2007: Comment on ‘The spatial extent of 20th-century warmth in the context of the past 1200 years


However, their finding that the spatial extent of 20th-century warming is exceptional ignores the effect of proxy screening on the corresponding significance levels. After appropriate correction, the significance of the 20th-century warming anomaly disappears. Osborn and Briffa (1) determined exceed-ance counts from 14 smoothed andnormalized Northern Hemisphere tem-perature proxies from 800 to 1995 and concluded that “the 20th century is the most anomalous interval in the entire analysis period, with highly significant occurrences of positive anomalies and positive extremes in the proxy records. ” The proxies were pooled from three previous studies (2–4) and screened for positive correlations to the associated grid cell temperature of the Climatic Research Unit’s CRUTEM2v data set (5). Signif-icance levels of the exceedance counts were de-termined by resampling. However, given the larg

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