Reconciling pollen-stratigraphical and tree-ring evidence for high- and low-frequency temperature variability in the past millennium

Climate change and variability assessments require an understanding of their long-term and period (low-frequency) and short-term and period (high-frequency) variations. Pollen data have conventionally been thought of as a proxy of low-frequency variation of past climates but of more limited applicability for studying high frequencies. Likewise, tree-rings are commonly supposed to reflect faithfully high-frequency variations, with additional uncertainties attributable to variations at lower frequencies. Here we challenge this view in the context of pollen and tree-ring based temperature reconstructions from high-latitude Europe. The two types of records are compared and it is shown that the pollen and tree-ring based reconstructions exhibit similar temperature variability on centennial and longer scales. These can be revealed using timescale-dependent filtering. The same method surmounts geochronological discrepancies, thus enabling reconciliation of the proxies. Resulting new reconstructions validate over the instrumental period. Over longer intervals, our new proxy-fusions cover the climatic reversals of the Medieval Climate Anomaly (during the 10th to 13th centuries), Little Ice Age (during the 14th to 19th centuries), and 20th century warming. The warmest spells occurred in association with the Medieval Climate Anomaly and during the 20th century. The coolest intervals occurred in relation to Little Ice Age conditions. The new reconstructions show that decadal temperature amplitude has been approximately 2.5 °C in the past and thus considerably larger than inferred from spatially large-scale estimates of temperature anomalies.