K. V. Kremenetski

Climate change and the northern Russian treeline zone

The Russian treeline is a dynamic ecotone typified by steep gradients in summer temperature and regionally variable gradients in albedo and heat flux. The location of the treeline is largely controlled by summer temperatures and growing season length. Temperatures have responded strongly to twentieth-century global warming and will display a magnified response to future warming. Dendroecological studies indicate enhanced conifer recruitment during the twentieth century. However, conifers have not yet recolonized many areas where trees were present during the Medieval Warm period (ca AD 800–1300) or the Holocene Thermal Maximum (HTM; ca 10 000–3000 years ago). Reconstruction of tree distributions during the HTM suggests that the future position of the treeline due to global warming may approximate its former Holocene maximum position. An increased dominance of evergreen tree species in the northern Siberian forests may be an important difference between past and future conditions. Based on the slow rates of treeline expansion observed during the twentieth century, the presence of steep climatic gradients associated with the current Arctic coastline and the prevalence of organic soils, it is possible that rates of treeline expansion will be regionally variable and transient forest communities with species abundances different from todays may develop.

Peatlands of the Western Siberian lowlands: current knowledge on zonation, carbon content and Late Quaternary history

The Western Siberian lowlands (WSL) are the world’s largest high-latitude wetland, and possess over 900,000 km 2 of peatlands. The peatlands of the WSL are of major importance to high-latitude hydrology, carbon storage and environmental history. Analysis of the existing Russian data suggests that the mean depth of peat accumulation in the WSL is 256 cm and the total amount of carbon stored there may exceed 53,836 million metric tons. A synthesis of published and unpublished radiocarbon dates indicates that the peatlands first developed at the end of the Last Glacial, with a rapid phase of initiation between 11,000 and 10,000 cal yr BP. Initiation slowed after 8000 cal yr BP and reached a nadir at 4000 cal yr BP. There has been renewed initiation, particularly south of 621N, following 4000 cal yr BP. The initial development of peatlands in the WSL corresponds with the warming at the close of the Pleistocene. Cooling after 4000 Cal yr BP has likely led to increased permafrost and increased peatland development particularly in central and southern regions. Cold and dry conditions in the far north may have inhibited peatland formation in the late Holocene. r 2002 Elsevier Science Ltd. All rights reserved.

Pattern of extinction of the woolly mammoth in Beringia

Extinction of the woolly mammoth in Beringia has long been subject to research and speculation. Here we use a new geo-referenced database of radiocarbon-dated evidence to show that mammoths were abundant in the open-habitat of Marine Isotope Stage 3 (∼45–30 ka). During the Last Glacial Maximum (∼25–20 ka), northern populations declined while those in interior Siberia increased. Northern mammoths increased after the glacial maximum, but declined at and after the Younger Dryas (∼12.9–11.5 ka). Remaining continental mammoths, now concentrated in the north, disappeared in the early Holocene with development of extensive peatlands, wet tundra, birch shrubland and coniferous forest. Long sympatry in Siberia suggests that humans may be best seen as a synergistic cofactor in that extirpation. The extinction of island populations occurred at ∼4 ka. Mammoth extinction was not due to a single cause, but followed a long trajectory in concert with changes in climate, habitat and human presence.

Prolonged California aridity linked to climate warming and Pacific sea surface temperature

California has experienced a dry 21st century capped by severe drought from 2012 through 2015 prompting questions about hydroclimatic sensitivity to anthropogenic climate change and implications for the future. We address these questions using a Holocene lake sediment record of hydrologic change from the Sierra Nevada Mountains coupled with marine sediment records from the Pacific. These data provide evidence of a persistent relationship between past climate warming, Pacific sea surface temperature (SST) shifts and centennial to millennial episodes of California aridity. The link is most evident during the thermal-maximum of the mid-Holocene (~8 to 3 ka; ka = 1,000 calendar years before present) and during the Medieval Climate Anomaly (MCA) (~1 ka to 0.7 ka). In both cases, climate warming corresponded with cooling of the eastern tropical Pacific despite differences in the factors producing increased radiative forcing. The magnitude of prolonged eastern Pacific cooling was modest, similar to observed La Niña excursions of 1o to 2 °C. Given differences with current radiative forcing it remains uncertain if the Pacific will react in a similar manner in the 21st century, but should it follow apparent past behavior more intense and prolonged aridity in California would result.

18. Indications of short-term climate warming at the very end of the Eemian in terrestrial records of Central and Eastern Europe

Geochemical and palynological studies of lacustrine sediments from the standard Eemian-Early Weichselian profiles Grobern, Neumark-Nord and Klinge (Germany, Central Europe) document at least two warming events during the transition from the Eemian to the Early Weichselian. The first pronounced warming phase takes place towards the very end of the Eemian Interglacial during pollen assemblage zone E7, just before the actual transition into the Weichselian Glacial period. Its amplitude is not on the scale of the Eemian climatic optimum, but is comparable with the conditions found in the first Early Weichselian Interstadial (Brorup). An additional event of climatic amelioration was detected within the period of the first Weichselian Stadial (Herning). In the high-resolution Eemian-Early Weichselian limnic sequence from Ples in the Upper Volga region (Russia, Eastern Europe), we also found indications of climate warming events at the very end of the Eemian during pollen assemblage zone E7 and within the Herning Stadial recorded both in palynological and in geochemical records. Furthermore, the δ18O results of the new Greenland ice core presented by the North Greenland Ice Core Project (NGRIP) members record ‘a hitherto unrecognised warm period initiated by an abrupt climate warming about 115 000 years ago (towards the end of the Last Interglacial), before glacial conditions were fully developed’. In this paper, we discuss possible correlations between our terrestrial results in Central and Eastern Europe and their possible connection to the NGRIP record. It appears that both in Central and Eastern Europe and in Greenland, warming phases towards the end of the Last Interglacial preceded the final transition to glacial conditions. Thus, natural warming episodes during the end of the Last Interglacial appear to be a global phenomenon for the Northern Hemisphere.