Abstract
Abstract. A fundamental problem in paleoclimatology is to take fully into account the various error sources when examining proxy records with quantitative methods of statistical time series analysis. Records from dated climate archives such as speleothems add extra uncertainty from the age determination to the other sources that consist in measurement and proxy errors. This paper examines three stalagmite time series of oxygen isotopic composition (δ18O) from two caves in western Germany, the series AH-1 from the Atta Cave and the series Bu1 and Bu4 from the Bunker Cave. These records carry regional information about past changes in winter precipitation and temperature. U/Th and radiocarbon dating reveals that they cover the later part of the Holocene, the past 8.6 thousand years (ka). We analyse centennial- to millennial-scale climate trends by means of nonparametric Gasser–Müller kernel regression. Error bands around fitted trend curves are determined by combining (1) block bootstrap resampling to preserve noise properties (shape, autocorrelation) of the δ18O residuals and (2) timescale simulations (models StalAge and iscam). The timescale error influences on centennial- to millennial-scale trend estimation are not excessively large. We find a "mid-Holocene climate double-swing", from warm to cold to warm winter conditions (6.5 ka to 6.0 ka to 5.1 ka), with warm–cold amplitudes of around 0.5‰ δ18O; this finding is documented by all three records with high confidence. We also quantify the Medieval Warm Period (MWP), the Little Ice Age (LIA) and the current warmth. Our analyses cannot unequivocally support the conclusion that current regional winter climate is warmer than that during the MWP.
Highlights
We can learn about the climatic variations over time during the past by examining natural archives. This task comprises (1) sampling an archive, (2) measuring on the samples a proxy variable indicative of climate, (3) dating the samples, and (4) statistically analysing the obtained proxy time series to infer the properties of the climatic process that generated the data
It is well established that the measurement error of δ18O in a speleothem is smaller than the “proxy error” because other considerable factors than climate influence δ18O (Fairchild and Baker, 2012). These error sources and internal variability of the complex, nonlinear climate system (Lorenz, 1963) already make an inference inexact and require us to report the uncertainty of a statistical analysis (Mudelsee, 2010)
With the knowledge generated by this procedure, we look again at the overall results, the regional climate during the Holocene, in a more quantitative manner (Sect. 4.4)
Summary
We can learn about the climatic variations over time during the past by examining natural archives. It is well established that the measurement error of δ18O in a speleothem is smaller than the “proxy error” because other considerable factors than climate influence δ18O (Fairchild and Baker, 2012). These error sources and internal variability of the complex, nonlinear climate system (Lorenz, 1963) already make an inference (of trends) inexact and require us to report the uncertainty of a statistical analysis (Mudelsee, 2010). The scientists measuring and establishing those records are mostly experts and do perform excellently when reporting the scope and limitations of the methods they develop and use.)
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