Overshooting tipping point thresholds in a changing climate

• Published in: Nature (London) Vol. 592; no. 7855; pp. 517 - 523
• Main Authors: Ritchie, Paul D. L., Clarke, Joseph J., Cox, Peter M., Huntingford, Chris
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 22.04.2021; Nature Publishing Group
• Subjects: 704/106/694/1108; 704/106/694/2786; 704/106/694/682; Analysis; Atlantic Meridional Overturning Circulation (AMOC); Atmospheric models; Climate change; Climate system; Climatic changes; Equilibrium; Fossil fuels; Global warming; Greenhouse effect; Greenhouse gases; Humanities and Social Sciences; Ice sheets; Measurement; multidisciplinary; Paleoclimatology; Review; review-article; Risk assessment; Science; Science (multidisciplinary); System theory; Thresholds; Transition temperature; United Kingdom
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/s41586-021-03263-2

Palaeorecords suggest that the climate system has tipping points, where small changes in forcing cause substantial and irreversible alteration to Earth system components called tipping elements. As atmospheric greenhouse gas concentrations continue to rise as a result of fossil fuel burning, human activity could also trigger tipping, and the impacts would be difficult to adapt to. Previous studies report low global warming thresholds above pre-industrial conditions for key tipping elements such as ice-sheet melt. If so, high contemporary rates of warming imply that exceeding these thresholds is almost inevitable, which is widely assumed to mean that we are now committed to suffering these tipping events. Here we show that this assumption may be flawed, especially for slow-onset tipping elements (such as the collapse of the Atlantic Meridional Overturning Circulation) in our rapidly changing climate. Recently developed theory indicates that a threshold may be temporarily exceeded without prompting a change of system state, if the overshoot time is short compared to the effective timescale of the tipping element. To demonstrate this, we consider transparently simple models of tipping elements with prescribed thresholds, driven by global warming trajectories that peak before returning to stabilize at a global warming level of 1.5 degrees Celsius above the pre-industrial level. These results highlight the importance of accounting for timescales when assessing risks associated with overshooting tipping point thresholds. Ongoing global warming is likely to cause tipping point thresholds to be passed, but an abrupt system change can still be avoided if the warming is reversed quickly relative to the timescale of the tipping element.