Questions: Climate Tipping Points and Critical Transitions

Question 1

The Greenland Ice Sheet crosses a tipping point due to warming, losing a critical fraction of its mass. If global temperatures are then reduced back to their pre-tipping-point level, what would you expect?

• A. Complete restoration of the ice sheet over decades as the cooler temperatures allow re-glaciation
• B. The ice sheet remains collapsed — the ice-elevation feedback keeps the remaining ice in too-warm air to rebuild
• C. Partial recovery to about 50% of original ice mass, stabilizing at a new intermediate state
• D. Rapid re-glaciation, since the same positive feedback that drove collapse now drives recovery

Answer: (B)

This is hysteresis. The Greenland Ice Sheet has an ice-elevation feedback: its high surface elevation keeps it in cold air, but as melting lowers the surface, ice encounters progressively warmer air at lower altitude, accelerating further melt. Once enough ice is lost, the remaining ice sits in air too warm to sustain the sheet — even at the original global temperature — because it's now at a lower elevation. Restoring the sheet would require cooling well below the original threshold. The positive feedback that drove collapse does NOT simply run in reverse on cooling; the system is locked into the ice-free basin.

Question 2

What distinguishes a climate tipping point from a typical amplifying (positive) climate feedback?

• A. Tipping points only involve atmospheric processes, while feedbacks include ocean and land surface interactions
• B. A tipping point is a threshold beyond which a self-sustaining transition occurs that may be irreversible, while feedbacks amplify warming proportionally without crossing into a qualitatively new state
• C. Tipping points are caused exclusively by human activity, while positive feedbacks are natural features of the climate system
• D. Tipping points reduce uncertainty in climate projections by locking in a specific outcome

Answer: (B)

Positive feedbacks amplify perturbations but still allow the system to settle at a new equilibrium proportional to the forcing. A tipping point occurs when positive feedbacks become so dominant that they overwhelm restoring forces entirely, driving the system to a qualitatively different stable state. The Greenland Ice Sheet, Amazon rainforest, and AMOC all have internal feedbacks that can become self-sustaining after a threshold is crossed — they don't just settle at a 'warmer but similar' state, they transition to a fundamentally different regime.

Question 3

Hysteresis in a climate tipping element means that reducing forcing back to its original level will restore the system to its original state.

• T. True
• F. False

Answer: False

Hysteresis is precisely the property that prevents this. Once a tipping element crosses its threshold, the internal positive feedbacks become self-sustaining. The system has moved into a different stable basin — a different 'valley' in the stability landscape. Returning to the original state requires pushing the system back over the energy barrier, which for climate systems means cooling well below the original tipping threshold. For the Greenland Ice Sheet, this could mean global temperatures 1–2°C below pre-industrial levels, sustained for centuries — effectively impossible without active intervention. The path forward is not the reverse of the path back.

Question 4

Tipping elements can interact such that crossing one tipping point may increase the likelihood of crossing others, potentially triggering a cascade of transitions.

• T. True
• F. False

Answer: True

Tipping elements are not independent. AMOC collapse could shift tropical rainfall belts, stressing the Amazon; Amazon dieback releases carbon, warming the climate further; that warming accelerates permafrost thaw, releasing methane; methane amplifies warming, potentially triggering more tipping elements. This cascade logic means the risk of crossing multiple thresholds is not the sum of individual risks but potentially much higher due to coupling. A 2018 analysis in PNAS coined the term 'Hothouse Earth' to describe a potential cascade pathway where several tipping elements collectively drive warming beyond what human emissions alone would cause.

Question 5

Explain why climate tipping points make linear projections of climate risk misleading, and what this implies for climate policy.

Model answer: Linear projections assume a roughly proportional relationship between emissions and outcomes: twice the warming causes twice the damage. Tipping points break this assumption because a small additional increment of warming could push a system across a threshold, triggering a large, irreversible transition. This means the true cost of each additional fraction of a degree of warming may be far greater than linear models suggest — not because the average response is larger, but because the tail risk of catastrophic, irreversible outcomes is higher. For policy, this implies that the expected cost of exceeding certain temperature targets (like 1.5°C or 2°C) may be dramatically underestimated by standard integrated assessment models, strengthening the case for aggressive early mitigation over gradual action.

The asymmetry of hysteresis also matters: warming that crosses a tipping point commits the world to consequences that no subsequent mitigation can undo. This makes prevention far more valuable than adaptation for tipping-element risks, which is qualitatively different from the logic that applies to proportional, reversible climate impacts.