Seminar Archive – Autumn 2024

Early opportunity signals of a tipping point in the EV market.

Shifting Diets: Positive Tipping Points in the Food System.

Uncertainty quantification for overshoots of tipping thresholds.

One common mechanism for tipping to occur is via forcing a nonlinear system beyond a critical threshold that signifies self-amplifying feedbacks inducing tipping. However, previous work has shown that it is possible to briefly overshoot a critical threshold and avoid tipping. For some cases, the peak overshoot distance and the time a system can spend beyond a threshold are governed by an inverse square law relationship. In the real world or complex models, critical thresholds and other system features are highly uncertain. In this presentation, we look at how such uncertainties affect the probability of tipping from the perspective of uncertainty quantification. We show the importance of constraining uncertainty in the location of the critical threshold and the linear restoring rate to the system’s stable state to reduce the uncertainty in the probability of tipping. We first prototypically analyse effects of an uncertain critical threshold location separately from effects due to an uncertain linear restoring rate. These uncertain system characteristics will be jointly analysed in a simple box model for the Atlantic Meridional Overturning Circulation (AMOC). Thereby, we highlight the need to constrain the highly uncertain diffusive timescale (representative of wind-driven fluxes) within the box model to reduce tipping uncertainty for overshoot scenarios of the AMOC. 

---

Positive Tipping Points in Practice: Co-Producing an Interactive Toolkit.

Over the last 18 months, Peter Lefort has been working with grassroots changemakers to co-produce a toolkit that is aimed at helping those working in environmental and social change to understand, explore, and apply the concepts of Positive Tipping Points in practice. In this session, Peter will share the story of this process, including what the toolkit contains, how it has been created, and what is happening next.

---

Resilience changes in the North Atlantic over the last millennium encoded in clam shells.

Amid the ongoing climate crisis, there is an increasing need to assess the vulnerability of different components of the climate system to change abruptly once a tipping point is crossed. An approach for obtaining early warning signals for tipping points involves detecting the loss of resilience associated with critical slowing down. This method requires long-term, regularly spaced time series —features that are rare among observational records, especially in the ocean. The recent development of proxy records with annual resolution based on information encoded in bivalve shells provides a unique opportunity for assessing resilience in the marine environment, particularly in the North Atlantic, where key processes of two candidate tipping elements occur. We explored resilience changes in the North Atlantic as encoded in a compilation of bivalve-derived records and found evidence of three significant periods of significant loss of resilience over the last millennium. The first period preceded an abrupt weakening of the subpolar gyre in the 14th century, providing empirical support for its bistability and offering insights into how the North Atlantic tipped into the Little Ice Age. The second destabilisation episode precedes the 1920s North Atlantic regime shift, which involved a strengthening of regional circulation. The most recent destabilisation episode is observed in recent decades in records that are highly sensitive to variability over the subpolar gyre region, suggesting that the subpolar North Atlantic circulation system has lost resilience and may be approaching a tipping point.

---

Quantifying tipping behaviour: geometric early warnings and quasipotentials for a box model of AMOC.

A non-autonomous system can undergo a rapid change of state in response to a small or slow change in forcing, due to the presence of nonlinear processes that give rise to critical transitions or tipping points. Such transitions are thought possible in various subsystems (tipping elements) of the Earth’s climate system. The Atlantic Meridional Overturning Circulation (AMOC) is considered a particular tipping element where models of varying complexity have shown the potential for bi-stability and tipping. We consider both transient and stochastic forcing of a simple but data-adapted model of the AMOC. We propose and test a geometric early warning signal to predict whether tipping will occur for large transient forcing, based on the dynamics near an edge state. For stochastic forcing, we quantify mean times between noise-induced tipping in the presence of stochastic forcing using an Ordered Line Integral Method (OLIM) of Cameron (2017) to estimate the quasipotential. We calculate minimum action paths (MAPs) between stable states for various scenarios. Finally, we discuss the problem of finding early warnings in the presence of both transient and stochastic forcing.