Important characteristics of climate with respect to repository safety are the timing and severity of future glaciations, as the related erosion may reduce the repository overburden thickness and thus modify the properties of the geological barrier (e.g. self-sealing properties, hydraulic conductivity; see Sections 6.4.1.4, 6.5). Hence, it is important to simulate future climate and analyse future climate scenarios explicitly with respect to these characteristics.
Parameters for orbital configurations can be calculated accurately for the past and future and are for example reported in Berger (1977, 1978) and Berger & Loutre (1997). However, the future atmospheric CO2 content cannot be accurately estimated as the additional anthropogenic emissions cannot be easily forecast. To accommodate this uncertainty, several emission scenarios were used to cover a range of likely scenarios (Talento & Ganopolski 2021). Palaeo-reconstructions of the past 800 kyr (global mean surface temperature, CO2 atmospheric concentration and normalised global landmass ice volume) were used as part of the training and validation set for the (predictive) model.
The first case is a natural scenario with no additional anthropogenic emissions (0 petagram Carbon [PgC]). This scenario also serves to determine the impact any additional anthropogenic emissions may have. For the period 1750 – 2017, fossil-fuel cumulative emissions and landuse in carbon equivalents are estimated to 660 PgC (Le Quéré et al. 2018). Future projections for additional cumulative emissions might reach between ~ 700 PgC (incorporating fossil-fuel reserves exploitable today) and ~ 3'000 PgC (considering resources that might become exploitable in the future) (McGlade & Ekins 2015). Accordingly, scenarios with additional cumulative emissions of 500, 1'000 and 3'000 PgC are analysed (for more detail see Nagra 2024j).
Talento & Ganopolski (2021) defined the occurrence of full glacial conditions and large glaciations at the 0.5 and 0.8 value of the normalised global landmass ice volume, respectively (see Fig. 6‑17 for best solution scenarios). With respect to the long-term geological evolution of the repository sites, the main interest is in the potential future timing of large glaciations, as they may reach the Alpine Foreland. This timing is expected to correspond to the peaks above the 0.8-unit ice volume threshold in Fig. 6‑17 (note that ~ 1 corresponds to the global equivalent of LGM ice volume).
In general, the higher the carbon concentration in the atmosphere, the later the next glaciation will occur (Fig. 6‑17 for best solutions). In the natural scenario (E = 0 PgC), the next full glacial condition is simulated to peak around ~ 110 kyr (Fig. 6‑17a). For the low emission scenario (E = 500 PgC), the next full glacial condition is postponed by ~ 90 – 100 kyr compared to the natural scenario, with a peak at ~ 220 kyr and simulated ice volumes of 0.8 (Fig. 6‑17b). For the intermediate emission scenario (E = 1'000 PgC), full glacial conditions only occur the first time at ~ 650 kyr (Fig. 6‑17c). Finally, with a cumulative emission of 3'000 PgC, simulations indicate mostly ice-free conditions for the next 800 kyr (Fig. 6‑17d). In this scenario, full glacial conditions are not expected to occur before 850 kyr. Once the first larger glaciation has occurred, all scenarios show a cyclicity of approximately 100 kyr, similar to the glacial/interglacial cycles, established after the Mid-Pleistocene Transition; see Section 3.5).
Fig. 6‑17:Future climate simulations based on orbital configurations and different Anthropogenic CO2 scenarios
The plots show the "best solution" simulations of global ice volume as proxy and are redrawn from Talento & Ganopolski (2021), using normalised global landmass ice volume as proxy for future glaciations. A value of 1 refers to approximately the same amount of ice as determined for the peak of the LGM. Full glacial conditions are suggested to occur at 0.5 units (filled parts), while a value of 0.8 may suggest a large glaciation (darker filled peaks). With respect to the long-term geological evolution of the repository sites, such large glaciations might reach the North Alpine Foreland. (a) Scenario with zero anthropogenic CO2 emis- sions. A large (potentially reaching the North Alpine Foreland) glaciation occurs only after ~ 100 kyr from now. (b) Scenario with cumulative emissions of 500 PgC. In such a scenario, the next glacial inception is significantly delayed, with the next large glaciation occurring only after ~ 200 kyr. (c) The 1'000 PgC scenario considers consumption of all presently exploitable fossil fuel reserves and will further delay future large glaciations. (d) 3'000 PgC represents a worst-case scenario, including fossil fuel reserves that might become exploitable in the future.