European seasonal temperature changes since the Last Glacial Maximum: Insights from model simulations

The Last Glacial Maximum (LGM, ~21 ka BP) is a key period for assessing climate sensitivity and evaluating Global Circulation Models (GCMs) used for future climate projections. However, determining whether regional cooling reflects a uniform decrease in monthly temperatures throughout the year (i.e., no change in seasonality) or a change in seasonality, would have significant implications for many paleotemperature reconstructions, including those based on paleo-glacier equilibrium lines.

In this study, we provide assessment of European climate seasonality during the LGM, using both GCMs and sensitivity experiments performed with the Earth system model of intermediate complexity iLOVECLIM downscaled over Europe. Models generally show a large dispersion in the pattern of differences between summer and winter, although some common features seem to emerge. Southern Europe shows a reduction in average seasonality during the LGM, in contrast to an amplification further north, relative to PI condition. Near coastal regions (low longitudes), models indicate a slight and consistent increase in seasonal anomalies, whereas eastern Europe shows a larger increase in seasonal anomalies, though with greater inter-model variability. We identify variations in LGM MTCO (the winter temperatures) as the primary drivers of both seasonality changes and inter-model discrepancies in both the GCM and iLOVECLIM outputs, with the largest disagreements occurring in northeastern Europe, over and near the Fennoscandian ice sheet.

Motivated by the fact that the iLOVECLIM model produces some features largely different from the GCM mean, we performed a series of sensitivity experiments. These include changes in greenhouse gas concentration, thermohaline circulation, albedo and topography of the Fennoscandian ice sheet and vegetation cover. We show that none of these processes reduces the mismatch between iLOVECLIM and the mean response of the GCMs. A significant reduction of this mismatch is achieved only by changing the vertical parametrisation in iLOVECLIM, suggesting that the lack of an explicit vertical representation in iLOVECLIM might bias the simulated seasonality changes at the LGM relative to PI.

Pollen-based reconstructions are generally consistent with model results. However, the regions that display the largest inter-model differences are also not covered by this type of data. European seasonality changes since the LGM therefore remain a key yet poorly constrained characteristic of LGM climates, calling for more single-model sensitivity experiments to improve our understanding of past and future seasonality changes.