1.5.3.2 Interactions during and since the last glacial period

Here, we discuss three important palaeoclimate candidates for tipping interactions during and since the last glacial period.

Dansgaard-Oeschger events

Rapid, decadal-timescale Northern Hemisphere warming transitions known as ‘Dansgaard-Oeschger’ (D/O) events (Figure 1.5.5) occurred repeatedly during glacial periods throughout much of the late Pleistocene prior to the Holocene (Ganopolski and Rahmstorf, 2001). In general, these events consist of an abrupt (in the order of decades) warming from glacial to interglacial conditions, followed by gradual cooling over the course of hundreds to a few thousand years, before a rapid transition back to cold glacial conditions. 

Evidence from Greenland ice cores and North Atlantic sediment records suggest that the abrupt cooling transitions were systematically preceded and possibly triggered by more gradual cooling across the high-latitude Northern Hemisphere (NGRIP project partners, 2004; Barker et al., 2015). The abrupt transitions from glacial to interglacial conditions were also preceded by more gradual changes elsewhere (for example, increasing Antarctic and deep ocean temperatures and decreasing dustiness; Barker and Knorr (2007)), leading to the idea that both types of transitions may be predictable to some extent (Lohmann, 2019; Barker and Knorr, 2016). Each event was also paired with rapid changes in ocean circulation, terrestrial hydroclimate, atmospheric composition and ocean oxygenation. The occurrence and interactions among many subsystems that show abrupt changes make it plausible then to consider it a cascade, and that such cascades are a common feature of late-Pleistocene climate variability.

During the abrupt warming phases of D/O cycles, an abrupt decrease of Arctic and North Atlantic sea ice cover likely contributed to the onset of convection and a rapid resurgence of a much weaker, and potentially even collapsed, AMOC (Gildor and Tziperman, 2003; Li et al., 2010: see 1.4.2.1). D/O-type changes in coupled climate models also feature a rapid disappearance of sea ice that precedes the abrupt AMOC strengthening (Vettoretti and Peltier, 2016; Zhang et al., 2014). Thus, the D/O warming events may potentially comprise a tipping cascade (Lohmann and Ditlevsen, 2021). However, such a cascading interaction may depend on the background climate state (i.e. only possible during glacial conditions), and it is unclear whether North Atlantic sea ice cover during the last glacial period can be considered a tipping system.

Bølling-Allerød

Towards the end of the last ‘ice age’ glacial period, a very prominent climate event is recorded in numerous geological archives. The Bølling-Allerød (B/A) started 14,700 years ago with abrupt warming in the Northern Hemisphere (with temperature increase in Greenland by 10-14^oC over a few years) in response to a reinvigoration of the AMOC (McManus et al., 2004) and lasted until 12,900 years ago. The B/A is an example of pronounced interactions between Earth system components and cascading impacts in the Earth system (Brovkin et al., 2021), potentially similar to a last D/O event during the ongoing deglaciation. 

At the onset of the B/A, atmospheric CO₂ and CH₄ concentrations rapidly increased over a few decades (Marcott et al., 2014) in response to abrupt Northern Hemisphere warming and permafrost thaw (Köhler et al., 2014) and moisture changes (Kleinen et al., 2023). This was followed by fast changes in precipitation (e.g. Zhang et al., 2017) and vegetation composition (Novello et al., 2017; Fletcher et al., 2010). The trigger for the rapid amplification of ocean circulation and the associated abrupt impacts at the B/A transition has been a focus of debate, with opinions divided between an essentially linear response to the (possibly abrupt) cessation of freshwater forcing (Liu et al., 2009) versus a non-linear response to more gradual forcing (i.e. a tipping point – Barker and Knorr (2021); Knorr and Lohmann (2007); Chiessi et al. (2008)).

Heinrich events

While the exact causes and mechanisms of the B/A transition and D/O events are still under debate, Heinrich events are better understood. They occurred during some of the cold glacial phases mentioned above and were associated with major reorganisation of ocean circulation in the North Atlantic (for a review, see Clement and Peterson (2008)). During Heinrich events, large masses of ice were released from the Laurentide Ice Sheet, which at that point covered most of northern North America, leading to a dramatic freshening of the North Atlantic Ocean and enhanced suppression of deep-water formation and the AMOC (Henry et al., 2016). They can be understood as a phenomenon involving two tipping systems – the Laurentide Ice Sheet and the AMOC (referred to as ‘binge/purge oscillator’ – MacAyeal (1993)).

Heinrich events provide some, albeit not fully consistent, insights into the response of the Amazon rainforest to reductions in rainfall, and therefore shed some light on its resilience. Using isotopes from sediments, savanna intrusions into the Amazon rainforest have been found during repeated Heinrich events (Häggi et al., 2017). These intrusions occurred in northern Amazonia (Zular et al., 2019; Häggi et al., 2017) and validate the suggested decrease in precipitation over that region in response to AMOC weakening (Campos et al., 2019; see 1.4.2.3). While further palaeoclimate evidence showed that large parts of the Amazon rainforest were stable even when precipitation was relatively low (Kukla et al., 2021; Prado et al., 2013), in the present climate it is unclear how additional effects from deforestation (Zemp et al., 2017), future climate change (Wunderling et al., 2022) and increasing chances of fires (Drüke et al., 2023) will affect the stability of the rainforest in the future (1.3.2.1).