1.5.4 Interactions between tipping systems and planetary-scale cascades

Assembling the individual links mentioned in the sections before gives rise to the possibility of domino effect-style tipping cascades involving more than two elements. The likelihood of such domino effects clearly depends on the strengths of interactions between the tipping systems. These could lead to large changes at the regional and even planetary scale. A plausible palaeoclimate example are D/O events (1.5.3.2).

While unlikely, a major concern regarding the future may be that a cascade involving several tipping systems and feedbacks could lock the Earth system on a pathway towards a ‘hothouse’ state, with conditions resembling that of the mid-Miocene or even Eocene (around 4-5^oC warmer, and sea level 10-60m higher compared to pre-industrial Holocene) (Burke et al., 2018; Steffen et al., 2018). Feedbacks that affect global temperature via albedo changes (through ice sheet or sea ice loss) and additional CO₂ and CH₄ emissions (through e.g. permafrost thawing or methane hydrates release) may lead to additional warming on medium to long timescales (Wunderling et al., 2020; Steffen et al., 2018). In a worst case (and unlikely) scenario, it has been speculated that a regional breakup of stratocumulus decks at atmospheric CO₂ levels above 1,200ppm could translate into a large-scale temperature feedback leading to a warming of roughly 8^oC (Schneider et al., 2019; see 1.4.2.4).

Timescales are crucial when discussing hothouse scenarios. A potential hothouse state in the next few centuries seems implausible in light of the current state of research. For example, in climate projections up to 2100, CMIP6 models show no evidence of nonlinear responses on the global scale. Instead, they show a near-linear dependence of global mean temperature on cumulative CO₂ emissions (Masson-Delmotte et al., 2021). Similarly, in a recent assessment, it is concluded that a tipping cascade with large temperature feedbacks over the next couple of centuries remains unlikely and that, while the combined effect of tipping systems on temperature is significant for those timescales, it is secondary to the choice of anthropogenic emissions trajectory (Wang et al., 2023). 

However, this does not completely rule out the possibility of a hothouse scenario in the longer term. Indeed, tipping events are not necessarily abrupt on human timescales. Positive/amplifying feedbacks could have negligible impacts by 2100, for example on global mean temperature and sea level rise, but still influence Earth system trajectories on a timescale of thousands of years (Kemp et al., 2022; Lenton et al., 2019; Steffen et al., 2018). Overall, this calls for experiments across the model complexity hierarchy. Earth system models of intermediate complexity in particular, and atmosphere-ocean general circulation models at coarse spatial resolution, offer an interesting trade-off as they include representations of most tipping systems while still allowing for long-term simulations.

Finally, spatial scales and patterns are relevant when it comes to risks of hothouse scenarios. Most examples of tipping cascades from palaeoclimate suggest that, while impacts are clearly global (e.g. greenhouse-icehouse transition, D/O events), the spatial expression of climate change (weather extremes, precipitation, seasonality) can vary greatly across the globe. Nevertheless, for societies, such cascades can be as dangerous as a global hothouse scenario, as are tipping cascades that do not lead to a hothouse but lock in other major harmful impacts such as a ‘wethouse’ scenario of tens of metres of sea level rise.