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The Spatial Extent of 20th-Century Warmth in the Context of the Past 1200 Years

By Timothy J. Osborn and Keith R. Briffa

Abstract:

Periods of widespread warmth or cold are identified by positive or negative deviations that are synchronous across a number of temperature-sensitive proxy records drawn from the Northern Hemisphere. The most significant and longest duration feature during the last 1200 years is the geographical extent of warmth in the middle to late 20th century. Positive anomalies during 890 to 1170 and negative anomalies during 1580 to 1850 are consistent with the concepts of a Medieval Warm Period and a Little Ice Age, but comparison with instrumental temperatures shows the spatial extent of recent warmth to be of greater significance than that during the medieval period.

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Popular summary as given in "Science":

A number of unusually warm or cold intervals can be seen in most proxy records of temperature of the last millennium, so how can we assess the relative magnitude of the current warm period? Osborn and Briffa (p. 841) compared the geographic extent of late 20th-century warming in the Northern Hemisphere to the distribution of both warm and cold intervals for the last 1200 years by adopting specific thresholds to define warm and cold periods in order to avoid questions about of the absolute magnitude of warm and cold events, and they considered only a subset of the data chosen specifically for its value as a temperature proxy. They find that the continuing warmth of the late 20th century is the most widespread and longest temperature anomaly of any kind since the 9th century A.D.

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