1.5.1 Introduction and definition

The tipping systems identified in the climate system generally operate not in isolation from each other, but connected either directly or mediated via changes in the overall climate (for example, global temperature) (Liu et al., 2023; Kriegler et al., 2009). Via such connections (see Figure. 1.5.1) tipping in one subsystem can therefore cause tipping in another, which we define as a tipping cascade (see Definition below) (Wunderling et al., 2021a; Klose et al., 2020; Dekker et al., 2018).

For example, disintegration of the Greenland Ice Sheet (GrIS) can lead to an abrupt shift in the Atlantic Meridional Overturning Circulation (AMOC), while an abrupt change in AMOC strength can lead to an intensification of the El Niño-Southern Oscillation (ENSO). Interactions between climate tipping systems could effectively lower the thresholds for triggering a tipping event as compared to those individual tipping systems in isolation (Wunderling et al., 2021a; Klose et al., 2020). Moreover, one or more tipping events could activate processes leading to additional CO₂ emissions into the atmosphere; permafrost thaw and forest dieback are typical examples of such additions of stored CO₂ into the atmosphere via positive amplifying feedbacks (Wunderling et al., 2020; Lenton et al., 2019; Steffen et al., 2018).

It is also conceivable that components of the Earth system, though not necessarily tipping systems in themselves, could mediate or amplify tipping in other components, thereby creating larger-scale impacts. As a result, some of these nonlinear components are also taken into account in this chapter. A prominent example is Arctic summer sea ice cover, which is not expected to show tipping behaviour (Lee et al., 2021, see 1.2.2.2), but can nevertheless trigger tipping events in the ocean-atmosphere-cryosphere system (Gildor and Tziperman, 2003). On the other hand, an abrupt transition in one tipping system may also stabilise other climate subsystems (Nian et al., 2023; Sinet et al., 2023) as is the case for a weakening AMOC decreasing local temperatures around Greenland (Jackson et al., 2015).

While most tipping systems that have been proposed so far are clearly regional (with some being large-scale), there are significant knowledge gaps with respect to their tipping probability, impact estimates and timescales, as well as their interactions. The potential of a tipping cascade that could lead to a global reorganisation of the climate system (Steffen et al., 2018; Hughes et al., 2013) remains therefore speculative. However, since multiple individual tipping point thresholds may be crossed during this century with ongoing global warming, and could lead to severe tipping system interactions and cascading transitions in the worst case, it is critical to review the current state of knowledge and reveal research gaps that need to be addressed (Armstrong McKay et al., 2022; Masson-Delmotte et al., 2021; Rocha et al., 2018).