The formation of a continent-scale ice sheet on Antarctica during the ‘Eocene-Oligocene Transition’ about 34 million years ago is known as Earth’s Greenhouse-Icehouse Transition. Following a cooling over tens of millions of years during the warm ‘Eocene’ period (c. 56 to 34 million years ago), this shift to a new cooler climate state in the ‘Oligocene’ period (c. 34 to 23 million years ago) would have been visible from space, as Antarctic forests were replaced by a blanket of ice and seawater receded from the continents, changing the shapes of coastlines worldwide. The climate transition had global consequences for Earth’s flora and fauna, both in the oceans and on land (Hutchinson et al., 2020; Coxall et al., 2005).
This climate transition has been identified as a possible palaeoclimate example of cascading tipping points in the Earth system (Dekker et al., 2018; Tigchelaar et al., 2011). Examples of climate tipping systems in this case consist of the global ocean circulatory system, the Antarctic ice sheet, polar sea ice, monsoon systems and tropical forests. In a conceptual model, the first part of the Eocene-Oligocene Transition is attributed to a major transition in global ocean circulation, while the second phase reflects the subsequent blanketing of Antarctica with a thick ice sheet (Tigchelaar et al., 2011). The glaciation of Antarctica also produced a sea level fall of several tens of metres, causing shallow seaways to recede, turning many marine regions into continental habitats (Toumoulin et al., 2022; Lear et al., 2008), see Figure 1.5.4.
The global ocean circulatory system was showing tentative signs of change a few million years before the climate transition, likely caused by changing ocean gateways in the north Atlantic (Coxall et al., 2018). Isotope measurements suggest that a precursor to North Atlantic Deep Water reached the southern hemisphere close to the Eocene-Oligocene Transition, perhaps signalling the first onset of AMOC (Via and Thomas, 2006), but the exact timing remains uncertain.
Biomes in Earth’s greenhouse state reflect warmer and wetter conditions than the icehouse state of the early Oligocene, but many of these seemed to have changed gradually as climate cooled in the Eocene, making it difficult to identify vegetation tipping systems following the glaciation of Antarctica (Hutchinson et al., 2020). The mammal fossil record, which is coupled to vegetation through diet, suggests more acute changes in the early Oligocene. The Grand Coupure (‘The Big Break’), is a long-known mammal extinction/origination event around the Eocene-Oligocene Transition, involving large-scale migrations of Asian mammals into Europe (Hooker et al., 2004). Thought to signal a combination of changing climate and floral changes, this abrupt faunal turnover might reflect the crossing of ecosystem tipping points caused by the crossing of a climate tipping point: a climate-biosphere tipping cascade.
In summary, Earth’s Greenhouse-Icehouse Transition was likely associated with a range of interactions between components of the Earth system that are debated as potential tipping systems. Determining the extent to which these reflect a cascading series will require a major data-modelling effort, with improved correlations between marine and terrestrial records, and better constraints on the rate and magnitude of change within a range of tipping systems.