Characterising tipping elements and thresholds for irreversible climate change
Earth’s climate system is characterised by highly complex interactions in time and space. Scientists have identified several essential components that are at risk of undergoing abrupt, irreversible transitions that could catalyse cascades of dramatic changes. Further complicating matters, critical transitions in one such ‘tipping element’ could impact others. Characterising these tipping elements and their interactions is necessary to develop predictive models, a fundamental step in preventing irreversible climate change. With the support of the Marie Skłodowska-Curie Actions(opens in new window) programme, the CriticalEarth(opens in new window) project trained 15 early-stage researchers to address this challenge. The work focused on the development of advanced statistical tools based on paleoclimate reconstructions and recent observations to help scientists identify ‘early warning signals’ of tipping.
Uncontrolled positive feedback
Most dynamical systems experience both positive and negative feedback. “When positive feedback begins to dominate, an uncontrollable amplification may result, causing a ‘control parameter’ to cross a critical threshold leading to an abrupt irreversible state change,” explains CriticalEarth project coordinator Peter Ditlevsen of the Niels Bohr Institute, University of Copenhagen(opens in new window). In the case of ice sheets, the lower the ice sheet’s height becomes as it melts, the warmer the atmosphere over it (temperatures are higher at lower altitudes) and the more the ice melts. At some critical atmospheric temperature threshold, this uncontrolled positive feedback cannot be reversed. Other potential tipping events include the irreversible change of rainforests to a savannah state and the shutdown of the Atlantic Meridional Overturning Circulation (AMOC) responsible for northern Europe’s temperate climate.
Edge states and early warning signals
Determining the exact values of tipping parameters turned out to be an extremely difficult task. “We applied advanced mathematical techniques to determine the boundaries between the present and the alternative tipped state in multidimensional climate models,” Ditlevsen says. The researchers characterised the so-called edge states on those boundaries, gaining information relevant to predictions of future transitions. They used new open-source statistical tools to detect early warning signals – statistical signatures in the time-series data that indicate the climate system is approaching a critical tipping point.
Revelations on Earth’s tipping elements
“Novel rare event and machine learning techniques enabled efficient estimation of transition probabilities in high-dimensional climate models, improving our understanding of abrupt shifts,” explains Ditlevsen. Surpassing original goals, CriticalEarth extrapolated its early warning signals to the future, revealing a collapse of the AMOC(opens in new window) in the mid-21st century. This refined prediction with striking near-term implications for northern Europe’s climate was reported in more than 2 500 news outlets around the world. The team identified key thresholds, bifurcations and tipping irreversibility in models of tipping elements and discovered new mechanisms of variability and resistance to tipping. Extension of bifurcation theory revealed how AMOC collapse would affect global precipitation and how coupled tipping can stabilise or destabilise key climate subsystems subjected to noise. Finally, predictive tools for climate response to perturbations shed light on factors that enhance or inhibit tipping and that shape transition statistics. CriticalEarth has provided predictive tools and crucial insights on critical climate transitions. Equally importantly, it has trained a cohort of 15 bright young scientists to continue the work to improve our early warning signal detection, supporting efforts to prevent irreversible climate changes and their effects on future generations.
