Why do global and hemispheric temperature anomalies differ from those quoted in the IPCC assessment and the media?
We have areally averaged grid-box temperature anomalies (using the HadCRUT3v dataset), with weighting according to the area of each 5° x 5° grid box, into hemispheric values; we then averaged these two values to create the global-average anomaly. However, the global and hemispheric anomalies used by IPCC and in the World Meteorological Organization and Met Office news releases were calculated using optimal averaging. This technique uses information on how temperatures at each location co-vary, to weight the data to take best account of areas where there are no observations at a given time. The method uses the same basic information (i.e. in future HadCRUT3v and subsequent improvements), along with the data-coverage and the measurement and sampling errors, to estimate uncertainties on the global and hemispheric average anomalies. Our alternative technique (used here) produces no estimates of uncertainties, but our results generally lie within the ranges estimated by optimum averaging. T
