Climate Proxies (climate + proxy)

Distribution by Scientific Domains


Selected Abstracts


Towards ice-core-based synoptic reconstructions of west antarctic climate with artificial neural networks

INTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 5 2005
David B. Reusch
Abstract Ice cores have, in recent decades, produced a wealth of palaeoclimatic insights over widely ranging temporal and spatial scales. Nonetheless, interpretation of ice-core-based climate proxies is still problematic due to a variety of issues unrelated to the quality of the ice-core data. Instead, many of these problems are related to our poor understanding of key transfer functions that link the atmosphere to the ice. This study uses two tools from the field of artificial neural networks (ANNs) to investigate the relationship between the atmosphere and surface records of climate in West Antarctica. The first, self-organizing maps (SOMs), provides an unsupervised classification of variables from the mid-troposphere (700 hPa temperature, geopotential height and specific humidity) into groups of similar synoptic patterns. An SOM-based climatology at annual resolution (to match ice-core data) has been developed for the period 1979,93 based on the European Centre for Medium-Range Weather Forecasts (ECMWF) 15-year reanalysis (ERA-15) dataset. This analysis produced a robust mapping of years to annual-average synoptic conditions as generalized atmospheric patterns or states. Feed-forward ANNs, our second ANN-based tool, were then used to upscale from surface data to the SOM-based classifications, thereby relating the surface sampling of the atmosphere to the large-scale circulation of the mid-troposphere. Two recorders of surface climate were used in this step: automatic weather stations (AWSs) and ice cores. Six AWS sites provided 15 years of near-surface temperature and pressure data. Four ice-core sites provided 40 years of annual accumulation and major ion chemistry. Although the ANN training methodology was properly designed and followed standard principles, limited training data and noise in the ice-core data reduced the effectiveness of the upscaling predictions. Despite these shortcomings, which might be expected to preclude successful analyses, we find that the combined techniques do allow ice-core reconstruction of annual-average synoptic conditions with some skill. We thus consider the ANN-based approach to upscaling to be a useful tool, but one that would benefit from additional training data. Copyright © 2005 Royal Meteorological Society. [source]


Climatic change during the last 4000 years in the southern Tarim Basin, Xinjiang, northwest China,

JOURNAL OF QUATERNARY SCIENCE, Issue 7 2007
Wei Zhong
Abstract In this study, a ca. 4000,cal. yr ancient lacustrine (or wetland) sediment record at the southern margin of Tarim Basin is used to reconstruct the history of climate change. Six radiocarbon dates on organic matter were obtained. ,18O and ,13C of carbonate, pollen and sediment particle size were analysed for climate proxies. The proxies indicate that a drier climate prevailed in the area before ca. 1010 BC and during period 1010 BC,AD 500 climate then changed rapidly and continuously from dry to moist, but after about AD 500 climate generally shows dry condition. Several centennial-scale climatic events were revealed, with the wettest spell during AD 450,550, and a relatively wetter interval between AD 930,1030. Pollen results show that regional climate may influence human agricultural activities. Spectral analysis of mean grain size (MGS) proxy reveals statistically pronounced cyclic signals, such as ca. 200,yr, ca. 120,yr, ca. 90,yr, ca. 45,yr and ca. 33 or 30,yr, which may be associated with solar activities, implying that solar variability plays an important role in the decadal- and centennial-scale climate variations in the study area. Copyright © 2007 John Wiley & Sons, Ltd. [source]


High-resolution stratigraphy of the northernmost concentric raised bog in Europe: Sellevollmyra, Andøya, northern Norway

BOREAS, Issue 3 2007
KARL-DAG VORREN
From the Sellevollmyra bog at Andøya, northern Norway, a 440-cm long peat core covering the last c. 7000 calendar years was examined for humification, loss-on-ignition, microfossils, macrofossils and tephra. The age model was based on a Bayesian wiggle-match of 35 14C dates and two historically anchored tephra layers. Based on changes in lithology and biostratigraphical climate proxies, several climatic changes were identified (periods of the most fundamental changes in italics): 6410,6380, 6230,6050, 5730,5640, 5470,5430, 5340,5310, 5270,5100, 4790,4710, 4890,4820, 4380,4320, 4220,4120, 4000,3810, 3610,3580, 3370,3340 (regionally 2850,2750; in Sellevollmyra a hiatus between 2960,2520), 2330,2220, 1950, 1530,1450, 1150,840, 730? and c. 600? cal. yr BP. Most of these climate changes are known from other investigations of different palaeoclimate proxies in northern and middle Europe. Some volcanic eruptions seemingly coincide with vegetation changes recorded in the peat, e.g. about 5760 cal. yr BP; however, the known climatic deterioration at the time of the Hekla-4 tephra layer started some decades before the eruption event. [source]


Trends in the boreal summer regional Hadley and Walker circulations as expressed in precipitation records from Asia and Africa during the latter half of the 20th century

INTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 5 2008
Hongxu Zhao
Abstract West African summer rainfall, north China summer rainfall and a new climate proxy, snow accumulation from the Dasuopu ice core in the southern Himalaya, have all experienced decreasing trends during the latter half of the 20th century. In this paper, we investigate the existence of a common mechanism that explains these geographically dispersed trends during the boreal summer. In particular, we explore the hypothesis that these trends are related to changes in the regional Hadley and Walker circulations. We show that the divergent circulation in the NCEP reanalysis indicates the existence of trends in these circulations that are consistent with the observed changes in the precipitation records. In addition, the regressions of the divergent circulation in the NCEP reanalysis against these precipitation records indicate that a similar globally coherent signal is associated with the time series and their linear trends while the regressions against the de-trended residuals do not contain statistically significant large-scale signals. These similarities lead us to conclude that the decreasing trends in the three precipitation time series during the latter half of the 20th century are consistent with large-scale changes in the global overturning circulation during the boreal summer. Copyright © 2007 Royal Meteorological Society [source]