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The Spatial Extent of Northern Hemisphere Temperature Anomalies, AD 800-1995


T.J. Osborn and K.R. Briffa

Complete Scientific Reference

A complementary approach to documenting temperature variability and trend over time, is to investigate the spatial extent of periods of warm and cold temperatures. In this recent study by Osborn and Briffa (2006), a collection of 14 high-resolution temperature-sensitive proxy records from the Northern Hemisphere were used to show the pattern of widespread temperature anomalies over the past 1200 years. These records, which include data from tree rings, ice cores, fossil shells, and historical documents, were selected on the basis of their correspondence with local instrumental temperature data. The set was used to infer regionally synchronous periods of warm and cold conditions by assessing the proportion of records with values exceeding a certain threshold. In the graph shown below, the fraction of records with values that are greater than the long-term mean are shown in light orange shading, and less than the long-term mean in light blue shading. The fraction that have values greater than one standard deviation from the mean are shown with dark orange shading (positive departures), and dark blue shading (negative departures).

When smoothed to highlight low frequency variations (longer than 20 years), these data suggest widespread temperature anomalies across the Northern Hemisphere. The period from about 890 to 1170 shows a higher proportion of positive anomalies, coinciding with what has been called the Medieval Warm Period, while the period of 1580-1850 shows a relatively greater fraction of records reflecting colder conditions. This period has often been called the Little Ice Age. In both cases, there are also a number of records that show the opposite anomaly, indicating that although widespread, these periods contained some spatial variability in temperatures. In contrast, the fraction of sites with temperatures much warmer (greater than one standard deviation) than the long-term mean is greater over almost the entire 20th century than at any time in the past 1200 years, inferring warm conditions across the Northern Hemisphere over much of the 20th century.

One question that may be asked is whether this small set of data does an adequate job of representing temperatures across the Northern Hemisphere. A comparison was done using the same type of analysis, but assessing instrumental records of gridded temperature data from all Northern Hemisphere data available and with grid boxes only in close proximity to the 14 proxy records. In both cases, the results were very similar to those from the proxy records, indicating that this set of data does represent hemispheric temperatures.

Osborn Briffa 2006 image

Osborn and Briffa 2006 data and information

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