What are the differences between HadCRUT3, GISS and NCDC global temperature analyses?
A. The datasets are largely based on the same raw data, but each analysis treats that data differently. HadCRUT3 is perhaps the simplest. The available data are averaged onto a regular grid. No attempt is made to fill grid boxes where there are no data, instead the empty boxes are treated as an additional source of uncertainty when area averages, such as the global average, are calculated. The GISS analysis uses an interpolated sea-surface temperature analysis, which fills in some of the gaps in the sea-surface temperature data. The land station data is also interpolated over data free regions (including over the oceans) to a maximum distance of 1200km. This has a particularly large effect over the Arctic and Antarctic where there are few data points and temperature variability is large. One important thing to note is that the difference between the GISS and HadCRUT3 analyses are smaller than the calculated uncertainties on the HadCRUT3 data set – the data sets are not inconsistent. Th
