Tipping points in the global climate system

“Tipping points” have become a widely-used shorthand for many aspects of non-linear changes in a complex system. What we now refer to collectively as “tipping points in the climate system” were first addressed in the Third Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) as “surprises” (Stocker et al., 2001) and subsumed under the “Reasons for Concern” as “large-scale singular events” or “discontinuities in the climate system” (IPCC, 2001). These tipping points have both global and regional consequences and include changes in the Atlantic Meridional Overturning Circulation (AMOC), the melting of polar ice sheets, the migration of large-scale weather and climate patterns, and dieback of the Amazon rainforest.

Tipping points with global consequences

Figure 1. Crossing tipping points associated with ice-sheet instabilities in Antarctica, or with rapid discharge from ice streams in Greenland, can have serious global impacts. (Terminus of Jakobshavn Isbrae, Greenland, Photo T.F. Stocker).

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AMOC is an important driver of the distribution of heat, salt, and water in the climate system, both regionally and globally. Based on paleoclimate proxy data, it has been suggested that AMOC may be weaker in the current climate than at any other time in the last millennium (Caesar et al., 2021). In addition, recent models consistently indicate that the AMOC will weaken as CO₂ continues to increase (Weijer et al., 2020). Although direct measurements since 2004 show no significant trends (Worthington et al., 2021), continuous long-term weakening of the AMOC, as robustly suggested by models (Jackson et al., 2022), may increase its vulnerability to other changes, such as freshwater delivery from melting ice sheets and glaciers. As a result, the continued study, identification, and observation of early warning signals of a potential tipping point in the AMOC is crucial (Boers, 2021).

The melting of the polar ice sheets on Greenland and Antarctica have been considered tipping elements for many years (Figure 1). Their tipping would be particularly dangerous as they would have global consequences due to substantial additional sea-level rise on the timescales of centuries to millennia (Clark et al., 2016). The IPCC Fifth Assessment Report communicated that crossing a critical global warming threshold between 1 °C and 4 °C would lead to significant and irreversible melting of the Greenland Ice Sheet (Stocker et al., 2013). However, this range was reassessed and found to be at or slightly above 1.5 to 2 °C – that is, the global warming limits of the Paris Agreement (Pattyn et al., 2018). At this warming level, the West Antarctic Ice Sheet would also be at increasing risk of irreversible ice loss (Garbe et al., 2020). While the underlying physical mechanisms are well-researched and theoretically understood, determination of the critical thresholds for the individual ice basins under realistic conditions and topography is very difficult, and large uncertainties remain (Pattyn and Morlighem, 2020).

Regional tipping points

A gradual migration of large-scale weather or climate patterns may be registered regionally as tipping into a new regime. The paleoclimate record, for example, has pointed to phases when the monsoon belt has shifted or changed in intensity in response to large-scale hemispheric climate changes during the last 30 000 years (Brovkin et al., 2021). A recent analysis suggests that future warming could lead to an intensification of the Indian monsoon and its variability, expressed possibly as shorter and heavier rains (Katzenberger et al., 2021).

In mid-latitude regions, changes in soil moisture can lead to threshold effects in evaporative regimes, and to an associated non-linear amplification of heat extremes (Seneviratne et al., 2010; Miralles et al., 2014; Vogel et al., 2018). Furthermore, the frequency of threshold-based climate extremes generally increases non-linearly with increasing global warming, with the largest relative changes for the most extreme events (Kharin et al., 2018). Changes in regional mean climate and the intensity of climate extremes tend to vary linearly as a function of global warming (Wartenburger et al., 2017). However, they can also lead to the crossing of regional ecosystem thresholds (Guiot and Cramer 2016; Warren et al., 2018; Ratnayake et al., 2019; Breshears et al., 2020) and to climate regime shifts in combination with vegetation changes and societal responses. An example of this is the dust bowl period in the United States (e.g., Cowan et al., 2020).

The marine environment is also prone to regional tipping. Marine heat waves, for example, could occur more frequently and more intensely (Frölicher et al., 2018). Ocean acidification, caused by the ocean’s absorption of increasing atmospheric carbon dioxide concentrations in its role as a carbon sink, could cross thresholds with consequent coral bleaching and other marine ecosystem impacts (Hoegh-Guldberg et al., 2019). Regional tipping points of marine systems due to warming, ocean acidification, and deoxygenation can, in combination, cause global impacts (Heinze et al., 2021).

The Amazon rainforest, a unique ecosystem of global significance and value, is under pressure from deforestation and anthropogenic climate change. Although projections of its future evolution are highly uncertain, studies point to the likelihood of further drying (Baker et al., 2021). More extended dry seasons and extreme drought events, and self-reinforcing feedbacks, could further reduce the forest extent (Zemp et al., 2017) with a potential approach to a tipping point (Boulton et al., 2022) where the forest is unsustainable. Loss of the Amazon rainforest would have potentially devastating consequences on regional climate, biodiversity and social systems as well as potentially wider impacts through changes of the hydrological and carbon cycles.

Figure 2. Thresholds and tipping points may be increasingly encountered in regional weather patterns and extremes with consequences for local communities and ecosystem services (Drought and developing storm in the Ebro Delta, Spain, 2020. Photo WMO/Agusti Descarrega Sola).

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Consequences of tipping points on human health and well-being

The impact of climate change on human health is receiving greater attention (Romanello et al., 2021) as the potential threats are multiple. The combined effects of higher temperatures and humidity during hot spells in some regions could reach dangerous levels in the next few decades (Pal and Eltahir, 2016), which could lead to physiological tipping points, or thresholds beyond which outdoor human labor is no longer possible without technical assistance. Already, a substantial fraction of heat-related mortality today can be attributed to anthropogenic warming (Vicedo-Cabrera et al., 2021) and this trend is increasing in extent and magnitude. Hence these events can cause tipping points and threshold behaviour in the Earth system, which includes the biosphere, the carbon cycle, and society, as socioeconomic impacts are expected to be strong and irreversible on intermediate timescales.

Taken together, tipping points in the climate system are a scientific topic of great public interest. The WCRP, for example, is addressing this issue in one of their Lighthouse Activities through an international platform to combine theoretical-mathematical approaches, observational moni-toring, and comprehensive climate modelling efforts. Non-linear processes in the climate system are at the origin of tipping elements, so a concerted international effort in high-resolution coupled Earth system modelling developing and utilizing exa-scale computing infrastructure (Slingo et al., 2022; Hewitt et al., 2022) will provide an improved representation of climate feedbacks and of dynamical responses responsible for tipping elements.

Finally, a formal scientific consensus on tipping points and irreversible climate change, which is central to estimating climate risk, yet fraught with deep uncertainty, is policy-relevant. The latest IPCC report has assessed tipping points and outlines the limits of the current status of knowledge. A cross-working group IPCC Special Report on “Climate Tipping Points and Consequences for Habitability and Resources” would help strengthening a consensus on this topic and trigger the much needed advances in scientific understanding to more comprehensively inform adaptation and mitigation strategies.