The instrumental record of surface temperature change is based on a combination of land air, marine air, and ocean surface temperature changes recorded over roughly the past century and a half. While several different datasets exist, the most widely used version has been produced by the Climatic Research Unit (CRU) of the University of East Anglia in the UK. Issues of consistency and homogeneity of the measurements through time have been taken into account in constructing this global surface temperature database, and measures have been taken to ensure that all non-climatic inhomogeneities (including Urban Heat Island effects) have been removed.
Over land regions, more than 3000 monthly station temperature time series are used. Coverage is denser over the more populated parts of the world, particularly, the United States, southern Canada, Europe and Japan. Coverage is sparsest over the interior of the South American and African continents and over the Antarctic. The number of available stations was small during the 1850s, but increases to over 3000 stations during the 1951-90 period. For marine regions sea surface temperature (SST) measurements taken on board merchant and some naval vessels are used. Coverage is reduced away from the main shipping lanes and is minimal over the Southern Oceans. For convenience, the global surface temperature data are interpolated onto a regular grid (e.g. 5 degree latitude/longitude gridboxes) and formed into ‘anomalies’ that represent relative departures from a base period (rather than absolute temperatures). These gridded data are, in turn, often spatially averaged to yield large-scale mean temperature estimates such as hemispheric or global mean temperature. The data and additional information about the surface temperature dataset including answers to frequently asked questions, are available at the CRU website.
See also MSU Temperature Record.