Abstract This study develops a novel general framework to project the permafrost fate with rigorous uncertainty quantification to assess dominant sources. Borehole temperature records from three sites in the Russian western Arctic are used to constrain the uncertainty of a high‐fidelity freeze‐thaw model. Projections from 9 Global Climate Models (GCM) are stochastically downscaled to generate future trajectories of surface ground heat flux. Under the two emission scenarios SSP2‐4.5 and SSP5‐8.5, the projected average thawing depths by 2100 vary from 0.4 to 14.4 m or 2.1 to 17.7 m, and the increase in the top 10 m average temperature from 2015 to 2100 is 1.2–2.7°C or 1.9–3.0°C. The results show that the freeze‐thaw model uncertainty can sometimes dominate over that of GCM outputs, calling for site‐specific information to improve model accuracy. The framework is applicable for understanding permafrost degradation and related uncertainties at larger scales.
Abstract High‐resolution continental temperature records from geological archives are crucial in order to evaluate temperature dynamics under fundamentally different climate conditions than today. Particularly the warm early to middle Eocene (∼56–40 million years ago) has become the focus of paleoclimate studies with the intention of quantifying temperatures under high atmospheric carbon dioxide ( p CO 2 ) levels. However, detailed proxy reconstructions of land temperature variability during the Eocene “greenhouse” are currently lacking for large parts of the continents. Here we present a ∼430 thousand‐year high‐resolution continental temperature record, reconstructed from terrestrial biomarkers preserved in the maar sediments of the UNESCO World Heritage Site “Messel Fossil Pit,” Germany during the earliest middle Eocene of Central Europe (∼47.7–47.2 million years ago). We found that continental temperatures ranged between ∼21 and 28°C, but shifted from a highly variable (fluctuations up to ∼5°C) temperature pattern (∼155 thousand years) toward a more constant temperature state (∼145 thousand years) through the time period covered here. The shift in temperature history was possibly associated with varying orbital configurations or the concurrent initiation of North Atlantic seaway changes. Moreover, we identified a pronounced warming at ∼47.5 million years, that coincides with negative benthic oxygen and carbon isotopic excursions in the time‐equivalent marine stacks, indicating that the maar lake recorded a previously unknown “hyperthermal‐like” global warming event.
In this study, a comparison is presented between two distinct approaches for interpreting validation results for light water reactor (LWR) fuel criticality safety assessments: one based on frequentist tolerance limits and the other on a Bayesian framework. In general, both the frequentist statistical methods and Bayesian models have inherent advantages and disadvantages, making it valuable to compare the results of the criticality safety evaluations (CSEs) obtained using both approaches. Of particular interest in this context is the application of CSE in conjunction with the burnup credit concept for LWR used nuclear fuel (UNF), whose composition differs significantly from that of fresh fuel, which is primarily used in validation studies worldwide. This paper aims to illustrate a comparative analysis of different CSE methodologies applied to a model of a UNF disposal canister filled with identical fuel assemblies as a function of burnup. The study found that the Bayesian approach yielded less penalizing results, leading, in the analyzed case, to a relaxation of the burnup requirement for UNF criticality safety by approximately ~2 to 3 GWd/tonnes heavy metal for pressurized water reactor fuel with an initial 235 U enrichment of 5 wt%. However, an interesting and somewhat counterintuitive behavior was observed in that the Bayesian-based results indicated a reduction in the safety margins as burnup increased, despite the absence of benchmarks with UNF in the employed validation suite. In any case, the observations and discussions presented suggest that the performance of both the frequentist and Bayesian methodologies, as applied in the context of the postulated CSE task and the employed nuclear data with associated uncertainties, requires further investigation before these approaches can be routinely and effectively adopted for licensing applications involving UNF.