Page "Temperature record of the past 1000 years" Paragraph 9

In general, the recent history of the proxy records is calibrated against local temperature records to estimate the relationship between temperature and the proxy.

The longer history of the proxy is then used to reconstruct temperature from earlier periods.

Proxy records must be averaged in some fashion if a global or hemispheric record is desired.

Considerable care must be taken in the averaging process ; for example, if a certain region has a large number of tree ring records, a simple average of all the data would strongly over-weight that region.

Hence data-reduction techniques such as principal components analysis are used to combine some of these regional records before they are globally combined.

An important distinction is between so-called ' multi-proxy ' reconstructions, which attempt to obtain a global temperature reconstructions by using multiple proxy records distributed over the globe and more regional reconstructions.

Usually, the various proxy records are combined arithmetically, in some weighted average.

More recently, Osborn and Briffa used a simpler technique, counting the proportion of records that are positive, negative or neutral in any time period.

This produces a result in general agreement with