Sensitivity Tests and Glaciation Scenarios of the Future with CLIMBER-2 – SICOPOLIS Model System for Olkiluoto

This report provides climate model simulations performed at the Finnish Meteorological Institute (FMI) in support of formulating climate scenarios for the safety analysis of the Olkiluoto repository. The simulations were run with an earth system model of intermediate complexity (EMIC) CLIMBER-2–SICOPOLIS. The sensitivity tests showed that the modelled ice volumes over the northern hemisphere (NH) and Fennoscandia are not highly sensitive to the factors and model parameters tested in this study. The ice thickness at Olkiluoto was found to be highly sensitive to the model set-up when Olkiluoto was located close to the ice sheet margin. When CLIMBER-2–SICOPOLIS simulations were compared against a land uplift model, the best fit between the two models was obtained when the time lag of relaxed asthenosphere was set to 4,000 years. A comparison to reconstruction data of the Fennoscandian ice sheet revealed that CLIMBER-2–SICOPOLIS tends to underestimate the ice extent over the Kara Sea during the earlier glaciations, but during the glaciation at 20 kyr BP, CLIMBER-2–SICOPOLIS reproduces well the maximum ice extent. CLIMBER-2–SICOPOLIS was also used to project future glaciations in the next 200 kyr under different assumptions of atmospheric CO2 concentrations. Under natural background CO2 conditions, variations in insolation make NH glaciation possible in 10– 20 kyr AP, 50–60 kyr AP, 90–100 kyr AP and 130–140 kyr AP, and at 20-30 kyr intervals after that. However, human activities have released large amount of additional carbon into the atmosphere and increased the global atmospheric CO2 concentration from ~270 ppm to close to 400 ppm within the past 250 years leading to delay the onset of the next glaciation by several tens of thousands of years. Assuming future scenarios with constant CO2 concentrations, CLIMBER-2‒SICOPOLIS predicted ice at Olkiluoto within the next 144 kyr only in the simulations with CO2 equal to or lower than 300 ppm. The timings of these ice events were 50-60 kyr, 95-105 kyr, and 125-135 kyr AP, and the predicted ice thicknesses at Olkiluoto 1600-1800 m for CO2 concentration of 270 ppm, and 800-1300 m for 300 ppm. Furthermore, the simulations showed that the total Fennoscandian ice volume correlates negatively with the atmospheric CO2 concentration, and the simulation with 500 ppm of CO2 prevented all glaciation events in Fennoscandia during the next 144 kyr. More realistic simulations with time-varying future CO2 concentrations were constructed by combining CO2 scenarios, atmospheric CO2 decline rates and CO2 from a new regression equation developed in this study. The regression equation was fitted using sea surface temperature as a predictor, and showed to be able to reproduce the historical CO2 record. Simulations conducted using the time-varying CO2 concentrations showed that the onset of a glaciation at 10–20 kyr AP is unlikely due to high atmospheric CO2 conditions. However, glaciation was predicted in ~100 kyr AP in all simulated scenarios, and in ~50 kyr AP in all but the business-as-usual scenario with continued high CO2 emissions in the coming centuries. The scenario runs predicted that ice extends Olkiluoto when the atmospheric CO2 concentration has dropped below 280 ppm. In the high-emission run the onset of significant ice formation in Olkiluoto is delayed until ~130 kyr AP. The maximum ice thicknesses in Olkiluoto are in all the scenarios 1,800-2,000 m. These simulations with CLIMBER-2‒SICOPOLIS highlight that the future concentration pathways of atmospheric CO2 can have large impacts on the ice formation over Olkiluoto in the next 100,000 years. However, the future emissions from both anthropogenic and natural sources are extremely challenging to foresee due to uncertainties in economical, technological and political developments as well as in the climate-change driven feedbacks in the natural carbon cycle.