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Monckton of Brenchley , Edinburgh   A:  In the early days of weather observations, there was rather poor coverage of the world, and most thermometers were in Europe and North America, and they were not necessarily well-sited using standardised enclosures. Interpreting the early data is therefore not straightforward, as there are various possible biases in the observations that should properly be allowed for, and there are also problems in dealing with the non-uniform spatial coverage.  Several groups routinely produce and maintain long-term estimates of global and regional temperature, and they all try to manage these difficulties in different ways