<P> Temperature anomalies are useful for deriving average surface temperatures because they tend to be highly correlated over large distances (of the order of 1000 km). In other words, anomalies are representative of temperature changes over large areas and distances . By comparison, absolute temperatures vary markedly over even short distances . </P> <P> The Earth's average surface absolute temperature for the 1961--1990 period has been derived by spatial interpolation of average observed near - surface air temperatures from over the land, oceans and sea ice regions, with a best estimate of 14 ° C (57.2 ° F). The estimate is uncertain, but probably lies within 0.5 ° C of the true value . Given the difference in uncertainties between this absolute value and any annual anomaly, it's not valid to add them together to imply a precise absolute value for a specific year . </P> <P> The period for which reasonably reliable instrumental records of near - surface temperature exist with quasi-global coverage is generally considered to begin around 1850 . Earlier records exist, but with sparser coverage and less standardized instrumentation . </P> <P> The temperature data for the record come from measurements from land stations and ships . On land, temperature sensors are kept in a Stevenson screen or a maximum minimum temperature system (MMTS). The sea record consists of surface ships taking sea temperature measurements from engine inlets or buckets . The land and marine records can be compared . Land and sea measurement and instrument calibration is the responsibility of national meteorological services . Standardization of methods is organized through the World Meteorological Organization (and formerly through its predecessor, the International Meteorological Organization). </P>

When does the reliable instrument record begin for climate variables