Geological long-term evolution: Climate and Iceflow

  pdf NAB 24-14 Geological long-term evolution: Climate and Iceflow.(71.42 MB)

This report summarises past and possible future climates and glacier conditions in Northern Switzerland. It provides the geoscientific basis to assess the impact of future climate change on erosion (Nagra 2024a), on transport in the geosphere (Nagra 2024b, 2024e) and biosphere (Nagra 2024d) and ultimately serves the comprehensive description of the post-closure evolution of a geological repository (Nagra 2024c and references therein). The report focuses on glacial climate conditions and related foreland glaciations.

The climate of the Quaternary showed a cyclicity of glacials and interglacials with a ~ 41-kyr cycle before the Mid Pleistocene Transition (MPT). After the MPT, which occurred between 1.25 and 0.7 Myr ago, the cyclicity changed to predominantly ~ 100 kyr. In the past, multiple glacial advances have occurred from the Alps into the Alpine Foreland in Switzerland. Of particular interest are the last glacial maximum (LGM), which peaked around ~ 24 ka ago, and the most extensive glaciation (MEG) which occurred sometime between 600 and 400 ka ago, with ice extents reaching the siting regions (LGM) or even further north to the Black Forest (MEG).

While past climates can be studied using proxy data, the determination of future climate conditions, especially for long timespans, can only be achieved using global climate simulations. The global climate simulations used here cover the next one million years and also the last ~ 800 kyr so that results of past simulated climates can be compared against known data e.g. from ice cores or pollen assemblages from sediments. The comparison of simulated past climates with proxy data shows that past climates are well reproduced by the simulations and, hence, future climate projections can be regarded as meaningful. Results from the global, long-term climate simulations show that, depending on emission scenarios of greenhouse gases, climate in the future will be significantly warmer than today and the onset of the next glaciation will be postponed. Even using preindustrial atmospheric greenhouse gas concentrations, the next glacial conditions will only peak at ~ 110 kyr with simulated global total ice-volumes below those of the LGM. The higher the estimated future emission of anthropogenic greenhouse gases, the more the next glacial inception will be postponed and, hence, the longer the current interglacial climate conditions will persist. In the most extreme emission scenario, the next glaciation is simulated to only peak after ~ 850 kyr.

Glaciations are of particular importance with respect to repository long-term evolution because the advance and retreat of glaciers in the foreland will alter the landscape in Northern Switzerland and may reduce the overburden of the repository by glacial erosion. Ice-flow simulations have been performed, using temperatures and precipitation of downscaled global climate simulations as inputs. Results of these simulations indicate temperate basal conditions below much of the glacier advancing onto the Alpine foreland during the LGM enabling sliding and, thus, providing conditions conducive to glacial erosion. However, the results also indicate a poor fit between the sliding distance, a proxy for glacial erosion by moving ice, and the location of existing overdeepened valleys. Calculation of the time-integrated flow accumulation area based on maps of hydraulic potential, a proxy for erosion by subglacial water flow, yields zones with significant subglacial water flow that favour enhanced sediment removal and increased fluvial erosive action. This therefore shows the importance of subglacial water flow in the formation of overdeepenings in the distal Swiss lowlands.

Geological field evidence and results of the ice-flow simulations show that two of the three possible repository sites were covered by ice during the last glacial maximum. It is therefore likely that, given glacial climate conditions, repository sites will in the future again be covered by glaciers, should they reach the lowlands of Northern Switzerland. Ice thickness of ~ 200 – 400 m and a duration of ice occupation of ~ 1'000 – 3'000 yr in the siting regions were simulated and are expected for future foreland glaciations.