Some mechanisms of mid-Holocene climate change in Europe, inferred from comparing PMIP models to data

We propose a new approach for comparing mid-Holocene climates from 18 PMIP simulations with climate reconstructions of winter and growing season temperatures and the annual water budget inferred from European pollen and lake-level data. A cluster analysis is used to extract patterns of multivariate climate response from the reconstructions; these are then compared to the patterns simulated by models. According to paleodata, summers during mid-Holocene were warmer-than-present in the north, and cooler-than-present in the south, while winters were colder-than-present in the southwest but milder-than-present in the northeast. Whereas warmer summers and colder winters may easily be explained as a direct response to the amplified seasonal cycle of insolation during the mid-Holocene, the other recorded responses are less straightforward to explain. We have identified, from the models that correctly simulate the recorded climate change, two important atmospheric and hydrological processes that can compensate for direct insolation effects. First, a stronger-than-present airflow from southwestern Europe that veers to the north over Eastern Europe, in winter, can consistently explain the reconstructed changes in this season's temperatures and water budget. Second, the increased winter soil moisture allows a shift of the partitioning of net radiative energy towards latent rather than sensible heat fluxes, thereby decreasing surface temperature during the following summer season. Our approach therefore solves one of the recurring problems in model-data comparisons that arises when a model simulates the correct response but in the wrong location (as a consequence, for instance, of model resolution and topography).