbon dioxide speeds up photosynthesis, so more carbon is taken up (negative).
- Water vapour: warmer air holds more water vapour, itself a greenhouse gas (positive).
- Today the positive loops are stronger, so overall they add to the warming.
Real example (HL): the ocean takes up about a quarter of our carbon dioxide, but warmer water dissolves less gas. So warming weakens this brake, turning part of a negative loop positive.
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A systems model joins the causes, the loops and the effects into one picture of cause and effect.
What a model does well
- Shows causes and effects in one picture
- Shows which loops add to or slow warming
- Lets scientists test 'what if' changes
Its limits
- Many loops act at once, with time lags
- The strength of each loop is uncertain
- It simplifies, so it can miss surprises
Real example (HL): a 2020 study checked 17 climate model projections made between 1970 and 2007. Most matched the warming that actually followed, which supports using such models.
How this comes up: Paper 2 Section A can ask you to construct a feedback loop diagram [4]: one mark per correct link in the loop. Words alone, with no diagram, score at most 3.
In 2023 wildfires burned more than 15 million hectares of Canada's boreal forest.
Construct a feedback loop diagram showing the impact of rising global temperature on wildfires in this forest.
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