Reconstruction of the mid-Holocene palaeoclimate of Siberia using a bioclimatic vegetation model

Abstract

Abstract A bioclimatic vegetation model is used to reconstruct the palaeoclimate of Siberia during the mid-Holocene, a warm, moist period also known as the Holocene climatic optimum. Our goal is to determine the magnitude of climatic anomalies associated with mapped changes in vegetation classes. Reconstructed anomalies are the logical outcome of the bioclimatic assumptions in the Siberia vegetation model operating on location-specific differences in the palaeomap of Khotinsky and the modern map of Isachenko. The Siberian vegetation model specifies the relationship between vegetation classes and climate using climatic indices (growing-degree days, dryness index, continentality index). These indices are then converted into parameters commonly used in climatic reconstructions: January and July mean temperatures, and annual precipitation. Climatic anomalies since the mid-Holocene are then displayed by latitude and longitude. An advantage of a model-based approach to climatic reconstruction is that grid cells can be modelled independently, without the need for interpolation to create smoothed temperature and precipitation contours. The resulting pattern of anomalies is complex. On average, Siberian winters in the mid-Holocene were 3.7°C warmer than now, with greater warming in higher latitudes. The major winter warming was concentrated in the Taiga zone on the plains and tablelands of East Siberia, where a warm and moist climate was necessary to support a broad expanse of shade-tolerant dark-needled Taiga. January temperatures averaged about 1°C warmer than now across southern Siberia, although large areas show no change. July temperature anomalies (0–5°C) are distributed mostly latitudinally, with anomalies increasing with latitude above 65°N. At latitudes below 65°N, July temperature was nearly the same as today across Siberia. Based on July temperatures, Siberian summers in the mid-Holocene were 0.7°C warmer than today's. Annual precipitation in Siberia was predicted to be 95 mm greater in the mid-Holocene than now. Most of the increase was concentrated in East Siberia (154 mm average increase). The precipitation anomalies are small in the south. Large precipitation anomalies are found in central and northeastern Siberia. This location corresponds rather closely to the large anomalies in January temperature in East Siberia. The annual precipitation increase was >200 mm more than present precipitation in Yakutia. This increase corresponds to the deep penetration of moisture-demanding dark-needled species ( Pinus sibirica, Abies sibirica, Picea obovata ) into East Siberia in the mid-Holocene, where currently only drought-resistant light-needled species ( Larix spp.) are found. Another area of increased precipitation was along the Polar Circle in West Siberia and at the base of the Taymyr Peninsula in East Siberia. In combination with 2–5 C warmer summers, moister climates there allowed forests to advance far northward into what is now the Tundra zone.