The big idea: A feedback loop is when the result of a change loops back and affects that same change again.
In positive feedback, the result makes the original change even bigger — the change feeds on itself and speeds up.
In climate change this is dangerous: a bit of warming can trigger loops that cause even more warming, which triggers the loops again — so the warming can run away.
- Feedback loop
- When the output of a process loops back to influence the same process again.
- Positive feedback
- Feedback that AMPLIFIES a change — the change drives an effect that makes the change even bigger (self-amplifying).
- Negative feedback
- Feedback that OPPOSES a change — the change drives an effect that reduces it, returning the system toward balance (self-correcting).
- Tipping point
- A threshold beyond which change becomes self-sustaining and very hard or impossible to reverse — the system 'tips' into a new state.
- Albedo
- How much sunlight a surface reflects. Bright ice/snow has high albedo (reflects heat); dark sea/soil has low albedo (absorbs heat).
'Positive' does NOT mean 'good': In feedback, positive means the loop adds to the change — it makes it bigger.
For climate, a positive feedback loop is actually bad news: it makes warming worse.
Negative feedback is the stabilising kind — it pushes the change back toward the start.
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Read each loop as a chain of cause and effect that ends back where it started — that loop-back is what makes it feedback.
The exam asks you to identify a process that contributes to a positive feedback loop, so know these three named loops and be able to trace the chain.
The ice–albedo loop, step by step
- Global warming melts bright white ice and snow.
- This exposes darker sea water or land underneath (a lower-albedo surface).
- The dark surface absorbs more heat instead of reflecting it.
- The extra absorbed heat causes more warming.
- More warming melts even more ice — so the loop repeats and amplifies.
The other two amplifying loops: Permafrost methane: warming thaws frozen permafrost soil, which releases methane and CO2 that were trapped inside. These greenhouse gases trap more heat → more warming → more thawing.
Ocean CO2 release: as the ocean warms, warm water holds less dissolved CO2, so CO2 is released to the air. The extra greenhouse gas traps more heat → the ocean warms further.
Every one of these is a loop where the effect feeds the cause — that is the signature of positive feedback.
| Positive feedback loop | The amplifying chain (cause → effect) |
|---|---|
| Ice–albedo (melting ice) | Warming melts bright white ice/snow → exposes darker sea or land → the dark surface absorbs MORE heat (lower albedo) → more warming → even more ice melts |
| Permafrost methane (thawing) | Warming thaws frozen permafrost soil → trapped methane and CO2 are released → these greenhouse gases trap more heat → more warming → more permafrost thaws |
| Ocean CO2 release (warming sea) | Warming warms the ocean → warm water holds LESS dissolved CO2 → CO2 is released to the air → the extra greenhouse gas traps more heat → the ocean warms further |
Tipping points and runaway warming: A tipping point is a threshold. Below it, the system can recover; past it, the positive feedback loops keep going on their own — change becomes self-sustaining and irreversible.
This is called runaway warming: enough ice melts, or enough permafrost thaws, that the loop drives more warming even if we stopped emitting.
Other tipping points include large-scale forest die-back (e.g. rainforest drying out) and collapse of major ice sheets.
Negative feedback (stabilising)
- Opposes the change
- Returns the system toward balance
- Output reduces the original change
- Example: more CO2 → more plant growth → some CO2 taken back
Positive feedback (amplifying)
- Amplifies the change
- Pushes the system further from balance
- Output increases the original change
- Example: melting ice → darker surface → more heat absorbed → more melting
A spot-the-loop trick: To check a process is positive feedback, finish the sentence: '…and that causes MORE of the thing we started with.'
Melting ice → darker surface → more warming → more melting. Yes — positive.
If instead it leads to LESS of the change (e.g. more plants soak up CO2), it is negative feedback.
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How this is tested: On Paper 1A (multiple choice) a 1-mark item asks you to identify a process that contributes to a positive feedback loop in global warming — pick the loop where the effect makes the warming even bigger (ice-albedo, permafrost methane, ocean CO2 release).
On Paper 2 you may be asked to explain how one such loop amplifies warming, or to distinguish positive from negative feedback. Score by tracing the chain and showing it loops back to more of the original change.
IB-style question — explain a positive feedback loop
As the climate warms, polar ice melts. Explain how the melting of ice acts as a positive feedback loop that amplifies global warming. [3]
How to score all three marks
- Start the chain. Warming melts bright (high-albedo) ice and snow, exposing the darker sea or land underneath.
- Effect of the darker surface. The dark, low-albedo surface absorbs more heat (sunlight) instead of reflecting it, so it causes more warming.
- Close the loop. That extra warming melts even more ice, which exposes more dark surface — the change feeds itself, so it is positive (amplifying) feedback. (Mark 1: melting exposes a darker/lower-albedo surface. Mark 2: dark surface absorbs more heat → more warming. Mark 3: more warming melts more ice — the loop amplifies / is positive feedback.)
Final answer
Melting ice exposes a darker, lower-albedo surface; that surface absorbs more heat and causes more warming; the extra warming melts even more ice — the effect feeds back to amplify the original change, so it is positive feedback.
Why this scores full marks: It is written as a loop, not a list: each step causes the next, and the last step leads back to more melting.
The word 'positive feedback' (or 'amplifies / self-reinforcing') is stated explicitly — examiners want the loop named, not just described.
| Negative feedback | Positive feedback | |
|---|---|---|
| What it does to a change | OPPOSES / reverses it | AMPLIFIES / drives it further |
| Effect on the system | Stabilises — returns toward the starting point | Destabilises — pushes ever further away |
| Direction of the loop | The output reduces the original change | The output increases the original change |
| Climate example | More CO2 → more plant growth → some CO2 removed | Melting ice → darker surface → more heat absorbed → more melting |
| Long-term result | Self-correcting, returns to balance | Self-amplifying, can run away to a tipping point |