Tipping Points and Abrupt Change

Thresholds in the climate system that, once crossed, trigger large, self-sustaining, and often irreversible change in ice sheets, ocean circulation, and ecosystems.

Definition

A climate tipping point is a critical threshold beyond which a part of the climate system reorganizes into a new state through self-reinforcing feedbacks, often abruptly and irreversibly on human timescales.

Scope

This topic covers climate tipping points and the potential for abrupt, nonlinear change under sustained forcing. It treats the major tipping elements, including the Greenland and West Antarctic ice sheets, the Atlantic overturning circulation, permafrost carbon, and the Amazon and boreal forests, the feedbacks that can lock in change once a threshold is passed, the evidence from past abrupt events, and the challenge of detecting early-warning signals of approaching transitions.

Core questions

• Which components of the climate system can pass tipping points?
• What feedbacks make a transition self-sustaining once triggered?
• At what warming levels might these thresholds be crossed?
• Can early-warning signals of an approaching tipping point be detected?

Key theories

Tipping elements and feedbacks

Certain large-scale components can be pushed past a threshold by gradual forcing, after which internal feedbacks drive them to a qualitatively different state largely independent of further forcing.

Critical slowing down as early warning

As a system nears a tipping point its recovery from small perturbations slows, producing rising autocorrelation and variance that may serve as statistical early-warning indicators.

Mechanisms

Positive feedbacks can make parts of the climate system bistable: for example, ice-albedo and elevation feedbacks can commit an ice sheet to continued loss once it retreats past a point, and freshwater input can weaken the Atlantic overturning circulation toward collapse. Near such thresholds the system recovers ever more slowly from disturbances, in principle leaving a detectable early-warning fingerprint before the abrupt transition occurs.

Clinical relevance

Tipping points represent low-probability but high-impact risks of large, abrupt, and irreversible change, which is why they feature prominently in risk assessments and arguments for limiting warming as a precaution.

Evidence & guidelines

The IPCC Sixth Assessment Report assesses several tipping elements as possible, though mostly low-likelihood at lower warming levels, and notes that the probability of abrupt and irreversible changes increases with the magnitude and rate of warming.

History

The recognition from paleoclimate that the system can change abruptly motivated the modern tipping-point framework, formalized by Lenton and colleagues in 2008, which catalogued the major tipping elements and stimulated research on thresholds and early-warning signals.

Debates

Proximity and likelihood of crossing tipping points

How close major tipping elements are to their thresholds, and how likely they are to be crossed at given warming levels, is debated because the thresholds are poorly constrained.

Key figures

• Timothy Lenton
• Stefan Rahmstorf
• Hans Joachim Schellnhuber
• Marten Scheffer

Seminal works

• lenton2008
• ipccar6wg1

Frequently asked questions

What is a climate tipping point?

It is a threshold beyond which part of the climate system, such as an ice sheet or ocean current, shifts into a new state through self-reinforcing feedbacks, often irreversibly.

Have any tipping points already been crossed?

It is not firmly established, but some assessments suggest parts of the West Antarctic and Greenland ice sheets and some warm-water coral reefs may already be near or past thresholds.