Temperature anomaly

For the recorded temperature change due to global warming, see Instrumental temperature record.
Various global surface temperature datasets originally had different reference periods, but for meaningful comparison have been adjusted to have the same "0 °C" reference temperature. Without such an adjustment, the traces would be vertically offset from each other. Here, the "0 °C" value is determined based the average for 1850-1900—considered to be the "pre-industrial" temperature—and does not indicate an absolute measured temperature of "0 °C".

Temperature anomaly is the difference, positive or negative, of a temperature from a base or reference value, normally chosen as an average of temperatures over a certain reference or base period. In atmospheric sciences, the average temperature is commonly calculated over a period of at least 30 years over a homogeneous geographic region, or globally over the entire planet.

Temperatures are obtained from surface and offshore weather stations or inferred from meteorological satellite data. Temperature anomalies can be calculated based on datasets of near-surface and upper-air atmospheric temperatures or sea surface temperatures.