Most of this topic assumes gradual change: a bit more gas, a bit more warming. Tipping points are where that assumption breaks. Past a certain threshold, a small extra push flips part of the Earth system into a different state that it will not simply slide back out of.
📚 What you need to know
A climate threshold is a point at which a small change in conditions triggers a large shift in an Earth system. Critical thresholds are called tipping points.
Once crossed, a new equilibrium may establish itself. The change can be rapid and is often very hard to reverse.
Positive feedback loops are the mechanism: they amplify the change and drive the system towards its new state.
Global tipping points include Antarctic and Greenland ice sheet melting, AMOC slowing, and the Amazon–Cerrado transition.
Local tipping points exist too, such as widespread coral reef death above a temperature threshold.
Tipping points can interact, forming a tipping cascade in which crossing one makes others more likely.
They can be biotic (forests, reefs) or abiotic (ice sheets, oceans, atmosphere), or a mixture of both.
Why “tipping” is the right word
Think of a ball sitting in a valley. Push it gently and it rolls partway up the side and comes straight back — the system is stable and resists change. Keep pushing and eventually you get the ball over the ridge. Now it rolls down into a completely different valley, and it settles there. To get it back you would have to push it all the way up the ridge again, which takes far more effort than the nudge that sent it over.
This is why “we can always cut emissions later” is a weak argument. Reversing the cause does not automatically reverse the effect once a threshold has been crossed.
Two features define a tipping point: the change is rapid compared with the change that triggered it, and it is difficult to reverse. Mention both when you define the term.
Global tipping points
Ice sheet melting
The mechanism is the albedo feedback in its most powerful form. Melting ice exposes darker rock or ocean, which reflects far less sunlight and absorbs more heat, which melts more ice. Antarctic and Greenland ice loss also raises sea level worldwide, endangering coastal ecosystems and the very large share of humanity that lives near a coast. Ice sheets are slow to build and, past a certain point, cannot regrow under current conditions — which is exactly what makes this a tipping point rather than a trend.
AMOC slowing
The Atlantic thermohaline circulation is driven by dense water sinking in the far North Atlantic. Melting Greenland ice adds fresh water there, which lowers salinity, which lowers density, which weakens the sinking motion that pulls the whole current system along. A weaker AMOC disrupts the global transport of heat and nutrients and could bring harsher winters to Europe and North America even while the planet as a whole warms.
The Amazon–Cerrado transition
The Amazon partly makes its own rain: trees release water vapour that falls again further inland. Remove enough trees through deforestation and warming, and that recycling breaks down, so the forest dries and shifts towards a drier savannah-like ecosystem resembling the Cerrado. Because forest stores far more carbon than savannah, the transition releases carbon dioxide, which accelerates the global warming that caused it in the first place.
Local tipping points
Thresholds are not only planetary. A coral reef has one of its own. Below a certain sea temperature the reef recovers between bleaching events; above it, and with bleaching events arriving too close together, the corals die faster than they can regrow and the reef collapses into rubble and algae. The reef supported a large share of local marine biodiversity, so its loss changes the whole local ecosystem — and it does not come back on any human timescale.
Tipping cascades
Individual tipping points are not isolated. Crossing one can push another closer to its own threshold, and that chain is a tipping cascade.
Read the chain out loud: less ice means a warmer Arctic, a warmer Arctic thaws permafrost, thawed permafrost releases greenhouse gas, which warms the Arctic again and speeds the Greenland melt. Every arrow is a positive feedback.
The IB source material sometimes uses the phrase “trophic cascade” here. Be careful: a trophic cascade is about food chains and predators. A tipping cascade is about thresholds triggering each other. Use the right one and you show you actually understand the difference.
Biotic and abiotic tipping points
Type
What is involved
Example
Why it tips
Biotic
Living components: forests, reefs, populations
Widespread tree loss in the Amazon rainforest
The forest recycles its own rainfall, so losing trees dries the region and kills more trees
Biotic
Living components under environmental stress
Coral reef collapse after repeated bleaching
Corals die faster than they regrow once heat events arrive too frequently
Permafrost thaw releasing methane and carbon dioxide
Released gas warms the atmosphere, which thaws more ground
Abiotic
Non-living components: ocean circulation
AMOC slowing as the North Atlantic freshens
Lower salinity reduces density, weakening the sinking that drives the current
WORKED EXAMPLE
Explain, using an example, how a positive feedback loop can drive a system past a tipping point. [4]
Define the term
A tipping point is a threshold beyond which a small further change produces a large, rapid and hard-to-reverse shift into a new state.
Name the example and start the loopArctic warming melts sea ice, exposing darker ocean in its place.Complete the loopDark ocean has a much lower albedo, so it absorbs more solar radiation, warms further and melts more ice.Link back to the thresholdBecause each cycle strengthens the warming, the system accelerates past the point where cooling could restore the ice cover.4 / 4Definition, example, loop, threshold. That structure fits almost any tipping point question.
WORKED EXAMPLE
Outline what is meant by a tipping cascade and explain why it increases uncertainty in climate predictions. [3]
Definition
A tipping cascade occurs when crossing one tipping point makes other tipping points in the system more likely to be crossed.
Example of the linkArctic ice loss warms the region, which thaws permafrost, which releases methane and CO₂ and accelerates Greenland ice melt.Why prediction becomes harderThe interactions add warming that models may not fully capture, so the pace and scale of future change become far less certain.3 / 3The third point is the one that answers the actual question. Do not stop after describing the cascade.
💡 Exam tip
Define a tipping point with both features: rapid change and difficult to reverse.
Every tipping point answer should contain a complete feedback loop that returns to its starting point.
Keep four global examples ready: ice sheets, AMOC, permafrost, Amazon–Cerrado. Add coral reefs as the local one.
Say albedo, salinity and density where they apply. Precise terms carry the marks.
Distinguish biotic from abiotic if the question uses those words, and note that many are a mixture.
Link tipping points back to models: they are a major source of uncertainty in projections.
⚠️ Common mix-up
Treating a tipping point as just “a lot of change”. The defining feature is the threshold and the difficulty of reversing it.
Saying tipping cascade when you mean trophic cascade, or the other way round.
Describing only half a feedback loop. If your answer does not come back to where it started, it is not a loop.
Claiming Greenland ice melt is dangerous only because of sea level. Its effect on AMOC through freshening is just as important.
Forgetting local tipping points exist. Coral reefs are the standard example and are often the one asked for.
Assuming reversing emissions reverses the tipping point. The system may settle in the new state regardless.
Up next: Who Caused It and Who Suffers — the last page of this sub-topic, and the one where the science turns into a question about fairness.
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