Some critics argue that the global record of land surface temperature over the 20th century could be to some extent corrupted by heat from towns and other factors. There is a clear warming, though, if a lesser one, in two other records made independently; that of the temperature of the oceans’ surface waters and of the night time air temperature over the oceans. While the larger warming over land could in part reflect some error of procedure, though that is not proven, the trends all have the same shape.

Source: The Economist on-line

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